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Investigation of the systems LaNi5Hx and LaNi5Dx by proton and deuteron nuclear magnetic resonance
483
Journal of the Less-Common Metals, 49 (1976) 483 - 502 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
INVESTIGATION OF THE SYSTEMS LaNisH, AND La&D, BY PROTON AND DEUTERON NUCLEAR MAGNETIC RESONANCE*
R. G. BARNES**,
W. C. HARPER,
S. 0. NELSON, D. K. THOME and D. R. TORGESON
Ames Laboratory, Energy Research and Development Administration of Physics and Metallurgy, Iowa State University, Ames, Iowa 50010
and Departments (U.S.A.)
(Received December 1, 197 5)
Summary We report on the results of a study of the LaNis hydride and deuteride systems by means of C.W. (wide-line) nuclear magnetic resonance (NMR). The NMR spectra of samples of LaNi,H, (2.1 < x < 6.8) and of LaNisD, (3.4 < x < 6.7) were obtained at resonance frequencies in the range 3 - 100 MHz for protons and at 16 MHz for deuterons, at temperatures in the range 4.2 380 K. The temperature dependence of the shape, line-width, and second moment of the proton resonance and of the nuclear quadrupole interaction parameters characterising the deuteron resonance spectrum reflect the effects of the rapid diffusive motion of the hydrogen species at temperatures above approximately 140 K. At low temperatures (4.2 - 140 K) the proton resonance shape is distinctly non-Gaussian, with a second moment of 12.78 + 0.26 Oe2, independent of composition, and the deuteron spectrum may be characterised by two or three quadrupole interactions, also independent of composition. Both observations indicate the existence of a single hydride phase possessing a narrow range of stoichiometry. At temperatures above approximately 270 K the deuteron spectrum reflects only a single, weak, though well-resolved, quadrupole interaction. These results are discussed in terms of several models for hydrogen (deuterium) interstitial site occupation. Measurements are also reported bearing on the existence of an adsorbed hydrogen surface layer at temperatures in the range 4.2 - 100 K.
*Presented at the Meeting on “Hydrogen in Metals”, held at the University of Birmingham, United Kingdom, on January 5 - 6, 1976, under the auspices of the Chemical Society (Faraday Division). **Address during 1975 - 76: Physikahsche Chemie III, Technische Hochschule, 61 Darmstadt, Petersenstr. 15, Federal Republic of Germany.
484
1. Introduction A variety of recent studies has shown that the inte~e~lic compound LaNi, can absorb almost seven hydrogen atoms per formula unit at essentially ambient temperature and pressure [ 1 -31. Hydrogen is rapidly absorbed and desorbed with only slight changes in pressure, making this system particularly interesting from the standpoint of potential hydrogen storage applications [4]. Proton and deuteron nuclear magnetic resonance (NMR) together provide a very useful probe for the study of hydrogen-containing solids as they sample both the magnetic and electrostatic environment of the nucleus. We report here the results of such a NMR investigation of compounds in the systems LaNi,H, (2.1 G x < 6.8) and LaNiSD, (3.4 G x G 6.7), in which the temperature dependence of the proton resonance lineshape and second moment, and of the deuteron electric qua~pole interaction have been measured over the temperature range 4.2 - 380 K, with the objective of determining the probable interstitial site locations of the hydrogen species at both low and high temperatures. The results of these measurements are discussed in the context of two structural arrangements which appear to be most probable. These are the hexagonal ~6/~~~ structure of LaNiS itself [S] and the trigonal P31m structure proposed by Bowman et al., on the basis of powder neutron diffraction measurements [6] . 2. Sample preparation and experimental details The starting LaNib was prepared by arc-melting high-purity Ames Laboratory lanthanum (99.99%) with nickel, also of nominal 99.99% purity, six or seven times to ensure homogeneity. The single phase character of the resulting material was verified by both X-ray and metallographic analysis. Following successive hydriding cycles to reduce the LaNiS to a very fine powder, the final hydride and deuteride com~sitio~ were prepared and sealed off in pyrex tubes at a temperature of 77 K. Compositions were determined from published pressure-composition isotherms and the measured quantity of hydrogen (deuterium) gas reacted. In the case of the deuteride samples, two grades of nickel were used to ascertain the effect of iron impurity content on the NMR parameters. Those samples of LaNi5D, with 3t = 3.4 and 5.75 contained -450 ppm Fe by weight, whereas all other samples contained -12 ppm Fe by weight as determined by spark-source mass spectrometry. All the NMR measurements were made with induction spectrometers employing crystal-stabilised radio-frequency sources and Varian Associates crossed-coil probes [ 7] . Conventional 400 channel nuclear physics analysers adapted for NMR applications were used for signal averaging. Proton (‘H) resonance data were obtained at frequencies ranging from 3 to 100 MHz, corresponding to magnetic field strengths in the range 720 - 23,800 Oe to check the dependence on applied field strength. However, most spectra were
485
recorded at a nominal frequency of 28 MHz. Deuteron (2D) resonances were recorded at a nominal frequency of 16 MHz, corresponding to an applied magnetic field strength of 23,400 Oe. Signal averaging for periods of four to five hours was typically required at most temperatures to obtain measurable spectra. Some spectrometer mode-mixing difficulty (i.e., mixing of the absorption and dispersion mode signals) was experienced due to the apparent high electrical conductivity of the compounds and the good electrical contact between the individual particles. This was encountered especially at the higher frequencies employed for some of the proton resonance measurements, where skin-depth problems would be expected to become more severe. Some samples of LaN& were mixed with an equal quantity of boron nitride before the final hydriding, and this was found to reduce substantially the modemixing effects. The nominal resonance frequency of 28 MHz used for the majority of the proton measurements represents a compromise between signal-sensitivity and avoidance of mode-mixing difficulties. For temperatures other than room temperature and the bath temperatures of 4.2 K and 77 K, a gas-flow cryostat system was employed. With this system, a temperature gradient of several degrees K was monitored across the length of the sample (-1.5 cm) by means of thermocouples attached to the sample tube. Experimental runs were characterised by the mean of the temperature extremes recorded during the run, the thermal gradient across the sample constituting the largest uncertainty in this temperature.
3. Proton
NMR measurements
(a) Resonance in the solid Qualitatively, the proton NMR spectra show the typical features of motional narrowing of the resonance line with increasing temperature. This behaviour was previously reported by Halstead for a sample having the composition LaNis.sH, [8]. The wide line observed at low temperature (4.2 140 K) narrows sharply at temperatures above approximately 140 K, and the narrowing process is essentially complete at 200 K. In the low-temperature regime (T < 140 K), the resonance shape, width, and second moment are constant, and no significant dependence on hydrogen concentration is observed in the range 2.1 < x < 6.8 which was investigated. As elaborated further below, the line-shape in this region is “squarer” than Gaussian. A typical example of the absorption-mode derivative signal from such a resonance is shown in Fig. 1 for the composition x = 6.8 at 77 K. This resonance is fitted very closely by a function of the form exp(--blH - H014) where Ha is the field at the resonance centre and b is a constant related to the line-width. In this example, the sample was effectively quenched from 200 K to 77 K by immersing it in the liquid nitrogen bath. The narrow line which appears approximately in the centre of the recording apparently results from this quenching, and we believe it represents a trapped hydrogen film on the sur-
ABSORPTION
DERIVATIVE
(ARBITRARY
UNITS)
_
The second moment of the proton resonance at low temperatures was obtained both by direct numerical integration of the recorded derivative traces, and by fitting the derivatives of the empirical line-shape function g(H - H,) = A exp(--b/H -HOI”), where n is permitted to vary continuously between 1.5 and 4.0, to the recorded derivative trace. For this function, the second moment is given by M2 = {n,(n - 1)}2’“f?(3/n)
(AH~2/~r(l/2)
(1)
where AH is the separation of the extrema of the derivative curve [ 12 J . For a Gaussian shape function, n = 2 and &f2 = (AH)2/4, whereas for a “rectangular” shape function, n = 4 and M2 = 0.098 (AJQ2. This method substitutes judgement of overall goodness of fit for judgement of where the tails of the derivatives vanish. For the resonances showing evidence of two components, this procedure also circumvents the difficulties associated with subtracting out the narrow component before calculating the second moment of the broad component. These results are summarized in Table 1, which lists the n-values, linewidths (as defined above), and second moments, for hydrogen concentrations ranging from it: = 2.1 to x = 6.8 at 77 K, and at a nominal resonance frequency of 28 MHz (corresponding to 6670 Oe). In all cases, the shape is seen to be rectangular or close to rectangular. The shape, width, and second moment of these resonances remain constant at temperatures below 140 K, down to and including 4.2 K. In the vicinity of 300 K, the proton reson~ce width is strongly dependent upon the applied magnetic field strength, as is to be expected for NMR in materials of high magnetic susceptibility. Drain has shown that, for a closelypacked powder, the field-dependent broadening is given approximately by 3xvH0 where xv is the volume susceptibility and H,, is the applied magnetic field strength [13]. At 300 K, the proton line-widths can be represented by expressions of the form AH = (AH), f- 3xvHa for applied field strengths to 24 kOe. The residual line-width (AH), = 0.25 Oe for x =: 2.1 and 0.07 Oe for x = 6.8, and xv ranges from 2.3 X 10m5 cme3 for x: = 2.1 to 1.1 X 10mm5 cmS3 for x = 6.8. From the measured density of the powder samples (-3.5 g cmS3), mass susceptibilities of 6.4 and 3.2 X 10mm6ggl are obtained for x^ = 2.1 and 6.8, respectively, These are in good agreement with a direct measurement of the susceptibility of La.Nis at 300 K which yielded 4.9 X lOme g-l. The Knight shift of the proton resonance was also measured at 300 K where the line width was sufficiently narrow to permit a sensible determination of this quantity. Defined as K = (HH - HM)/HR for measurements at constant frequency, where H, and HM are the fields for resonance in a reference material (water) and in the metal (LaNi&I,), respectively, the shift was found to have the value K = +(0.005 + O.OOl)% for the composition range 4.4 < x < 6.8 at 300 K. {b) Surface film resonance As seen both in Fig. 1 and in the last tracing of Fig. 2, samples quenched from 300 to 77 K showed a narrow resonance super~pos~ on the broad
488 TABLE
1
Summary of proton resonance parameters in LaNisH, at 77 K and at a resonance frequency of 28 MHz Hydrogen cone.
Shape function exponent
Line-width
X
n
g
2.1 4.4 5.2 6.2 6.7 6.8
3.4 3.6 4.0 3.8 4.0 4.0*
10.7 11.2 11.4 11.2 11.4 11.6*
*Me~urement
at 54 MHz.
f 0.5)
Second moment M2
(Oe f 0.3) 12.4 13.0 12.6 12.7 12.8 13.2*
line background of the bulk material. This resonance, as seen in Fig. 1, is broader than its true width, due to the excessive modulation amplitude employed in the scan (appropriate to the broad line). However, by scanning only over the central region of the broad line, the narrow component could be properly measured. Table 2 lists the width of the narrow resonance at a nominal resonance frequency of 51 MHz as a function of decreasing temperature below 77 K. The major part of the measured width in the range 20 < 7’ G 77 must be ascribed to the bulk susceptib~ity effect discussed above in Section 3(a). However, below 20 K the line broadens in qualitative agreement with the behaviour of solid hydrogen [ 141. Below roughly 10 K, the narrow line merges into the broad line and can no longer be distinguished. Efforts to observe a Fake doublet structure of the resonance at 4.2 K, as has been observed for hydrogen adsorbed on zeolite by Monod et aZ. [15], were unsuccessful. The Knight shift of the narrow resonance is, however, consistently negative, K = -(0.005 f O.OOl)%, by contrast to the positive shift of the bulk signal at room temperature. In one series of experiments, hydrogen gas was sealed in a sample tube with LaNi6 powder at 77 K. Only the narrow resonance could then be observed at 77 K. When the sample tube was warmed to 300 K, absorption of the gas occurred, as evidenced by the resonance shift and width at 300 K (appropriate to an x = 2, as determined by the quantity of gas enclosed). Quenching the sample to 77 K then yielded the typical broad-line plus narrowline spectrum, similar to that of Fig. 1. In repetitions of this experiment, a structure was sometimes observed in the initial narrow line at 77 K, suggestive of shift anisotropy. Differences in sample preparation details and history apparently affect the characteristics of the narrow line si~ific~tly.
489
TABLE
2
Line-width of the narrow component of the proton resonance to 77 K, at a nominal resonance frequency of 51 MHz Temperature (K)
Line-width (Oe)
67.0 42.9 29.8 21.3 17.0 13.9 10.3 7.2 4.2
0.64 f 0.1 0.61 f 0.1 0.67 f 0.1 0.62 + 0.1 0.83 + 0.1 1.32 f 0.3 not observed not observed not observed
in LaNigH6.8
quenched
4. Deuteron NMR measurements (a) Resonance in the solid In non-cubic materials, the NMR spectrum of the deuteron reflects the interaction of its electric quadrupole moment with the gradient of the total electrostatic field at the deuteron. In view of the small magnitude of the deuteron quadrupole moment (Q = 0.2875 + 0.0020 X 1O-26 cm2 [ 16]), the effects of this interaction on the NMR spectrum are adequately described by simple first-order perturbation theory. A spectrum may be characterised by the strength of the quadrupole interaction and by the asymmetry of the electric field gradient tensor (efg tensor) [ 171. In conventional notation, the elements of the efg tensor in its principal axis system are expressed in terms of the “field gradient”, q, and asymmetry parameter, 77 eqzz = a2v/az2
eqrv = a2v/ay2 eqxx = a2v/ax2
= eq = -+ (1 + v)eq =-$ (l-_7))eq,
where V is the net electrostatic potential at the deuteron site and e is the magnitude of the electron charge. Thus, 7 = (q,, - qyy)/qzz, with the prescription that lq,,l > lqyyl > Iq,..l, so that 0 < 71< 1. With polycrystalline (powder) samples, the observed spectrum is an average over all orientations of the efg tensor with regard to the applied magnetic field [ 171. The powder spectrum possesses three distinctive features: a pair of strong singularity peaks separated by Au2 = f YQ (1 a (1 + 7]), and a pair of steps separated pair of shoulders separated by Au i = 3 Here, the frequency, (3/2)(e2qQ/h), is a convenient meaby Avs = sure of the quadrupole interaction strength. In addition, an important “nonfeature” of the deuteron spectrum is the absence of a central peak such as occurs in the quadrupole perturbed spectra of half-integer spin nuclei. Such q),
vQ
VQ.
vQ
=
MODULATION It-O25Oe AMPLITUDE
/
I
E
P
a
2
I
I 23 705
I 23 715
23 710 MAGNETIC
FIELD
I 23725
23720
IN
(4
KOe
r
I La N’5
?
15 499
3 &
1395
D5 75 693
I AH,6
MHz
~88
2 Oe
nHz5=46.5
K
nHj4=20
Oe 7 Oe
2 Hc:23.712
!j+ ; s 6z x
148
8 KOe
MINUTES
MODULATION
21 SCANS
AMPLITUDE
L 0 k : m
9
4 I
I
23.650
V
MAGNETIC
FIELD
I 23750
23.700 IN KOe
I 23.800
(b)
Fig. 3. (a) Absorption derivative signal of deuteron NMR in LaNigD6.7 at 300 K. The splitting of the major peaks corresponds to a quadrupole coupling of 7.2 kHz. (b) Absorption derivative signal of deuteron NMR in LaNigD5.75 at 139.5 K.
a central peak can only appear in deuteron NMR when the effective quadrupole interaction strength is zero. Typical examples of deuteron spectra observed in LaNi5D, are shown in Fig. 3(a) and (b). As seen in Fig. 3(a), the 300 K spectra show well-resolved singularity and step features characteristic of a zero or very small asymmetry parameter interaction [ 171. For T < 300 K, the spectra become progressively less well-resolved, until in the range 160 - 200 K they appear as broad, unsplit lines. At still lower temperatures (T < 160 K), recognisable features (splittings) are again observed, as shown in Fig. 3(b). This spectrum remains unaltered in general appearance down to 4.2 K. All deuterium concentrations with x > 3 exhibited these features and temperature dependence, and, in
491
;-- --~ LaNi5D67 vop=15 5 MHz
,-I I-l__-L0
200 TEMPERBTC!RE IO@
1
I
300 400 IN K
Y
23700
23710 MAGNETIC
23720 FIELD (KOe)
23 730
Fig. 4. Temperature dependence of the quadrupole splitting of the deuteron NMR spectrum in LaNi$,. Fig. 5. Sequence of deuteron resonance derivative recordings illustrating the extinction of the quadrupolar spectrum in the temperature range 300 Q T < 380 K. The recordings were made sequentially with increasing temperature.
general, the greater the deuterium concentration the more clearly were the spectral features resolved. It should be noted that the broad lines observed at intermediate and low temperatures are much too broad to result from nuclear dipolar broadening. A plot of quadrupole splitting us. temperature for each sample is displayed in Fig. 4. The splitting refers to the main (singularity) peaks of Fig. 3(a) and to the innermost peaks (AH%) of Fig. 3(b). The increased AH,, values for the x = 3.4 and 5.75 samples are very probably a consequence of their relatively high iron content contributing an anomalous width. The samples containing less iron yield substantially smaller splittings both at 100 K and at 4.2 K. At 4.2 K the various splittings are essentially independent of composition, and have the values: AH,, = 21.9 + 2.5 Oe, AH,, = 39.8 + 2.0 Oe, and AH,, = 82.8 f 2.0 Oe, corresponding to frequency splittings of Au, = 14.3 + 1.5 kIIz, Ava5 = 26.0 f 1.3 kHz, and Avr6 = 54.1 + 1.3 kHz. In the temperature range T 2 300 K, the 2D quadrupolar spectrum was observable up to an “extinction” temperature, Tc . In this range, the spectrum consists of a single, narrow line superimposed on the centre of the quadrupolar pattern. As shown in Fig. 5, the intensity of the quadrupolar pattern is extinguished in this range, leaving only the narrow line at high
492 TABLE
3
Quadrupole interaction parameters used to generate synthetic spectra which match the experimental spectrum of Fig. 3(b) Parameters
Synthetic spectrum number 1
VQl 1)l 61
2
772 62
63
16.3 0.95 0.35
17.0 0.95 0.32
17.3 0.01 0.43 1.00
1.00
44.4 0.01 0.35
35.9 0.95 0.25
58.2 0.28 0.12
58.2 0.28 0.12
1.4
1.5
116.6 0.01 0.05
109.5 0.01 0.06
Intensity VQ3 ‘173
4
15.4 0.01 0.44
Intensity vQ2
3
Values of vQ are given in kHz, and the field gradient inhomogeneity, 6, is defined by AvQIvQ, where AvQ is the full-width of a Gaussian distribution of efg values centred on VQ-
temperature. Extinction temperatures fall in the range 360 < Tc < 380 for x values in the range 3.6 < x < 6.7. Within this range, the quadrupole splitting increases linearly with temperature, as can be seen at the high temperature end of Fig. 4, the increase for x = 6.7 being 27%, for example. The low-temperature spectra were fitted (but not in the least-squares sense) by synthetic spectra generated by folding a Gaussian broadening function into the powder pattern distribution function [ 171. Qualitatively, as remarked above, the broad character of the 2D spectra at low temperatures cannot arise from the nuclear dipolar interaction, as in the proton case, because of the much smaller deuteron magnetic moment strength. Instead, we ascribe this broadening to a Gaussian distribution of quadrupole interaction strengths, as a first approximation. In fact, there very probably exists a distribution of both vo and 7) values, and in our treatment we have, for simplicity, ignored the latter. The parameters employed in four different synthetic spectra, each of which approximates fairly well to the experimental spectrum at low temperatures, are listed in Table 3. Synthetic spectra (1) and (2) comprise three different quadrupole interactions; in the case of (1) all the interactions have essentially zero asymmetry, in the case of (2) two have large asymmetry. Spectra (3) and (4) comprise two different interactions; in the case of (3) with small asymmetries, and in the case of (4) one interaction has a large asymmetry. The relative intensities of the component spectra in the latter two syntheses are also listed. One may argue, in a least-structure sense, that synthetic spectra (3) and (4) are to be preferred over (1) and (2) because fewer interaction para-
493
WITH 85psi OVER PRESSURE
23.720 23.716 23.718 MAGNETIC FIELD IN KOe
Fig. 6. Absorption mode derivative recording of deuteron NMR in sample of LaNis exposed to deuterium gas at pressure of 85 psi at 77 K.
meters are required to achieve the same quality of fit-to-experiment. Spectrum (4) is somewhat preferable to (3) because it necessitates a slightly smaher field-gradient inhomogeneity. The contrast is not striking, however. (b) Surface film resonance The counterpart of the narrow proton resonance observed in hydride samples (Fig. 1) was not detected in deuterium samples quenched to 77 K. This may very well be attributed to the much weaker signal intensity of the 2D resonances in general. However, sample tubes containing deuterium gas sealed over LaNi, powder at 77 K yielded deuteron NMR traces similar to the proton resonances described for similar conditions in Section 3(b). As in the proton NMR, structure was observed in some experiments indicative of shift anisotropy or the occurrence of two closely spaced resonances. An example of such a structure is shown in Fig. 6, from a sample of deuterium gas sealed over LaNi, powder at 85 psi at 77 K. The width of the principal resonance (as in Fig. 6) was consistently found to be AH = 0.65 f 0.10 Oe, the same as that of the narrow proton line at 77 K. This is consistent with the interpretation that the observed width of this narrow line results from the bulk susceptibility contribution, since this quantity is independent of the magnetic moment of the resonant nucleus. Similarly, the Knight shift of the narrow resonance was found to be Ir: = -(0.005 f O.OOl)%, in agreement with the proton measurements.
5. Discussion (a) Proton NMR in the solid We may compare the proton second moment measurements at low temperatures with theoretical expectations based on two structural models for hydrogen interstitial site occupation in LaNi, hydride. These are the ~/mm~ structure of LaNis itself [5], and the P31m structure proposed by
. H(m) La
Ni5
STRUCTURE,
C-AXIS
VIEW
.
H(n)
A
Ii
(0)
0.5 011.0 89 0 27,o 73
Fig. 7. Plan view of PG/mmm structure (four unit cells) showing location of hydrogen sites (m), (n), and (o), as well as the metal ions.
Bowman et al. [6]. The unit cell of the PG/mmm structure contains a total of 38 potential interstitial sites [8], distributed among six groups of 1, 3, 4, 6, 12, and 12 sites each, respectively, and designated (b), (f), (h), (m), (n), and (0) in the Wycoff notation. However, on the basis of hydrogen-hydrogen distances, only the last two groups of sites appear to be probable locations 16981. A plan view of the P6/mmm structure is shown in Fig. 7 (4 unit cells are shown for clarity), with the (m), (n), and (0) sites indicated. These account for 30 of the 38 interstitial sites. As discussed by Bowman et al. [6], the 12 (n) sites occur in three sets of four each, having very short (0.8 - 1.1 A) hydrogen-hydrogen distances within each such grouping. In addition, the (f) sites (not shown in Fig. 7) occupy the centres of these groups. Accordingly, one hydrogen is assumed to occupy one of the (n) sites of each group in an ordered manner. As seen in Fig. 7, the (0) sites form two rings of six within the unit cell, and in their analysis of powder neutron diffraction data for LaNisD,.,, Bowman et al. [6] assigned the remaining 3.8 deuterons randomly over the ring furthest from occupied the (n) sites. The same considerations are appropriate in the present analysis, since the observed rigid lattice proton second moment is quite small, indicating that the protons are rather far apart. The P31m structure is a lower symmetry derivative of the PG/mmm structure and contains, in addition to the metal atoms, nine potential hydrogen sites. Three of these, designated (c), correspond to three of the (n) sites of the PG/mmm structure, and the remaining six sites (designated (d)) correspond to one ring of (0) sites in P6/mmm. The metal atom structure of P31m is essentially identical to that of PG/mmm. An obvious difficulty posed by P31m is the very short distance that arises between two of the (d) sites (1.32 a), since simultaneous occupancy of these two sites leads at once to a very large contribution to the proton second moment.
The theoretical second moments for the proton resonance were calculated with the Van Vleck formula [18] appropriate to a powder using the generalisation introduced by Stalinski et al. [19] to take account of the contributions from inequivalent lattice sites to the total moment. Although the contribution from the 61Ni nuclei can be ignored (small isotopic abundance and small nuclear moment), that from the 13’La cannot. In the notation of Weaver [ 111 and of Halstead [ 81, the rigid lattice second moment is given by
M2 =
(2)
fiM$'
5 i
where fi is the fraction of all protons located in sites of type i and M$’ is the second moment of the spectrum resulting from protons located in i sites. We denote by aj the probability of occupation of a j site and by wj the relative number of j sites, so that (3)
wkak
k=l
where N is the number of inequivalent proton sites. The second moment, Mij’, of the protons on an i site is Mt)=
CI g
aj Sij + CFS;
(4)
j=l
where C, = (3/5)(~~h)~Z(Z + 1) for contributions from like nuclei (other protons) and CF = (4/15)(~~h)~Z~(Z~ + 1) for the contributions from unlike nuclei (here lsaLa, with IF = 7/2). S’ij denotes the lattice sum zrriy6 where the origin is taken at an i site and the summation extends over all j sites. Similarly, S;j denotes the sum over La sites with the origin taken at the i-th proton site (all La sites are assumed to be occupied). Combining eqns. (2) and (4), we have the general result M2 = CI 2 i.j
fiaj
Sij + CF s fiS:j.
(5)
i= 1
For the evaluation of the lattice sums, the unit cell parameters were taken as those given by van Vucht et al. [1] for LaNieHs (a = 5.44 A, and c/u = 0.792) which were also used by Halstead [8]. For the PG/mmm’ structure the atomic coordinates within the unit cell were also those used by Halstead: (n) with (x, y, z) = (0.40, 0, O.ll), and (0) with (x, y,z) = (0.20, 0.40, 0.27). These values differ negligibly from those of Bowman et al. [6] : (n) with (x, y, z) = (0.43, 0, 0.12) and (0) with (x, y, z) = (0.21, 0.42,0.29). The coordinates of Bowman et al. [6] were used for the P31m structure: (c) with (x, y, z) = (0.50, 0,0.08) and (d) with (x, y, z) = (0.25,0.86,0.58). The results of second moment calculations are summarised in Table 4. For illustrative purposes, and for comparison with the experimental results of Table 1, the rigid lattice second moments are listed for two x values (6.0 and 6.8) for each of the two structures considered. The first three hydrogens
496 TABLE
4
Calculated second moments of the proton resonance in LaNi5H, for several arrangements of hydrogen on interstitial sites in the PGlmmm and P3lm structures Structure
Hydrogen ordering on sites
Second moment ( Oe2)
x=6
x = 6.8
P6lmmm
ordered (n) ordered (0)
14.8
-
PGlmmm
ordered (n) random (0)
18.8
20.9
P31m
ordered ordered ordered random
11.1
-
10.8
12.0
P31m
(c) (d) (c) (d)*
*Shortest hydrogen-hydrogen
distance (1.32 A) excluded.
are assumed to occupy only the three (n) or (c) sites in PG/mmm or P31m, respectively. For x = 6 the additional three hydrogens are placed either in an ordered arrangement on three of the (0) or (d) sites, or randomly over these sites. For x = 6.8, only a random distribution over the (0) or (d) sites was assumed. For the P6/mmm structure, M2 = 14.8 0e2 for x = 6, assuming an ordered arrangement on both the (n) and (0) sites. With a random distribution over the (0) sites, M2 = 18.8 Oe2, due to contributions from shorter proton-proton separations. Similarly, on the basis of the P31m structure, M, = 11.1 0e2 for x = 6, with an ordered arrangement on the (d) sites. In this case, a random distribution over the (d) sites leads to an anomalously large second moment (26.3 0e2) because of the one very short (1.32 A) spacing which occurs between two of the (d) sites. However, if simultaneous occupancy of these two sites is excluded, and the three hydrogens are permitted to occupy five of the six (d) sites randomly, then M, = 10.8 Oe2 for x = 6. For the composition x = 6.8, 3.8 hydrogens are placed randomly on the (0) or (d) sites, but again excluding the simultaneous occupancy of the two close (d) sites in the P31m structure. For PG/mmm, this yields M2 = 20.9 0e2, and for P31m,M, = 12.0 0e2. The P31m structure leads to reduced second moment values because of the increased separation of the (c) and (d) sites compared with PG/mmm, and through the device of excluding simultaneous occupancy of the close pair of (d) sites. Some reduction of the second moments could also be achieved by permitting some degree of proton motion to continue at low temperatures (e.g., tunnelling). For example, each of the three protons fixed on (n) or (c) sites in this calculation might move constantly over the subgroup of four (n) or (c) sites, since the distances are so short (0.8 - 1.1 A) within each sub-group. This would have an averaging effect on M,. Similarly,
497
in the context of the present calculations, it may be that a proton effectively oscillates between the two close (d) sites in P31m. The present calculation has been aimed only at assessing whether reasonable agreement can be obtained between experimental and calculated second moments on a rigid lattice basis. It appears more reasonable to us that relaxation time measurements, for example, should be made before more exotic calculations are performed. Although undue quantitative significance should not be attached to the calculated second moments, in particular because of the substantial uncertainty in position coordinates of the P31m structure [20], the results support qualitatively the concept of a hydride structure of lower symmetry than that of LaNiS itself. Certainly, the absence of any significant dependence of the experimental second moments on hydrogen concentration, coupled with the calculated values, indicates a rather narrow range of stoichiometry for the hydride phase. Conversely, one may ask at what composition the models discussed here yield the observed value of 12.78 0e2. With a random distribution over the (0) or (d) sites, again excluding the close pair in P31m, the required hydrogen concentrations are x = 4.8 in PG/mmm and x = 6.9 in P31m. (b) Deuteron
NMR in the solid
As a first step towards correlating the observed quadrupole splittings with deuteron locations in the lattice, a calculation of the ion-lattice contribution to the electric field gradient at the different interstitial sites in the expanded lattice was made, taking the La ion charge to be +3 and the nickel charge to be zero. The latter assumption is reasonable in view of the Pauliparamagnetic nature of LaNi5. For the sake of simplicity, a calculation of the deuteron contribution to the efg was not made, since we expect that the efg is dominated by the La3+ contribution and, since screening effects are also neglected, the deuteron contribution will be highly uncertain. Qualitatively, we infer that the considerable inhomogeneity of the coupling constants at low temperatures reflects the inhomogeneity of the deuteron contribution, although it is possible that some of the observed inhomogeneity results from disorder on the La sites (see further below). The Sternheimer anti-shielding effect has also been neglected since no estimates of this parameter have been given for the deuteron in metallic systems. Inasmuch as only the contribution from the La3+ ions is considered, the z component of the ion lattice efg tensor in its principal axis system is given by 42z = 3 jz,
(3zZj
-
rZj)
r;;j5,
(6)
with similar equations for qxx and qy y, where the sum runs over all La3+ sites, the origin being taken at a deuteron site. In this manner the efg tensors (characterised by qZL and 17) were calculated for all the interstitial sites in PG/mmm and in P31m, using the same atom coordinates as for the proton second moment calculations of Section 5(a). The results of these calculations are listed in Table 5 for PG/mmm and in Table 6 for P31m.
498 TABLE 5 Calculated ion-lattice contributions to the electric field gradient tensor at each deuteron site in the PG/mmm structure in the low-temperature (rigid lattice) and high-temperature (motionally averaged) limits Site
Rigid lattice -_ Qrr
1)
VQ
b f h m n
+ + + +
0
+
0 0 0 0.58 0.05 0.22
172.7 77.5 5.7 72.0 90.8 72.0
1.15 0.52 0.04 0.48 0.61 0.48
no m,n,o all
Motionally averaged _ 9ZZ VQ + + -
1.15 0.26 0.04 0.48 0.28 0.05 0.16 0.04 0.03
172.7 38.7 5.7 72.0 42.5 8.1 24.6 5.7 4.8
The qzz values are in units of 1O24 cm3. In the motionally averaged limit, the efg z axis coincides with the crystalline c axis and 7) = 0 for all sites. TABLE 6 Calculated ion lattice contributions to the electric field gradient tensor at each deuteron site in the P31m structure in the low-temperature (rigid lattice) and high-temperature (motionally averaged) limits Site
C
d c,d
Rigid lattice
Motionally averaged --
QZZ
rl
+ 0.52 + 0.24
0.07 0.93
VQ 77.5 35.8
92.2 - 0.33 + 0.12 - 0.03
G 47.0 17.1 4.3
The qrz values are in units of 1O24 cm3 and vQ values are in kHz. In the motionally averaged limit the efg z axis coincides with the crystalline c axis and 17= 0 for all sites.
In view of the proton NMR results which demonstrate the presence of rapid proton motion at temperatures T 2 140 K, the well-resolved deuteron quadrupolar spectrum at 300 K must represent an average interaction over some or all of the interstitial sites [17]. In this respect, we note that the symmetry operations of PG/mmm cause the average efg tensor for each of the six different sets of sites to be axially symmetric (77= 0) with qzr aligned along the crystalline c axis. Without diffusive averaging motion, this is only true for sites (b) and (h). Table 5 also lists the averaged qzz values, (zz), for each of the six sets of sites, and for averages over several groups of sites. For ready comparison with the experimental data, the resultant vQ values in kHz are given for all of these cases. Similar considerations apply in the case of P31m, and the average over the (c) and (d) sites is included in Table 6.
499
Referring now to the quadrupole interaction parameters needed to fit the low-temperature deuteron spectra (Table 3), it is clear that the synthetic spectra (1) and (2), which require three different interaction strengths, must be interpreted in terms of the P6jmmm structure. However, the calculated qzr and ho values in Table 5 show relatively little variation in the rigid lattice case among the various sites. This is especially true for sites (m), (n), and (0) which account for 30 of the 38 sites. Site (h) has a very small interaction, and site (b), of which there is only one per unit cell, has a much stronger interaction. Only site (m) has a substantial asymmetry parameter value, as would be required by the synthetic spectrum (2). Even allowing for the approximate features of the model, the PG/mmm structure does not appear to offer much likelihood of accounting for these synthetic spectra. Conversely, synthetic spectra (3) and (4) require only two different interactions, and could follow from either the P6/mmm or P31m ~ngemen~. However, taking the proton second moment calculations as a guide, we may assume that it is primarily the (n) and (0) sites of PG/mmm and the (c) and (d) sites of P31m that are of relevance. It then appears that the P31m model yields ho and Q estimates which are substantially closer to, and in about the right ratio to, the values required by the synthetic spectra. Moreover, the average over all the (c) and (d) sites is also in reasonable agreement with the observed value of vo = 11 kHz at 300 K. The relative intensities of the two interactions required by synthetic spectra (3) and (4) are not in good agreement with this arrangement, however. In the second moment calculations, 3.8 protons were assigned to the (d) sites and 3 to the (c) sites, so that the intensity of the (d) site interaction should be somewhat greater than that of the (c) site interaction. However, Table 3 shows that the interaction with the larger ho value (hence the (c) site) contributes greater intensity than the weaker interaction (the (d) site). The substantial efg inhomogeneities required by the synthetic spectra, and evident from the experimental data (Fig. 3(b)), may be a consequence of the randomness of the deuteron contribution brought about by the random distribution over the (d) sites. However, it is also possible that a small degree of disorder on the lanthanum sites could have this effect. Although the X-ray data indicated a composition of LaNis for the starting material, the work of Buschow and van Mal [21] has shown that the compound exists over a fairly broad range of composition at high temperature. Efforts to observe the 13’La NMR in the starting material have not been successful, and this tends to support the view that there is inhomogeneity on the La sites themselves. Finally, the temperature dependence of the quadrupole interaction strength in the temperature range 300 G T < 380 appears puzzling. The linear increase, 27% in the case of x = 6.7, of the coupling constant matches almost exactly the 25% volume expansion of LaNiS upon hyd~ding, and it is tempting to ascribe the increase in coupling constant to the lattice contraction accompanying the loss of deuterium at the higher temperatures. However, this would imply that a continuous range of intermediate compositions could exist within each unit cell of the compound, an implication that does not appear
500
to be borne out by any other aspect of the experimental data. Hence, the correspondence between coupling constant and lattice contraction appears to be a fortuitous one. (c) Surface film resonance Both proton and deuteron measurements indicate that an adsorbed surface film can be stabilised on LaN&, powder at temperatures as high as 77 K, either by quenching from 300 K, or simply by exposing clean LaNi, powder to hydrogen (deuterium) gas at moderate pressure at 77 K. The close similarity of the proton line-width behaviour to that of solid hydrogen [ 141 at temperatures in the 10 - 20 K range suggests that we are observing a physisorbed rather than a chemisorbed film. Both the line-width and shift of the film resonances differ substantially from those in the bulk material and in Hz and Dz gas alone. Using identical spectrometer settings with those used for the film resonances, the proton and deuteron resonances were observed in the Hz and Dz gas above the LaNiB sample at 85 psi and 77 K. The line-widths were found to be 0.25 f 0.05 Oe (due to excessive modulation amplitude), and the Knight shift was 0.000 f O.OOl%, i.e., zero within the experimental uncertainty. The proton resonance in quenched samples has an intensity approximately 1O-3 that of the broad line of the bulk material. The particle size of cycled and activated LaNie hydride powder has been reported to be in the range 1 - 4 pm [ 21. Scanning electron microscope pictures reveal small, highly fractured particles (resembling popcorn), consistent with this estimate. van Mal has reported the specific area of a well-activated sample to be 1.17 m2gP1 [4]. For a typical NMR sample of roughly 2 g, a monolayer coverage of molecular hydrogen would amount to 2 X 102’ protons, approximately 0.016 the number in the bulk LaNi,Hs. Thus, it appears likely that the observed signals arise from less than full monolayer coverage. Although a substantial shift difference exists between the bulk and surface resonances, the measurements are not readily susceptible to quantitative interpretation. This is because of the significant contribution to the shift resulting from the high magnetic susceptibility of LaNi, and the overall nonspherical geometry of the powder samples [22] . For the proton resonance in the bulk material, we estimate the contribution to the measured shift, due to this effect, to fall in the range + 0.0011% to - 0.0045%, depending on individual particle geometry. The net effect is an average over the particles in the sample [22]. The resonance from a surface film experiences a different bulk susceptibility effect, since the film behaves analogously to a thin cavity in a solid [23] . Qualitatively, the shift difference is consistent with the difference in origin (i.e., bulk and surface) of the two signals.
6. Conclusions The salient features of the experimental NMR measurements on the LaNiSH, and LaNi&, systems are as follows:
501
(1) At low temperatures (4.2 < T < 140 K) the proton resonance shape is distinctly squarer than Gaussian, with a second moment of 12.78 + 0.26 Oe2 practically independent of composition within the limits 2.1< x < 6.8. (2) In the same temperature range, the deuteron NMR spectrum may be characterised by two or three quadrupole interactions, also independent of composition within the limits 3.4 < x d 6.7. (3) At temperatures in the range 270 < T G 300 the deuteron spectrum reflects only a single, weak, though well-resolved, axially symmetric quadrupole interaction. (4) At still higher temperatures, 300 < T < 380, this quadrupolar spectrum is extinguished and is replaced by a single, narrow, unsplit resonance. (5) Both proton and deuteron experiments show evidence supporting the existence of an adsorbed molecular surface layer at temperatures below 77 K in samples of LaNisH, quenched from 300 K, or in samples of LaNis exposed to H2 or D2 gas at 77 K. The rigid lattice proton second moments and the deuteron quadrupole interactions have been shown to be in reasonable agreement with expectations based on the lower symmetry P31m structure proposed by Bowman et al. [6] for the LaNi5 deuteride. Interpretation of the experimental data within the context of the PG/mmm structure of LaNiB does not appear to be possible. The occurrence of an adsorbed surface layer of hydrogen may not seem surprising in view of the high nickel content of LaNis and the well-known propensity of nickel itself to maintain adsorbed hydrogen layers [24].
Acknowledgements The authors are indebted to Dr. T. Pinter and Dr. Y. S. Hwang of the Ames Laboratory for assistance with the various computations, and to Professor K. Gschneidner for very helpful discussions. One of the authors (R.G.B.) acknowledges the support of the Alexander von Humboldt Foundation and the hospitality of the Technische Hochschule, Darmstadt, where this manuscript was completed.
References 1 J. H. N. van Vucht, F. A. Kuijpers and H. C. A. M. Bruning, Philips Res. Rep., 25 (1970) 133. 2 K. H. J. Buschow and H. H. van Mal, J. Less-Common Met., 29 (1972) 203. 3 H. H. van Mal, K. H. J. Buschow and A. R. Miedema, J. Less-Common Met., 35 (1974) 65. 4 H. H. van Mal, Chem.-Ing.-Tech., 45 (1973) 80. 5 J. H. Wernick and S. Geller, Acta Crystallogr., 12 (1959) 662. 6 A. L. Bowman, J. L. Anderson and N. G. Nereson, Proc. Rare Earth Res. Conf., lOth, Carefree, Arizona, 1972, p. 485. 7 D. R. Torgeson, Rev. Sci. Instr., 38 (1967) 612.
502 8 T. K. Halstead, J. Solid State Chem., 11 (1974) 114. 9 J. F. Hon, J. Chem. Phys., 36 (1962) 759. 10 0. J. Zogal and B. Stalinski, Proc. Colloque Ampere, 14th, North-Holland Publ. Co., Amsterdam, 1967, p. 432. 11 H. T. Weaver, J. Chem. Phys., 56 (1972) 3193. 12 W. C. Harper and R. G. Barnes, J. Magn. Reson., 21 (1976) 507. 13 L. E. Drain, Proc. Phys. Sot., London, 80 (1962) 1380. 14 G. W. Smith and R. M. Housley, Phys. Rev., 117 (1960) 732. 15 P. Monod, J. A. Cowen and W. N. Hardy, J. Phys. Chem. Solids, 27 (1966) 727. 16 R. V. Reid, Jr. and M. L. Vaida, Phys. Rev. Lett., 29 (1972) 494. 17 R. G. Barnes, in J. A. S. Smith (ed.), Advances in Nuclear Quadrupole Resonance, Heydon, London, 1974, p. 335. 18 J. H. Van Vleck, Phys. Rev., 74 (1948) 1168. 19 B. Stalinski, C. K. Coogan and H. S. Gutowsky, J. Chem. Phys., 34 (1961) 1191. 20 A. L. Bowman, private communication. 21 K. H. J. Buschow and H. H. van MaI, J. Less-Common Met., 29 (1972) 203. 22 D. S. Schreiber and L. D. Graham, J. Chem. Phys., 43 (1965) 2573. 23 J. Q. Adams, Rev. Sci. Instr., 37 (1966) 1099. 24 L. H. Germer and A. U. MacRae, J. Chem. Phys., 37 (1962) 1382.
Journal of the Less-Common Metals, 49 (1976) 483 - 502 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
INVESTIGATION OF THE SYSTEMS LaNisH, AND La&D, BY PROTON AND DEUTERON NUCLEAR MAGNETIC RESONANCE*
R. G. BARNES**,
W. C. HARPER,
S. 0. NELSON, D. K. THOME and D. R. TORGESON
Ames Laboratory, Energy Research and Development Administration of Physics and Metallurgy, Iowa State University, Ames, Iowa 50010
and Departments (U.S.A.)
(Received December 1, 197 5)
Summary We report on the results of a study of the LaNis hydride and deuteride systems by means of C.W. (wide-line) nuclear magnetic resonance (NMR). The NMR spectra of samples of LaNi,H, (2.1 < x < 6.8) and of LaNisD, (3.4 < x < 6.7) were obtained at resonance frequencies in the range 3 - 100 MHz for protons and at 16 MHz for deuterons, at temperatures in the range 4.2 380 K. The temperature dependence of the shape, line-width, and second moment of the proton resonance and of the nuclear quadrupole interaction parameters characterising the deuteron resonance spectrum reflect the effects of the rapid diffusive motion of the hydrogen species at temperatures above approximately 140 K. At low temperatures (4.2 - 140 K) the proton resonance shape is distinctly non-Gaussian, with a second moment of 12.78 + 0.26 Oe2, independent of composition, and the deuteron spectrum may be characterised by two or three quadrupole interactions, also independent of composition. Both observations indicate the existence of a single hydride phase possessing a narrow range of stoichiometry. At temperatures above approximately 270 K the deuteron spectrum reflects only a single, weak, though well-resolved, quadrupole interaction. These results are discussed in terms of several models for hydrogen (deuterium) interstitial site occupation. Measurements are also reported bearing on the existence of an adsorbed hydrogen surface layer at temperatures in the range 4.2 - 100 K.
*Presented at the Meeting on “Hydrogen in Metals”, held at the University of Birmingham, United Kingdom, on January 5 - 6, 1976, under the auspices of the Chemical Society (Faraday Division). **Address during 1975 - 76: Physikahsche Chemie III, Technische Hochschule, 61 Darmstadt, Petersenstr. 15, Federal Republic of Germany.
484
1. Introduction A variety of recent studies has shown that the inte~e~lic compound LaNi, can absorb almost seven hydrogen atoms per formula unit at essentially ambient temperature and pressure [ 1 -31. Hydrogen is rapidly absorbed and desorbed with only slight changes in pressure, making this system particularly interesting from the standpoint of potential hydrogen storage applications [4]. Proton and deuteron nuclear magnetic resonance (NMR) together provide a very useful probe for the study of hydrogen-containing solids as they sample both the magnetic and electrostatic environment of the nucleus. We report here the results of such a NMR investigation of compounds in the systems LaNi,H, (2.1 G x < 6.8) and LaNiSD, (3.4 G x G 6.7), in which the temperature dependence of the proton resonance lineshape and second moment, and of the deuteron electric qua~pole interaction have been measured over the temperature range 4.2 - 380 K, with the objective of determining the probable interstitial site locations of the hydrogen species at both low and high temperatures. The results of these measurements are discussed in the context of two structural arrangements which appear to be most probable. These are the hexagonal ~6/~~~ structure of LaNiS itself [S] and the trigonal P31m structure proposed by Bowman et al., on the basis of powder neutron diffraction measurements [6] . 2. Sample preparation and experimental details The starting LaNib was prepared by arc-melting high-purity Ames Laboratory lanthanum (99.99%) with nickel, also of nominal 99.99% purity, six or seven times to ensure homogeneity. The single phase character of the resulting material was verified by both X-ray and metallographic analysis. Following successive hydriding cycles to reduce the LaNiS to a very fine powder, the final hydride and deuteride com~sitio~ were prepared and sealed off in pyrex tubes at a temperature of 77 K. Compositions were determined from published pressure-composition isotherms and the measured quantity of hydrogen (deuterium) gas reacted. In the case of the deuteride samples, two grades of nickel were used to ascertain the effect of iron impurity content on the NMR parameters. Those samples of LaNi5D, with 3t = 3.4 and 5.75 contained -450 ppm Fe by weight, whereas all other samples contained -12 ppm Fe by weight as determined by spark-source mass spectrometry. All the NMR measurements were made with induction spectrometers employing crystal-stabilised radio-frequency sources and Varian Associates crossed-coil probes [ 7] . Conventional 400 channel nuclear physics analysers adapted for NMR applications were used for signal averaging. Proton (‘H) resonance data were obtained at frequencies ranging from 3 to 100 MHz, corresponding to magnetic field strengths in the range 720 - 23,800 Oe to check the dependence on applied field strength. However, most spectra were
485
recorded at a nominal frequency of 28 MHz. Deuteron (2D) resonances were recorded at a nominal frequency of 16 MHz, corresponding to an applied magnetic field strength of 23,400 Oe. Signal averaging for periods of four to five hours was typically required at most temperatures to obtain measurable spectra. Some spectrometer mode-mixing difficulty (i.e., mixing of the absorption and dispersion mode signals) was experienced due to the apparent high electrical conductivity of the compounds and the good electrical contact between the individual particles. This was encountered especially at the higher frequencies employed for some of the proton resonance measurements, where skin-depth problems would be expected to become more severe. Some samples of LaN& were mixed with an equal quantity of boron nitride before the final hydriding, and this was found to reduce substantially the modemixing effects. The nominal resonance frequency of 28 MHz used for the majority of the proton measurements represents a compromise between signal-sensitivity and avoidance of mode-mixing difficulties. For temperatures other than room temperature and the bath temperatures of 4.2 K and 77 K, a gas-flow cryostat system was employed. With this system, a temperature gradient of several degrees K was monitored across the length of the sample (-1.5 cm) by means of thermocouples attached to the sample tube. Experimental runs were characterised by the mean of the temperature extremes recorded during the run, the thermal gradient across the sample constituting the largest uncertainty in this temperature.
3. Proton
NMR measurements
(a) Resonance in the solid Qualitatively, the proton NMR spectra show the typical features of motional narrowing of the resonance line with increasing temperature. This behaviour was previously reported by Halstead for a sample having the composition LaNis.sH, [8]. The wide line observed at low temperature (4.2 140 K) narrows sharply at temperatures above approximately 140 K, and the narrowing process is essentially complete at 200 K. In the low-temperature regime (T < 140 K), the resonance shape, width, and second moment are constant, and no significant dependence on hydrogen concentration is observed in the range 2.1 < x < 6.8 which was investigated. As elaborated further below, the line-shape in this region is “squarer” than Gaussian. A typical example of the absorption-mode derivative signal from such a resonance is shown in Fig. 1 for the composition x = 6.8 at 77 K. This resonance is fitted very closely by a function of the form exp(--blH - H014) where Ha is the field at the resonance centre and b is a constant related to the line-width. In this example, the sample was effectively quenched from 200 K to 77 K by immersing it in the liquid nitrogen bath. The narrow line which appears approximately in the centre of the recording apparently results from this quenching, and we believe it represents a trapped hydrogen film on the sur-
ABSORPTION
DERIVATIVE
(ARBITRARY
UNITS)
_
The second moment of the proton resonance at low temperatures was obtained both by direct numerical integration of the recorded derivative traces, and by fitting the derivatives of the empirical line-shape function g(H - H,) = A exp(--b/H -HOI”), where n is permitted to vary continuously between 1.5 and 4.0, to the recorded derivative trace. For this function, the second moment is given by M2 = {n,(n - 1)}2’“f?(3/n)
(AH~2/~r(l/2)
(1)
where AH is the separation of the extrema of the derivative curve [ 12 J . For a Gaussian shape function, n = 2 and &f2 = (AH)2/4, whereas for a “rectangular” shape function, n = 4 and M2 = 0.098 (AJQ2. This method substitutes judgement of overall goodness of fit for judgement of where the tails of the derivatives vanish. For the resonances showing evidence of two components, this procedure also circumvents the difficulties associated with subtracting out the narrow component before calculating the second moment of the broad component. These results are summarized in Table 1, which lists the n-values, linewidths (as defined above), and second moments, for hydrogen concentrations ranging from it: = 2.1 to x = 6.8 at 77 K, and at a nominal resonance frequency of 28 MHz (corresponding to 6670 Oe). In all cases, the shape is seen to be rectangular or close to rectangular. The shape, width, and second moment of these resonances remain constant at temperatures below 140 K, down to and including 4.2 K. In the vicinity of 300 K, the proton reson~ce width is strongly dependent upon the applied magnetic field strength, as is to be expected for NMR in materials of high magnetic susceptibility. Drain has shown that, for a closelypacked powder, the field-dependent broadening is given approximately by 3xvH0 where xv is the volume susceptibility and H,, is the applied magnetic field strength [13]. At 300 K, the proton line-widths can be represented by expressions of the form AH = (AH), f- 3xvHa for applied field strengths to 24 kOe. The residual line-width (AH), = 0.25 Oe for x =: 2.1 and 0.07 Oe for x = 6.8, and xv ranges from 2.3 X 10m5 cme3 for x: = 2.1 to 1.1 X 10mm5 cmS3 for x = 6.8. From the measured density of the powder samples (-3.5 g cmS3), mass susceptibilities of 6.4 and 3.2 X 10mm6ggl are obtained for x^ = 2.1 and 6.8, respectively, These are in good agreement with a direct measurement of the susceptibility of La.Nis at 300 K which yielded 4.9 X lOme g-l. The Knight shift of the proton resonance was also measured at 300 K where the line width was sufficiently narrow to permit a sensible determination of this quantity. Defined as K = (HH - HM)/HR for measurements at constant frequency, where H, and HM are the fields for resonance in a reference material (water) and in the metal (LaNi&I,), respectively, the shift was found to have the value K = +(0.005 + O.OOl)% for the composition range 4.4 < x < 6.8 at 300 K. {b) Surface film resonance As seen both in Fig. 1 and in the last tracing of Fig. 2, samples quenched from 300 to 77 K showed a narrow resonance super~pos~ on the broad
488 TABLE
1
Summary of proton resonance parameters in LaNisH, at 77 K and at a resonance frequency of 28 MHz Hydrogen cone.
Shape function exponent
Line-width
X
n
g
2.1 4.4 5.2 6.2 6.7 6.8
3.4 3.6 4.0 3.8 4.0 4.0*
10.7 11.2 11.4 11.2 11.4 11.6*
*Me~urement
at 54 MHz.
f 0.5)
Second moment M2
(Oe f 0.3) 12.4 13.0 12.6 12.7 12.8 13.2*
line background of the bulk material. This resonance, as seen in Fig. 1, is broader than its true width, due to the excessive modulation amplitude employed in the scan (appropriate to the broad line). However, by scanning only over the central region of the broad line, the narrow component could be properly measured. Table 2 lists the width of the narrow resonance at a nominal resonance frequency of 51 MHz as a function of decreasing temperature below 77 K. The major part of the measured width in the range 20 < 7’ G 77 must be ascribed to the bulk susceptib~ity effect discussed above in Section 3(a). However, below 20 K the line broadens in qualitative agreement with the behaviour of solid hydrogen [ 141. Below roughly 10 K, the narrow line merges into the broad line and can no longer be distinguished. Efforts to observe a Fake doublet structure of the resonance at 4.2 K, as has been observed for hydrogen adsorbed on zeolite by Monod et aZ. [15], were unsuccessful. The Knight shift of the narrow resonance is, however, consistently negative, K = -(0.005 f O.OOl)%, by contrast to the positive shift of the bulk signal at room temperature. In one series of experiments, hydrogen gas was sealed in a sample tube with LaNi6 powder at 77 K. Only the narrow resonance could then be observed at 77 K. When the sample tube was warmed to 300 K, absorption of the gas occurred, as evidenced by the resonance shift and width at 300 K (appropriate to an x = 2, as determined by the quantity of gas enclosed). Quenching the sample to 77 K then yielded the typical broad-line plus narrowline spectrum, similar to that of Fig. 1. In repetitions of this experiment, a structure was sometimes observed in the initial narrow line at 77 K, suggestive of shift anisotropy. Differences in sample preparation details and history apparently affect the characteristics of the narrow line si~ific~tly.
489
TABLE
2
Line-width of the narrow component of the proton resonance to 77 K, at a nominal resonance frequency of 51 MHz Temperature (K)
Line-width (Oe)
67.0 42.9 29.8 21.3 17.0 13.9 10.3 7.2 4.2
0.64 f 0.1 0.61 f 0.1 0.67 f 0.1 0.62 + 0.1 0.83 + 0.1 1.32 f 0.3 not observed not observed not observed
in LaNigH6.8
quenched
4. Deuteron NMR measurements (a) Resonance in the solid In non-cubic materials, the NMR spectrum of the deuteron reflects the interaction of its electric quadrupole moment with the gradient of the total electrostatic field at the deuteron. In view of the small magnitude of the deuteron quadrupole moment (Q = 0.2875 + 0.0020 X 1O-26 cm2 [ 16]), the effects of this interaction on the NMR spectrum are adequately described by simple first-order perturbation theory. A spectrum may be characterised by the strength of the quadrupole interaction and by the asymmetry of the electric field gradient tensor (efg tensor) [ 171. In conventional notation, the elements of the efg tensor in its principal axis system are expressed in terms of the “field gradient”, q, and asymmetry parameter, 77 eqzz = a2v/az2
eqrv = a2v/ay2 eqxx = a2v/ax2
= eq = -+ (1 + v)eq =-$ (l-_7))eq,
where V is the net electrostatic potential at the deuteron site and e is the magnitude of the electron charge. Thus, 7 = (q,, - qyy)/qzz, with the prescription that lq,,l > lqyyl > Iq,..l, so that 0 < 71< 1. With polycrystalline (powder) samples, the observed spectrum is an average over all orientations of the efg tensor with regard to the applied magnetic field [ 171. The powder spectrum possesses three distinctive features: a pair of strong singularity peaks separated by Au2 = f YQ (1 a (1 + 7]), and a pair of steps separated pair of shoulders separated by Au i = 3 Here, the frequency, (3/2)(e2qQ/h), is a convenient meaby Avs = sure of the quadrupole interaction strength. In addition, an important “nonfeature” of the deuteron spectrum is the absence of a central peak such as occurs in the quadrupole perturbed spectra of half-integer spin nuclei. Such q),
vQ
VQ.
vQ
=
MODULATION It-O25Oe AMPLITUDE
/
I
E
P
a
2
I
I 23 705
I 23 715
23 710 MAGNETIC
FIELD
I 23725
23720
IN
(4
KOe
r
I La N’5
?
15 499
3 &
1395
D5 75 693
I AH,6
MHz
~88
2 Oe
nHz5=46.5
K
nHj4=20
Oe 7 Oe
2 Hc:23.712
!j+ ; s 6z x
148
8 KOe
MINUTES
MODULATION
21 SCANS
AMPLITUDE
L 0 k : m
9
4 I
I
23.650
V
MAGNETIC
FIELD
I 23750
23.700 IN KOe
I 23.800
(b)
Fig. 3. (a) Absorption derivative signal of deuteron NMR in LaNigD6.7 at 300 K. The splitting of the major peaks corresponds to a quadrupole coupling of 7.2 kHz. (b) Absorption derivative signal of deuteron NMR in LaNigD5.75 at 139.5 K.
a central peak can only appear in deuteron NMR when the effective quadrupole interaction strength is zero. Typical examples of deuteron spectra observed in LaNi5D, are shown in Fig. 3(a) and (b). As seen in Fig. 3(a), the 300 K spectra show well-resolved singularity and step features characteristic of a zero or very small asymmetry parameter interaction [ 171. For T < 300 K, the spectra become progressively less well-resolved, until in the range 160 - 200 K they appear as broad, unsplit lines. At still lower temperatures (T < 160 K), recognisable features (splittings) are again observed, as shown in Fig. 3(b). This spectrum remains unaltered in general appearance down to 4.2 K. All deuterium concentrations with x > 3 exhibited these features and temperature dependence, and, in
491
;-- --~ LaNi5D67 vop=15 5 MHz
,-I I-l__-L0
200 TEMPERBTC!RE IO@
1
I
300 400 IN K
Y
23700
23710 MAGNETIC
23720 FIELD (KOe)
23 730
Fig. 4. Temperature dependence of the quadrupole splitting of the deuteron NMR spectrum in LaNi$,. Fig. 5. Sequence of deuteron resonance derivative recordings illustrating the extinction of the quadrupolar spectrum in the temperature range 300 Q T < 380 K. The recordings were made sequentially with increasing temperature.
general, the greater the deuterium concentration the more clearly were the spectral features resolved. It should be noted that the broad lines observed at intermediate and low temperatures are much too broad to result from nuclear dipolar broadening. A plot of quadrupole splitting us. temperature for each sample is displayed in Fig. 4. The splitting refers to the main (singularity) peaks of Fig. 3(a) and to the innermost peaks (AH%) of Fig. 3(b). The increased AH,, values for the x = 3.4 and 5.75 samples are very probably a consequence of their relatively high iron content contributing an anomalous width. The samples containing less iron yield substantially smaller splittings both at 100 K and at 4.2 K. At 4.2 K the various splittings are essentially independent of composition, and have the values: AH,, = 21.9 + 2.5 Oe, AH,, = 39.8 + 2.0 Oe, and AH,, = 82.8 f 2.0 Oe, corresponding to frequency splittings of Au, = 14.3 + 1.5 kIIz, Ava5 = 26.0 f 1.3 kHz, and Avr6 = 54.1 + 1.3 kHz. In the temperature range T 2 300 K, the 2D quadrupolar spectrum was observable up to an “extinction” temperature, Tc . In this range, the spectrum consists of a single, narrow line superimposed on the centre of the quadrupolar pattern. As shown in Fig. 5, the intensity of the quadrupolar pattern is extinguished in this range, leaving only the narrow line at high
492 TABLE
3
Quadrupole interaction parameters used to generate synthetic spectra which match the experimental spectrum of Fig. 3(b) Parameters
Synthetic spectrum number 1
VQl 1)l 61
2
772 62
63
16.3 0.95 0.35
17.0 0.95 0.32
17.3 0.01 0.43 1.00
1.00
44.4 0.01 0.35
35.9 0.95 0.25
58.2 0.28 0.12
58.2 0.28 0.12
1.4
1.5
116.6 0.01 0.05
109.5 0.01 0.06
Intensity VQ3 ‘173
4
15.4 0.01 0.44
Intensity vQ2
3
Values of vQ are given in kHz, and the field gradient inhomogeneity, 6, is defined by AvQIvQ, where AvQ is the full-width of a Gaussian distribution of efg values centred on VQ-
temperature. Extinction temperatures fall in the range 360 < Tc < 380 for x values in the range 3.6 < x < 6.7. Within this range, the quadrupole splitting increases linearly with temperature, as can be seen at the high temperature end of Fig. 4, the increase for x = 6.7 being 27%, for example. The low-temperature spectra were fitted (but not in the least-squares sense) by synthetic spectra generated by folding a Gaussian broadening function into the powder pattern distribution function [ 171. Qualitatively, as remarked above, the broad character of the 2D spectra at low temperatures cannot arise from the nuclear dipolar interaction, as in the proton case, because of the much smaller deuteron magnetic moment strength. Instead, we ascribe this broadening to a Gaussian distribution of quadrupole interaction strengths, as a first approximation. In fact, there very probably exists a distribution of both vo and 7) values, and in our treatment we have, for simplicity, ignored the latter. The parameters employed in four different synthetic spectra, each of which approximates fairly well to the experimental spectrum at low temperatures, are listed in Table 3. Synthetic spectra (1) and (2) comprise three different quadrupole interactions; in the case of (1) all the interactions have essentially zero asymmetry, in the case of (2) two have large asymmetry. Spectra (3) and (4) comprise two different interactions; in the case of (3) with small asymmetries, and in the case of (4) one interaction has a large asymmetry. The relative intensities of the component spectra in the latter two syntheses are also listed. One may argue, in a least-structure sense, that synthetic spectra (3) and (4) are to be preferred over (1) and (2) because fewer interaction para-
493
WITH 85psi OVER PRESSURE
23.720 23.716 23.718 MAGNETIC FIELD IN KOe
Fig. 6. Absorption mode derivative recording of deuteron NMR in sample of LaNis exposed to deuterium gas at pressure of 85 psi at 77 K.
meters are required to achieve the same quality of fit-to-experiment. Spectrum (4) is somewhat preferable to (3) because it necessitates a slightly smaher field-gradient inhomogeneity. The contrast is not striking, however. (b) Surface film resonance The counterpart of the narrow proton resonance observed in hydride samples (Fig. 1) was not detected in deuterium samples quenched to 77 K. This may very well be attributed to the much weaker signal intensity of the 2D resonances in general. However, sample tubes containing deuterium gas sealed over LaNi, powder at 77 K yielded deuteron NMR traces similar to the proton resonances described for similar conditions in Section 3(b). As in the proton NMR, structure was observed in some experiments indicative of shift anisotropy or the occurrence of two closely spaced resonances. An example of such a structure is shown in Fig. 6, from a sample of deuterium gas sealed over LaNi, powder at 85 psi at 77 K. The width of the principal resonance (as in Fig. 6) was consistently found to be AH = 0.65 f 0.10 Oe, the same as that of the narrow proton line at 77 K. This is consistent with the interpretation that the observed width of this narrow line results from the bulk susceptibility contribution, since this quantity is independent of the magnetic moment of the resonant nucleus. Similarly, the Knight shift of the narrow resonance was found to be Ir: = -(0.005 f O.OOl)%, in agreement with the proton measurements.
5. Discussion (a) Proton NMR in the solid We may compare the proton second moment measurements at low temperatures with theoretical expectations based on two structural models for hydrogen interstitial site occupation in LaNi, hydride. These are the ~/mm~ structure of LaNis itself [5], and the P31m structure proposed by
. H(m) La
Ni5
STRUCTURE,
C-AXIS
VIEW
.
H(n)
A
Ii
(0)
0.5 011.0 89 0 27,o 73
Fig. 7. Plan view of PG/mmm structure (four unit cells) showing location of hydrogen sites (m), (n), and (o), as well as the metal ions.
Bowman et al. [6]. The unit cell of the PG/mmm structure contains a total of 38 potential interstitial sites [8], distributed among six groups of 1, 3, 4, 6, 12, and 12 sites each, respectively, and designated (b), (f), (h), (m), (n), and (0) in the Wycoff notation. However, on the basis of hydrogen-hydrogen distances, only the last two groups of sites appear to be probable locations 16981. A plan view of the P6/mmm structure is shown in Fig. 7 (4 unit cells are shown for clarity), with the (m), (n), and (0) sites indicated. These account for 30 of the 38 interstitial sites. As discussed by Bowman et al. [6], the 12 (n) sites occur in three sets of four each, having very short (0.8 - 1.1 A) hydrogen-hydrogen distances within each such grouping. In addition, the (f) sites (not shown in Fig. 7) occupy the centres of these groups. Accordingly, one hydrogen is assumed to occupy one of the (n) sites of each group in an ordered manner. As seen in Fig. 7, the (0) sites form two rings of six within the unit cell, and in their analysis of powder neutron diffraction data for LaNisD,.,, Bowman et al. [6] assigned the remaining 3.8 deuterons randomly over the ring furthest from occupied the (n) sites. The same considerations are appropriate in the present analysis, since the observed rigid lattice proton second moment is quite small, indicating that the protons are rather far apart. The P31m structure is a lower symmetry derivative of the PG/mmm structure and contains, in addition to the metal atoms, nine potential hydrogen sites. Three of these, designated (c), correspond to three of the (n) sites of the PG/mmm structure, and the remaining six sites (designated (d)) correspond to one ring of (0) sites in P6/mmm. The metal atom structure of P31m is essentially identical to that of PG/mmm. An obvious difficulty posed by P31m is the very short distance that arises between two of the (d) sites (1.32 a), since simultaneous occupancy of these two sites leads at once to a very large contribution to the proton second moment.
The theoretical second moments for the proton resonance were calculated with the Van Vleck formula [18] appropriate to a powder using the generalisation introduced by Stalinski et al. [19] to take account of the contributions from inequivalent lattice sites to the total moment. Although the contribution from the 61Ni nuclei can be ignored (small isotopic abundance and small nuclear moment), that from the 13’La cannot. In the notation of Weaver [ 111 and of Halstead [ 81, the rigid lattice second moment is given by
M2 =
(2)
fiM$'
5 i
where fi is the fraction of all protons located in sites of type i and M$’ is the second moment of the spectrum resulting from protons located in i sites. We denote by aj the probability of occupation of a j site and by wj the relative number of j sites, so that (3)
wkak
k=l
where N is the number of inequivalent proton sites. The second moment, Mij’, of the protons on an i site is Mt)=
CI g
aj Sij + CFS;
(4)
j=l
where C, = (3/5)(~~h)~Z(Z + 1) for contributions from like nuclei (other protons) and CF = (4/15)(~~h)~Z~(Z~ + 1) for the contributions from unlike nuclei (here lsaLa, with IF = 7/2). S’ij denotes the lattice sum zrriy6 where the origin is taken at an i site and the summation extends over all j sites. Similarly, S;j denotes the sum over La sites with the origin taken at the i-th proton site (all La sites are assumed to be occupied). Combining eqns. (2) and (4), we have the general result M2 = CI 2 i.j
fiaj
Sij + CF s fiS:j.
(5)
i= 1
For the evaluation of the lattice sums, the unit cell parameters were taken as those given by van Vucht et al. [1] for LaNieHs (a = 5.44 A, and c/u = 0.792) which were also used by Halstead [8]. For the PG/mmm’ structure the atomic coordinates within the unit cell were also those used by Halstead: (n) with (x, y, z) = (0.40, 0, O.ll), and (0) with (x, y,z) = (0.20, 0.40, 0.27). These values differ negligibly from those of Bowman et al. [6] : (n) with (x, y, z) = (0.43, 0, 0.12) and (0) with (x, y, z) = (0.21, 0.42,0.29). The coordinates of Bowman et al. [6] were used for the P31m structure: (c) with (x, y, z) = (0.50, 0,0.08) and (d) with (x, y, z) = (0.25,0.86,0.58). The results of second moment calculations are summarised in Table 4. For illustrative purposes, and for comparison with the experimental results of Table 1, the rigid lattice second moments are listed for two x values (6.0 and 6.8) for each of the two structures considered. The first three hydrogens
496 TABLE
4
Calculated second moments of the proton resonance in LaNi5H, for several arrangements of hydrogen on interstitial sites in the PGlmmm and P3lm structures Structure
Hydrogen ordering on sites
Second moment ( Oe2)
x=6
x = 6.8
P6lmmm
ordered (n) ordered (0)
14.8
-
PGlmmm
ordered (n) random (0)
18.8
20.9
P31m
ordered ordered ordered random
11.1
-
10.8
12.0
P31m
(c) (d) (c) (d)*
*Shortest hydrogen-hydrogen
distance (1.32 A) excluded.
are assumed to occupy only the three (n) or (c) sites in PG/mmm or P31m, respectively. For x = 6 the additional three hydrogens are placed either in an ordered arrangement on three of the (0) or (d) sites, or randomly over these sites. For x = 6.8, only a random distribution over the (0) or (d) sites was assumed. For the P6/mmm structure, M2 = 14.8 0e2 for x = 6, assuming an ordered arrangement on both the (n) and (0) sites. With a random distribution over the (0) sites, M2 = 18.8 Oe2, due to contributions from shorter proton-proton separations. Similarly, on the basis of the P31m structure, M, = 11.1 0e2 for x = 6, with an ordered arrangement on the (d) sites. In this case, a random distribution over the (d) sites leads to an anomalously large second moment (26.3 0e2) because of the one very short (1.32 A) spacing which occurs between two of the (d) sites. However, if simultaneous occupancy of these two sites is excluded, and the three hydrogens are permitted to occupy five of the six (d) sites randomly, then M, = 10.8 Oe2 for x = 6. For the composition x = 6.8, 3.8 hydrogens are placed randomly on the (0) or (d) sites, but again excluding the simultaneous occupancy of the two close (d) sites in the P31m structure. For PG/mmm, this yields M2 = 20.9 0e2, and for P31m,M, = 12.0 0e2. The P31m structure leads to reduced second moment values because of the increased separation of the (c) and (d) sites compared with PG/mmm, and through the device of excluding simultaneous occupancy of the close pair of (d) sites. Some reduction of the second moments could also be achieved by permitting some degree of proton motion to continue at low temperatures (e.g., tunnelling). For example, each of the three protons fixed on (n) or (c) sites in this calculation might move constantly over the subgroup of four (n) or (c) sites, since the distances are so short (0.8 - 1.1 A) within each sub-group. This would have an averaging effect on M,. Similarly,
497
in the context of the present calculations, it may be that a proton effectively oscillates between the two close (d) sites in P31m. The present calculation has been aimed only at assessing whether reasonable agreement can be obtained between experimental and calculated second moments on a rigid lattice basis. It appears more reasonable to us that relaxation time measurements, for example, should be made before more exotic calculations are performed. Although undue quantitative significance should not be attached to the calculated second moments, in particular because of the substantial uncertainty in position coordinates of the P31m structure [20], the results support qualitatively the concept of a hydride structure of lower symmetry than that of LaNiS itself. Certainly, the absence of any significant dependence of the experimental second moments on hydrogen concentration, coupled with the calculated values, indicates a rather narrow range of stoichiometry for the hydride phase. Conversely, one may ask at what composition the models discussed here yield the observed value of 12.78 0e2. With a random distribution over the (0) or (d) sites, again excluding the close pair in P31m, the required hydrogen concentrations are x = 4.8 in PG/mmm and x = 6.9 in P31m. (b) Deuteron
NMR in the solid
As a first step towards correlating the observed quadrupole splittings with deuteron locations in the lattice, a calculation of the ion-lattice contribution to the electric field gradient at the different interstitial sites in the expanded lattice was made, taking the La ion charge to be +3 and the nickel charge to be zero. The latter assumption is reasonable in view of the Pauliparamagnetic nature of LaNi5. For the sake of simplicity, a calculation of the deuteron contribution to the efg was not made, since we expect that the efg is dominated by the La3+ contribution and, since screening effects are also neglected, the deuteron contribution will be highly uncertain. Qualitatively, we infer that the considerable inhomogeneity of the coupling constants at low temperatures reflects the inhomogeneity of the deuteron contribution, although it is possible that some of the observed inhomogeneity results from disorder on the La sites (see further below). The Sternheimer anti-shielding effect has also been neglected since no estimates of this parameter have been given for the deuteron in metallic systems. Inasmuch as only the contribution from the La3+ ions is considered, the z component of the ion lattice efg tensor in its principal axis system is given by 42z = 3 jz,
(3zZj
-
rZj)
r;;j5,
(6)
with similar equations for qxx and qy y, where the sum runs over all La3+ sites, the origin being taken at a deuteron site. In this manner the efg tensors (characterised by qZL and 17) were calculated for all the interstitial sites in PG/mmm and in P31m, using the same atom coordinates as for the proton second moment calculations of Section 5(a). The results of these calculations are listed in Table 5 for PG/mmm and in Table 6 for P31m.
498 TABLE 5 Calculated ion-lattice contributions to the electric field gradient tensor at each deuteron site in the PG/mmm structure in the low-temperature (rigid lattice) and high-temperature (motionally averaged) limits Site
Rigid lattice -_ Qrr
1)
VQ
b f h m n
+ + + +
0
+
0 0 0 0.58 0.05 0.22
172.7 77.5 5.7 72.0 90.8 72.0
1.15 0.52 0.04 0.48 0.61 0.48
no m,n,o all
Motionally averaged _ 9ZZ VQ + + -
1.15 0.26 0.04 0.48 0.28 0.05 0.16 0.04 0.03
172.7 38.7 5.7 72.0 42.5 8.1 24.6 5.7 4.8
The qzz values are in units of 1O24 cm3. In the motionally averaged limit, the efg z axis coincides with the crystalline c axis and 7) = 0 for all sites. TABLE 6 Calculated ion lattice contributions to the electric field gradient tensor at each deuteron site in the P31m structure in the low-temperature (rigid lattice) and high-temperature (motionally averaged) limits Site
C
d c,d
Rigid lattice
Motionally averaged --
QZZ
rl
+ 0.52 + 0.24
0.07 0.93
VQ 77.5 35.8
92.2 - 0.33 + 0.12 - 0.03
G 47.0 17.1 4.3
The qrz values are in units of 1O24 cm3 and vQ values are in kHz. In the motionally averaged limit the efg z axis coincides with the crystalline c axis and 17= 0 for all sites.
In view of the proton NMR results which demonstrate the presence of rapid proton motion at temperatures T 2 140 K, the well-resolved deuteron quadrupolar spectrum at 300 K must represent an average interaction over some or all of the interstitial sites [17]. In this respect, we note that the symmetry operations of PG/mmm cause the average efg tensor for each of the six different sets of sites to be axially symmetric (77= 0) with qzr aligned along the crystalline c axis. Without diffusive averaging motion, this is only true for sites (b) and (h). Table 5 also lists the averaged qzz values, (zz), for each of the six sets of sites, and for averages over several groups of sites. For ready comparison with the experimental data, the resultant vQ values in kHz are given for all of these cases. Similar considerations apply in the case of P31m, and the average over the (c) and (d) sites is included in Table 6.
499
Referring now to the quadrupole interaction parameters needed to fit the low-temperature deuteron spectra (Table 3), it is clear that the synthetic spectra (1) and (2), which require three different interaction strengths, must be interpreted in terms of the P6jmmm structure. However, the calculated qzr and ho values in Table 5 show relatively little variation in the rigid lattice case among the various sites. This is especially true for sites (m), (n), and (0) which account for 30 of the 38 sites. Site (h) has a very small interaction, and site (b), of which there is only one per unit cell, has a much stronger interaction. Only site (m) has a substantial asymmetry parameter value, as would be required by the synthetic spectrum (2). Even allowing for the approximate features of the model, the PG/mmm structure does not appear to offer much likelihood of accounting for these synthetic spectra. Conversely, synthetic spectra (3) and (4) require only two different interactions, and could follow from either the P6/mmm or P31m ~ngemen~. However, taking the proton second moment calculations as a guide, we may assume that it is primarily the (n) and (0) sites of PG/mmm and the (c) and (d) sites of P31m that are of relevance. It then appears that the P31m model yields ho and Q estimates which are substantially closer to, and in about the right ratio to, the values required by the synthetic spectra. Moreover, the average over all the (c) and (d) sites is also in reasonable agreement with the observed value of vo = 11 kHz at 300 K. The relative intensities of the two interactions required by synthetic spectra (3) and (4) are not in good agreement with this arrangement, however. In the second moment calculations, 3.8 protons were assigned to the (d) sites and 3 to the (c) sites, so that the intensity of the (d) site interaction should be somewhat greater than that of the (c) site interaction. However, Table 3 shows that the interaction with the larger ho value (hence the (c) site) contributes greater intensity than the weaker interaction (the (d) site). The substantial efg inhomogeneities required by the synthetic spectra, and evident from the experimental data (Fig. 3(b)), may be a consequence of the randomness of the deuteron contribution brought about by the random distribution over the (d) sites. However, it is also possible that a small degree of disorder on the lanthanum sites could have this effect. Although the X-ray data indicated a composition of LaNis for the starting material, the work of Buschow and van Mal [21] has shown that the compound exists over a fairly broad range of composition at high temperature. Efforts to observe the 13’La NMR in the starting material have not been successful, and this tends to support the view that there is inhomogeneity on the La sites themselves. Finally, the temperature dependence of the quadrupole interaction strength in the temperature range 300 G T < 380 appears puzzling. The linear increase, 27% in the case of x = 6.7, of the coupling constant matches almost exactly the 25% volume expansion of LaNiS upon hyd~ding, and it is tempting to ascribe the increase in coupling constant to the lattice contraction accompanying the loss of deuterium at the higher temperatures. However, this would imply that a continuous range of intermediate compositions could exist within each unit cell of the compound, an implication that does not appear
500
to be borne out by any other aspect of the experimental data. Hence, the correspondence between coupling constant and lattice contraction appears to be a fortuitous one. (c) Surface film resonance Both proton and deuteron measurements indicate that an adsorbed surface film can be stabilised on LaN&, powder at temperatures as high as 77 K, either by quenching from 300 K, or simply by exposing clean LaNi, powder to hydrogen (deuterium) gas at moderate pressure at 77 K. The close similarity of the proton line-width behaviour to that of solid hydrogen [ 141 at temperatures in the 10 - 20 K range suggests that we are observing a physisorbed rather than a chemisorbed film. Both the line-width and shift of the film resonances differ substantially from those in the bulk material and in Hz and Dz gas alone. Using identical spectrometer settings with those used for the film resonances, the proton and deuteron resonances were observed in the Hz and Dz gas above the LaNiB sample at 85 psi and 77 K. The line-widths were found to be 0.25 f 0.05 Oe (due to excessive modulation amplitude), and the Knight shift was 0.000 f O.OOl%, i.e., zero within the experimental uncertainty. The proton resonance in quenched samples has an intensity approximately 1O-3 that of the broad line of the bulk material. The particle size of cycled and activated LaNie hydride powder has been reported to be in the range 1 - 4 pm [ 21. Scanning electron microscope pictures reveal small, highly fractured particles (resembling popcorn), consistent with this estimate. van Mal has reported the specific area of a well-activated sample to be 1.17 m2gP1 [4]. For a typical NMR sample of roughly 2 g, a monolayer coverage of molecular hydrogen would amount to 2 X 102’ protons, approximately 0.016 the number in the bulk LaNi,Hs. Thus, it appears likely that the observed signals arise from less than full monolayer coverage. Although a substantial shift difference exists between the bulk and surface resonances, the measurements are not readily susceptible to quantitative interpretation. This is because of the significant contribution to the shift resulting from the high magnetic susceptibility of LaNi, and the overall nonspherical geometry of the powder samples [22] . For the proton resonance in the bulk material, we estimate the contribution to the measured shift, due to this effect, to fall in the range + 0.0011% to - 0.0045%, depending on individual particle geometry. The net effect is an average over the particles in the sample [22]. The resonance from a surface film experiences a different bulk susceptibility effect, since the film behaves analogously to a thin cavity in a solid [23] . Qualitatively, the shift difference is consistent with the difference in origin (i.e., bulk and surface) of the two signals.
6. Conclusions The salient features of the experimental NMR measurements on the LaNiSH, and LaNi&, systems are as follows:
501
(1) At low temperatures (4.2 < T < 140 K) the proton resonance shape is distinctly squarer than Gaussian, with a second moment of 12.78 + 0.26 Oe2 practically independent of composition within the limits 2.1< x < 6.8. (2) In the same temperature range, the deuteron NMR spectrum may be characterised by two or three quadrupole interactions, also independent of composition within the limits 3.4 < x d 6.7. (3) At temperatures in the range 270 < T G 300 the deuteron spectrum reflects only a single, weak, though well-resolved, axially symmetric quadrupole interaction. (4) At still higher temperatures, 300 < T < 380, this quadrupolar spectrum is extinguished and is replaced by a single, narrow, unsplit resonance. (5) Both proton and deuteron experiments show evidence supporting the existence of an adsorbed molecular surface layer at temperatures below 77 K in samples of LaNisH, quenched from 300 K, or in samples of LaNis exposed to H2 or D2 gas at 77 K. The rigid lattice proton second moments and the deuteron quadrupole interactions have been shown to be in reasonable agreement with expectations based on the lower symmetry P31m structure proposed by Bowman et al. [6] for the LaNi5 deuteride. Interpretation of the experimental data within the context of the PG/mmm structure of LaNiB does not appear to be possible. The occurrence of an adsorbed surface layer of hydrogen may not seem surprising in view of the high nickel content of LaNis and the well-known propensity of nickel itself to maintain adsorbed hydrogen layers [24].
Acknowledgements The authors are indebted to Dr. T. Pinter and Dr. Y. S. Hwang of the Ames Laboratory for assistance with the various computations, and to Professor K. Gschneidner for very helpful discussions. One of the authors (R.G.B.) acknowledges the support of the Alexander von Humboldt Foundation and the hospitality of the Technische Hochschule, Darmstadt, where this manuscript was completed.
References 1 J. H. N. van Vucht, F. A. Kuijpers and H. C. A. M. Bruning, Philips Res. Rep., 25 (1970) 133. 2 K. H. J. Buschow and H. H. van Mal, J. Less-Common Met., 29 (1972) 203. 3 H. H. van Mal, K. H. J. Buschow and A. R. Miedema, J. Less-Common Met., 35 (1974) 65. 4 H. H. van Mal, Chem.-Ing.-Tech., 45 (1973) 80. 5 J. H. Wernick and S. Geller, Acta Crystallogr., 12 (1959) 662. 6 A. L. Bowman, J. L. Anderson and N. G. Nereson, Proc. Rare Earth Res. Conf., lOth, Carefree, Arizona, 1972, p. 485. 7 D. R. Torgeson, Rev. Sci. Instr., 38 (1967) 612.
502 8 T. K. Halstead, J. Solid State Chem., 11 (1974) 114. 9 J. F. Hon, J. Chem. Phys., 36 (1962) 759. 10 0. J. Zogal and B. Stalinski, Proc. Colloque Ampere, 14th, North-Holland Publ. Co., Amsterdam, 1967, p. 432. 11 H. T. Weaver, J. Chem. Phys., 56 (1972) 3193. 12 W. C. Harper and R. G. Barnes, J. Magn. Reson., 21 (1976) 507. 13 L. E. Drain, Proc. Phys. Sot., London, 80 (1962) 1380. 14 G. W. Smith and R. M. Housley, Phys. Rev., 117 (1960) 732. 15 P. Monod, J. A. Cowen and W. N. Hardy, J. Phys. Chem. Solids, 27 (1966) 727. 16 R. V. Reid, Jr. and M. L. Vaida, Phys. Rev. Lett., 29 (1972) 494. 17 R. G. Barnes, in J. A. S. Smith (ed.), Advances in Nuclear Quadrupole Resonance, Heydon, London, 1974, p. 335. 18 J. H. Van Vleck, Phys. Rev., 74 (1948) 1168. 19 B. Stalinski, C. K. Coogan and H. S. Gutowsky, J. Chem. Phys., 34 (1961) 1191. 20 A. L. Bowman, private communication. 21 K. H. J. Buschow and H. H. van MaI, J. Less-Common Met., 29 (1972) 203. 22 D. S. Schreiber and L. D. Graham, J. Chem. Phys., 43 (1965) 2573. 23 J. Q. Adams, Rev. Sci. Instr., 37 (1966) 1099. 24 L. H. Germer and A. U. MacRae, J. Chem. Phys., 37 (1962) 1382.