Mössbauer and time-differential perturbed angular correlation studies of the hydrides and deuterides of vanadium

Journal of the Less-Common

Metals, I30 (1987)

173 - 180

173

Mi)SSBAUER AND TIME-DIFFERENTIAL PERTURBED ANGULAR CORRELATION STUDIES OF THE HYDRIDES AND DEUTERIDES VANADIUM* L. IANNARELLA?, Physik-Departmen

M. BAIER, M. ZELGER t, Technische

Universitiit

OF

and F. E. WAGNER

Miinchen,

D-8 046 Garching (F. R. G.)

(Received May 28,1986)

Summary Hydrides and deuterides of vanadium were studied by source and absorber Miissbauer experiments with the resonances of 99R~, 1931r and 19’Au and by perturbed angular correlation experiments with the 353 - 90 keV ‘y-y cascade in 99Ru . The observed isomer shifts and electric quadrupole interactions are discussed in terms of the local environment of the probes in the different hydride and deuteride phases and compared with similar results for the Nb-H system.

1. Introduction Both Mijssbauer spectroscopy and the time-differential perturbed angular correlation (TDPAC) technique are microscopic nuclear methods, by which mainly the local environment of the probe atoms can be studied [l, 21. They are thus well suited to probe the hydrogen environment of impurities in metal-hydrogen systems [ 1 - lo]. In non-magnetic systems, the information obtained by Miissbauer spectroscopy stems from the isomer shift, which measures the electron density at the probe nucleus, and from the electric quadrupole interaction, which is sensitive to deviations of the charge distribution around the nucleus from cubic symmetry. Another important parameter is the recoil-free fraction [ 11,121, but the implications of this parameter will not be considered in this paper. The TDPAC technique can only measure the electric quadrupole interaction, but it has the advantage that it is not restricted to low temperatures, whereas Miissbauer spectroscopy with the isotopes used in this work is restricted to low temperatures because of the high 7 ray energies which lead to forbiddingly small recoil-free fractions at temperatures much above that of liquid helium. *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubuisson, France, May 25 - 30,1986. ?On leave from Universidade Federal Rural de Rio de Janeiro, Brasil. 0 Elsevier Sequoia/Printed

in The Netherlands

174

The Mijssbauer resonances of 99Ru (90 keV), 1931r(73 keV) and 19’Au (77 keV), as well as the 353 - 90 keV TDPAC cascade in 9gRuphave previously been used in studies of the Pd-H and Nb-H systems. We now present the first results of a study of the hydrides and deuterides of vanadium with these probes. For the TDPAC experiments, the radioactive parent isotope 99Rh is incorporated into the hydride lattice, where it decays to 99Ru. The environment of the ruthenium probes will thus still be that of the rhodium parent at low temperatures, where no hydrogen jumps take place within the time elapsing between the radioactive decay of 99Rh and the decay of the 90 keV state of 99R~, whose quadrupole interaction is measured and which has a lifetime of 7, = 30 ns. At higher temperatures, however, one expects hydrogen jumps to be sufficiently fast for the environment to adjust towards the equilibrium for a ruthenium impurity, if this is different from that for rhodium. Mijssbauer experiments with the isotopes used in this work are limited to low temperatures. Measurements with hydride absorbers obviously probe the environment of the Mijssbauer elements themselves, whereas measurements with sources in which the radioactive parent isotope has been incorporated into the hydride systems will be representative of the environment of the source elements, i.e. of rhodium, osmium and platinum, because at low temperatures the rearrangement of the interstitials is too slow to take place before the emission of the Miissbauer y ray. Any differences found between source and absorber data for the same hydride system thus reveal different environments for the source and absorber element.

2. Experimental details The sources and absorbers of the various isotopes in VH, and VD, were made by gas phase loading of the respective alloys. These were prepared by arc melting in an argon atmosphere. The absorbers were made of alloys containing 0.5 at.% natural iridium, gold, or ruthenium enriched to 92% in the g9Ru isotope. The alloys for the source experiments with 1931rand 19’Au contained 0.1 at.% isotopically enriched 19*Os (97%) or 196Pt(87%). They were activated by reactor irradiation with thermal neutrons after hydrogenation. The 99Rh activity was produced by the “‘Ru(d, 3n)99Ru reaction with enriched (91%) iooRu as the target material. After irradiation, alloys contaming 0.3 at.% looRu in vanadium were prepared and then loaded with hydrogen. The hydrogen content was determined with an accuracy of about 1% by weighing before and after hydrogenation and by outgassing after the experiments. For hydrogen concentrations above x = 1 only the outgassing method was used, since the samples were handled at liquid nitrogen temperatures after being removed from the loading vessel to avoid losses of hydrogen. The sources used in the absorber experiments were made of metallic 196Ptand of an alloy of 0.5 at.% 19*Os in niobium, which both give single, lines, as well as of metallic looRu , which is hexagonal and therefore has a

175

small unresolved quadrupole interaction [ 131. The absorbers in the experiments with the hydride sources were metallic iridium, gold and ruthenium. All Miissbauer experiments were performed at 4.2 K. The spectra were fitted with suitable superpositions of lorentzian lines. In the “Ru spectra, the unresolved quadrupole splitting [13] in the ruthenium metal source or absorber was taken into account in the fitting procedure. The TDPAC measurements were made with a standard four-detector fast-slow coincidence arrangement. The sources, which were small fractions of the respective Miissbauer sources, were cooled in a gas flow cryostat, in which they could also be heated above ambient temperature. The spectra measured to delay times of 160 ns could all be fitted with a single quadrupole frequency when a rather strong gaussian damping [9] was taken into account.

3. Results and discussion A few of the Mijssbauer spectra are shown in Fig. 1. The respective Miissbauer parameters are summarized in Tables 1 - 3. Most of the spectra consist of a single peak with a shape indicating either a small, unresolved quadrupole splitting, or several unresolved components with different isomer shifts. For these spectra an interpretation in terms of symmetric lOO.O-

99.0. i

98.0

2 -

97.0~

98.0 99.0:: 100-o-

100.0

99.8.

I

98.04

I

I

-1.0

-0.5 VELOCITY

0 0.5 (mm/s1

1.0

1

/

-4

-2 VELOCIIY

0

2 imm/s)

b

Fig. 1. Miissbauer spectra of g9Ru, 1931r and 19’Au in vanadium and its hydrides and deuterides.

176 TABLE 1 Summary of results obtained with the 90 keV Mijssbauer resonance in 99Ru Source

Absorber

x

AS (mm

99Rh :Ru

99Ru :VH,

0.50 0.67 0.82 1.85

-0.048(3) -0.013(3) -0.021(2) -0.710(6)

0.11(l) 0.09(l) 0.10(l) -

0.20(l) 0.20(l) 0.19(l) 0.23(3)

99Rh :Ru

99Ru :VD,

0.51 0.79 1.09

-0.069(3) -0.042(3) -0.045(3) -0.656(5) -0.045(3) -0.644(4) -0.047(3) -0.649(3)

0.11(l) 0.06(l) 0.06(l) 0.01(3) 0.06(l) 0.07(3) 0.06(l) 0.07(l)

0.18(l) 0.18(l) 0.18(l) 0.18(l) 0.17(l) 0.17(l) 0.18(l) 0.18(l)

0.87(l) 0.13(l) 0.79(l) 0.21(l) 0.56(l) 0.44(l)

-0.072(3) -0.031(3) -0.259(9) -0.025(3) -0.241(5)

0.11(l) 0.10(l) -

0.22(l) 0.20(l) 0.20(l) 0.21(l) 0.21(l)

0.90(5) 0.10(5) 0.90(5) 0.10(5)

0.23(l) 0.23(l) 0.23(3) 0.23(3) 0.20(l) 0*20(l) 0.20(l) 0.20(l) 0.20(l) 0.20(l)

0.90(5) 0.10(5) 0.96(2) 0.04(2) 0.79(l) 0.10(2) 0.11(2) 0.48(l) 0.26(3) 0.26(3)

1.20 1.48

99Rh :VH,

Ru

0.50 0.68 0.83

99Rh:VDX

Ru

0.80 0.99

1.14

1.65

-0.053(3) -0.204(5) -0.055(3) -0.607(10) -0.059(3) -0.543(5) -0.713(5) -0.056(3) -0.564(5) -0.714(5)

6-l)

AE,

(mm s-r)

0.10(l) 0.08(l) 0.02(2) 0.06(l) 0.08(2) -

W (mm s-l)

A/A,,,

AS is the isomer shift relative to ruthenium in hydrogen-free vanadium ; the shift of Ru :V relative to ruthenium metal is +0.160(2) mm s-l. For the source experiments, the sign of the shifts has been inverted to facilitate the comparison with the absorber data. AE, is the electric quadrupole splitting of the 90 keV state with spin 312, and W is the full linewidth at half-maximum. A/A,, gives the relative intensity of the different components listed for the same spectrum.

quadrupole doublets was adopted, although resolved doublets are observed only in the case of 1931r(Fig. 1). Concentrations of x 2 1 have so far only been studied with 99Ru. In the V-D system one finds a second, well-separated peak attributable to e-VD, at concentrations in the 6 + E mixed phase region. In the source spectra, the peak attributable to E-VD2 consists of several unresolved components, which were attributed to species with different isomer shifts (Table 1) rather than to electric quadrupole interaction, because the TDPAC data for these cases showed no electric quadrupole

177 TABLE 2 Summary of results obtained with the 73 keV Mijssbauer resonance in lWIr Source

Absorber

x

AS (mm s-r)

AE~ (mm s-l)

W (mm s-r)

1930s :Nb

Ir :VH,

0.56 0.82

-0.04(4) -0.02(4)

0.58(3) 0.98(4)

0.86(4) 0.82(5)

1930s :VH,

Ir

0.62 0.74

-0.05(2) -0.05(2)

0.73(2) 0.83(l)

0.83(3) 0.87(2)

1930s :VD,

Jr

0.74 0.75

-0.14(2) -0.15(2)

0.13(5) 0.29(4)

0.94(5) 0.84(4)

AS is the isomer shift relative to iridium in hydrogen-free vanadium; the shift of Ir:V relative to iridium metal is +1.68(l) mm s-I. For the source experiments, the sign of the shifts has been inverted to facilitate the comparison with the absorber data. AEQ is the electric quadrupole splitting of the 73 keV excited state with spin 312, and W is the full linewidth at half maximum.

TABLE 3 Summary of results obtained with the 77 keV Miissbauer resonance in r9’Au Source

Absorber

x

AS (mm s-l)

AE, (mm s-r)

W (mm s-l)

‘9%

Au :VH,

0.50 0.56 0.58 0.73

-0.79(l) -0.80(l) -0.83(l) -0.83(l)

0.52(3) 0.62(4) 0.62(3) 0.82(l)

1.95(2) 1.96(4) 1.94(2) 1.99(l)

‘97pt

Au :VD,

0.81 0.87

-0,89(l) -1.29(l)

0.87(3) 0.82(3)

1.95(4) 1.99(2)

197Pt:VH,

Au

0.49 0.59 0.73

-0.37(l) -0.48(l) -0.38(l)

0.58(4) 0.43(10) 0.47(7)

1.95(3) 2.10(6) 1.96(4)

19% :VD,

Au

0.77 0.79

-0.69(l) -0.78(l)

0.31(27) 0.34(25)

2.12(10) 2.01(10)

AS is the isomer shift relative to gold in hydrogen-free vanadium; the shift of Au:V relative to gold metal is +6.78(2) mm s-‘. For the source experiments, the sign of the shifts has been inverted to facilitate the comparison with the absorber data. AEQ is the electric quadrupole splitting of the 77 keV excited state with spin 312, and W is the full linewidth at half maximum.

interactions of sufficient magnitude to explain the structure in the Mossbauer spectra in terms of quadrupole splittings. The species with different isomer shifts are presumably probe atoms surrounded by different numbers of nearest hydrogen neighbours. The spectrum of ruthenium in VH,.ss shows only the peak attributed to ^/-VH2 with an isomer shift of -0.71 mm s-l. A

178

slight asymmetry in the 3c = 0.8 was interpreted shift of about -0.25 mm The TDPAC results interaction frequency of eratures these quadrupole

source spectra of hydrides at concentrations near as an additional weak component with an isomer s-l with respect to the unloaded sample (Table 1). for the temperature dependence of the quadrupole the V-H system are shown in Fig. 2. At low teminteractions are only slightly smaller than those

40

%I

Ah Imm/sl

100 200 300 TEMPERATURE

400 [Kl

500

Fig. 2. Temperature dependence of the quadrupole frequency wc = eQV,,/2h, where Q is the nuclear quadrupole moment of the 90 keV state of *Ru and V,, is the electric field gradient, for 99Ru in VH, at different hydrogen concentrations. On the right-hand scale the quadrupole interaction of the 89 keV state of 99Ru is expressed in velocity units to facilitate a comparison with the Miissbauer data of Table 1,

found for 99Ru in NbH, [ 91. In VD, , the temperature dependence of the quadrupole interaction frequencies was measured at several concentrations between x = 0.59 and 1.65. The quadrupole interactions found at low temperatures are between 10 and 20 Mrad s-l, i.e. even smaller than those in the hydrides (Fig. 1). Like these, they decrease when the temperature increases. The TDPAC data support the Mijssbauer results indicating that the electric quadrupole interactions are small at all concentrations in both the hydrides and the deuterides, but the accuracy of the TDPAC results is substantially better than that of the Mossbauer results. The changes of the isomer shifts caused by hydrogenation are all in the sense of decreasing electron densities at the probe nuclei, as has been found in virtually all systems studied so far (see, e.g. refs. 1, 3 - 9, 11, 12). Apart from the rather large shifts found for 99Ru in y-VH2 and e-VD2, they are, however, very small. In stoichiometric VH, and VD2, which have a fluorite structure [ 141, a metal atom is expected to be surrounded by eight nearest hydrogen neighbours. If one assigns the largest shift of about -0.7 mm s-i (Table 1) found in the VH, and VD1 hosts to such a configuration and assumes that the shift depends linearly on the number of nearest neighbours [6], one derives a mean isomer shift of about -0.09 mm s-l per

179

nearest hydrogen neighbour. About the same value is obtained from 99Ru Mossbauer data on the Pd-H system [3, 81. Comparing this value with the much smaller shifts of Table 1, one concludes that both ruthenium and rhodium in VH, and VD, mostly have less than one nearest neighbour for 0.5 5 x 5 1.0. Since the small negative shifts found in these cases may also arise from volume expansion, both ruthenium and rhodium appear to be so strongly antitrapping that they never or rarely have nearest hydrogen or deuterium neighbours. The electric quadrupole interactions must then be attributed mainly to the lattice distortions caused by the superlattice ordering of the interstitials. It is then easily understandable that the quadrupole interactions are larger in the hydrides, where the occupation of octahedral sites accompanies large distortions [ 141, than in the deuterides, where, except in @-VDo.s, tetrahedral interstitial sites are occupied and the distortions are small [ 141. Comparing the isomer shifts of Table 1 with those found for 99Ru in NbH, [7, 91, one finds that for x 5 0.8 both host metals yield very small shifts, whereas for 0.8 5 x ;5 0.9 the shift for sources of 99Rh in NbH, decreases rapidly to - 0.22 mm s-i at x = 0.9 [ 91, This indicates that at high hydrogen concentrations hydrogen goes into the vicinity of rhodium impurities. There is an indication that this also happens, albeit very weakly, for rhodium in VH,68 and VH,,ss, where a weak component with a shift near -0.25 mm s-l is found (Table 1). Very small isomer shifts are also typical for the results obtained with 1931r. Measurements with this isotope in PdH, [5] and NbH, [7] suggest that for this resonance the shift induced by one nearest hydrogen neighbour is about -0.25 mm s-l, i.e. much greater than any of the shifts in Table 2. Thus one again concludes that the interstitials do not go into the vicinity of the iridium and osmium probes at the concentrations studied so far or, at least, that sites next to these probes have a strongly reduced probability of being populated. It is interesting to note that the isomer shifts for iridium in NbH, [7] are also small for x 6 0.8, but then increase more rapidly; in can already be seen from 6-NbH2 which has a NbHo.99, a small contribution substantial isomer shift. The electric quadrupole interactions for iridium in VH, are rather well resolved (Fig. 1). Since we have concluded that there are no nearest hydrogen neighbours around iridium or osmium, these quadrupole interactions must be attributed to the lattice distortion [ 141. As in the 99Ru case, the quadrupole interaction is larger in the hydrides than it is in the deuterides (Table 2). The results obtained with the 19’Au resonance (Table 3) are not very different from those obtained with this isotope in NbH, [ 71. Using the results for 19’Au in PdH, [4], one estimates that the shift induced by a single hydrogen neighbour next to a gold probe causes a shift of between 0.3 and 0.5 mm s-l. If the shift induced by a single neighbour in VH, and VD, is similar, the results compiled in Table 3 do not exclude the possibility that gold has nearest neighbours in these systems; the same is true for gold in NbH, [ 71. One notes, however, that the shifts for sources of 19’Pt in VH, are smaller than those for absorbers of gold in VH, at the same concentration.

180

This indicates antitrapping in the platinum case. The rapid change of the shift for gold impurities in VD, between x = 0.81 and x = 0.89 (Table 3) indicates that gold is also antitrapping, albeit somewhat less so than platinum. The electric quadrupole interactions of gold are quite small compared with the linewidth and hence too unreliable to warrant serious discussion. 4. Conclusions All probe nuclei studied appear to be antitrapping in VH, as well as in VD, , except in the VH, and VD, phases, where ruthenium and rhodium are

definitely surrounded by interstitials. Corresponding experiments with other isotopes will be interesting, not only to obtain information on the behaviour of probes in these concentrated phases, but also to obtain isomer shift data which permit better values for the isomer shift per nearest neighbour to be derived. This would considerably facilitate the interpretation of Mijssbauer data for such systems. Acknowledgment We thank the staff of the cyclotron laboratory of the Kernforschungszentrum Karlsruhe for the production of the “Rh sources used in this work. One of us (L.I.) would like to thank the CNPq for financial support. This work was supported by the Bundesministerium fur Forschung und Technologie. References 1 F. E. Wagner and G. Wortmann, in G. Alefeld and J. Vijlkl (eds.), Hydrogen in Metals I, Top. Appl. Phys., 28 (1978) 131. 2 A. Weidinger, to be published. 3 M. Karger and F. E. Wagner, Hyperfine Interact., 9 (1981) 553. 4 M. Karger, F. Probst, B. Schiittler and F. E. Wagner, in T. N. Veziroglu (ed.), MetalHydrogen Systems, Pergamon, Oxford, 1982, p. 187. 5 F. E. Wagner, M. Karger, F. Probst and B. Schiittler, in P. Jena and C. B. Satterthwaite (eds.), Electronic Structure and Properties of Hydrogen in Metals, Plenum, New York, 1983,~. 581. 6 F. Probst, F. E. Wagner and M. Karger, J. Less-Common Met., 88 (1982) 201. 7 R. Wordel and F. E. Wagner, J. Less-Common Met., 101 (1984) 427. 8 J. Trager, M. Karger, T. Butz and F. E. Wagner, Hyperfine Zntemct., 15 - 16 (1983) 795. 9 M. Berneis, J. Trager, R. Wordel, M. Zelger, F. E. Wagner and T. Butz, 2. Phys. Chem. NY,, 145 (1985) 129. 10 P. Peretto, G. Teisseron and J. Berthier, J. Phys. {Paris), 44 (1983) 109. 11 R. Wordel, F. J. Litterst and F. E. Wagner, J. Phys. P, 15 (1985) 2525. 12 R. Wordel and F. E. Wagner, J. Less-Common Met., 130 (1986). 13 J. Kotthaus and R. Vianden, Hyperfine Interact., 14 (1983) 99. 14 T. Schober and H. Wenzl, in G. Alefeld and J. Vijlkl (eds.), Hydrogen in Metals IZ, Top. Appl. Phys., 29 (1978) 11.