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γ-γ Perturbed angular correlation studies of dysprosium hydrides
Journal of the Less-CommonMetals,
130 (1987)
155 - 162
‘y-y PERTURBED ANGULAR CORRELATION DYSPROSIUM HYDRIDES* L. P. FERREIRAa, G. TEISSERONb, P. VULLIETb, A. P. DE LIMABand N. AYRES DE CAMPOSa
aDepartamento
155
STUDIES
OF
J. M. GILa, P. J. MENDESa,
de Fisica, Universidade de Coimbra, 3000 Coimbra (Portugal)
bCentre d%tudes Nucle’aires, DRF/Service de Physique/Magnetisme et Diffusion par Interactions Hyperfines, 85 X, 38041 Grenoble Cedex, and Universite’ Scientifique, Technologique et Medicale de Grenoble, Grenoble (France) (Received
May 9,1986)
Summary Perturbed angular correlation measurements have been performed on two dysprosium hydrides (DyH, with x = 2 and x = 3) using the lslTa nucleus as a substitutional probe. When x = 2, the observed electric field gradients result from hydrogen vacancies in a cubic lattice. The contribution of the conduction electron density to the electric field gradient is discussed. In the trihydride the electric field gradient has an axial symmetry.
1. Introduction Rare earth hydrides have long been known to exhibit interesting electronic and magnetic properties. In the heavy rare earths, addition of hydrogen induces crystallographic changes: rare earth metals and rare earth trihydrides have an h.c.p. structure, whereas rare earth dihydrides show an f.c.c. structure [ 11. As well as these structural differences, the density of conduction electrons varies with the hydrogen concentration: rare earth and rare earth dihydrides are good conductors, rare earth trihydrides are semiconductors or insulators [ 2, 31. Among the numerous experimental methods which can give information on structural or electronic changes, hyperfine techniques such as the Mijssbauer effect and y-y perturbed angular correlation (PAC) are of special interest. They allow measurements of local electric and/or magnetic fields
*Paper presented at the International of Metal Hydrides V, Maubuisson, France,
Symposium on the Properties May 25 - 30,1986.
0022-5033/87/$3.50
@ Elsevier
Sequoia/Printed
and Applications
in The Netherlands
156
and thus are a priori sensitive to any change in the neighbourhood of the probe atom. Although the Mijssbauer effect has been widely used in RE hydrides studies (see refs. 3 - 5), we were unable to find any paper in the literature in which the PAC method has been used. However, it has been successfully employed to determine electric field gradients (EFG) in metals and particularly in rare earth metals [ 6,7]. In this paper, we report preliminary results on dysprosium hydrides using “lTa as the PAC probe nucleus.
2. Experimental
details
lslHf labelled dysprosium ingots are prepared by irradiating about 3 mg of natural hafnium in a neutron flux of 4 X 1013 neutrons cme2 s-l for 3 days and then by melting this radioactive material with dysprosium(4N) in an induction furnace under a very pure argon stream. The final hafnium concentration in dysprosium is about 700 ppm. When necessary, thermal annealing of the radioactive samples is made in a furnace in a vacuum of 2 X lO-‘j Torr. Hydrogenation is performed in a quartz vessel. A vacuum of lO-‘j Ton is achieved before heating the sample. Hydrides which have a hydrogen to dysprosium ratio of about two are obtained by introducing the appropriate number of moles of hydrogen into the reacting cell (volume, about 1 1). The temperature of the ingot is maintained at 520 “C for 4 h and then slowly decreased to room temperature in half an hour. To prepare hydrogen saturated samples (which have a hydrogen to dysprosium ratio of about 3), hydrogen is admitted at a pressure of one atmosphere. The temperature of the ingot is stabilized at 570 “C for 2 h and then gradually decreased to room temperature in 4 h. The DyH, compound is very sensitive to oxygen and water, so it is immediately transferred to sealed quartz tubes. The uncertainty in the hydrogen content is 0.03. The experimental 7-y perturbed angular correlation set-up is a conventional one with three detectors which has time resolution of 2 ns. The -y-y cascade of the “iTa nucleus follows the P_ disintegration of the parent lelHf nucleus (mean lifetime of 44 days). For the I = 5/2 intermediate nuclear level of islTa, the theoretical perturbation factor G,(t) for a randomly oriented electric quadrupole interaction is given by [8]
Szn and fn(r)) are tabulated coefficients. r) and o. are the asymmetry parameter of the EFG and the quadrupole frequency respectively. The exponential factor takes into account a possible distribution of the EFG which has a relative width 6. o. is related to the main component of the EFG V,, by: o. = 6eQVz,/4J(21l)fi, where Q is the quadrupole moment of the I = 5/2
157
lslTa level (Q = 2.51 b [9]). It should be noted that conventional PAC measurements do not permit the sign of V,, to be determined. If the i81Ta nuclei experience more than one hyperfine interaction, the following expression is fitted to the data: G,(t) = CCiG,‘(t) i
where Ci is the fraction
of the ith hyperfine
interaction.
3. Results The time dependence of the perturbation factor G2(t) of i8iTa in dysprosium which has been annealed at 500 “C for 4 h is shown in Fig. l(a). The observed spectrum is best fitted with two hyperfine interactions: a welldefined one with a quasi-axial symmetry (11= 0), whose proportion Ci amounts to 89% of the total spectrum and a non-perturbed one (wO(*) = 0
l.W
0.75
0.50
0.25
0.00
0.
10.
0.
10.
20.
30.
20.
IO.
LO.
50.
II I nr
LO.
3.
Ill ns
I
I
I
I
I
I
0.
10.
20.
30.
40.
xl.
I
ItI nc
l.W
0.75
0.50
0.25
0.00
Fig.
1. Room
temperature
PAC spectra
of (a) Dy(Hf),
(b) Dy(Hf)Hl_sx,
(c) Dy(Hf)Hz.ps.
158
Mrad s-l) with Cz = 11% (see Table 1). Another sample annealed at the same temperature for 16 h gives the same results. Either without annealing or with annealing at 750 “C for 4 h, higher values of Cz are observed (30% and 40% respectively). The presence of a second interaction has been observed in similar experiments and has been attributed to hafnium impurities on nonregular lattice sites only to the low solubility of hafnium in dysprosium [ 11, 121. In the present case, the value oot2) = 0 results from ‘*lTa nuclei in cubic symmetry. It is not known if interstitial sites available for hafnium impurities (with a lower atomic volume than the host) offer the proper symmetry. This special symmetry may come from the cubic high temperature phase of RE [13], locally quenched during the very rapid cooling process of the melted ingot. Taking into account the observed evolution of a fraction of non-regular lattice sites with temperature and the time of annealing, we assume that this fraction might be reduced to a vanishing value by carefully choosing the annealing conditions. Dysprosium(hafnium) hydrides have been prepared from ingots annealed at 500 “C for 4 h. The room temperature G,(t) spectrum of Dy(‘81Ta)H,.,s is shown in Fig. l(b). The best fit is obtained if we suppose the presence of four different axially symmetric quadrupole interactions, i.e. four types of tantalum nuclei experiencing different axially symmetric EFGs, the parameter values of which are listed in Table 1. The high frequency of about 1000 Mrad s-’ is responsible for the rapid decrease of the G,(t) factor in the first nanosecond. In Fig. l(c), the PAC spectrum of Dy(181Ta)H2.s5 at room temperature is shown. A good fit to the data is obtained with three quadrupole interactions with axial symmetry. The main fraction (about 80%) has a broad frequency distribution (6 = 15%), indicating that there are many more strains and much more local disorder in the trihydride than in the dihydride. 4. Discussion The total EFG V,, at nuclear site in non-cubic conducting materials is usually considered to arise from: (i) a lattice EFG Vzzlat due to the positively charged lattice ions, multiplied by the Stemheimer enhancement factor (1- ym) and (ii) an electronic contribution Vzzel caused by the non-spherical charge density of the conducting electrons v,, = (1 - y,)V/
+ vztei
Although Vrzht can be computed easily from a point-charge model, the electronic EF.G contribution is far from fully understood. In all pure h.c.p. rare earths as well as at substitutional impurity sites in h.c.p. rare earth hosts, it has been shown that Vzzht and Vzzel have the same positive sign (see ref. 14 and the references therein). This means that the conduction electron density must be concentrated closer to the hexagonal plane than between the hexagonal planes. A measurement of the electronic contribution relatively to the lattice contribution is given by
1 2 3
Dy(Hf)Hr.ss
1 2 3
,,Ci)
0 0 0 0
298(10) 123(8) O(2)
0 0 0
0.04(l) 0
va
O(2)
1007(100) 138(E) 32(5)
367(4) O(d)
(Mrad s-l)
15(l) 5(5) 0
0
5(2) 4(2) 7(2)
l(O.2) 0
6a (%)
b
78(3) 7(2) 15(2)
18(2)
9(3) 10(5) 63(3)
89(2) ll(2)
f&)
5.21(17) 2.15(14) O(O.04)
O(O.04)
17.6(1.8) 2.41(14) 0.56(9)
6.41(7) 0
IV ?
(1tP V m-2)
5.90
15.7 (V,) 2.26 (V,) 0.94 (Va) 0.56 (V,)
1.60
(1 -YmNzz cdc
(102’ V mea)
0.88(3)
1.12(11) 1.06(6) 0.66(11)e
4.00(4)
I VZLil
Cry= (1 - r,)Vzzca’
Dy(Hf)H r.ss and Dy(Hf)Ha.as and corresponding
%hen no error is given, the parameter has been fixed in the fitting procedure to make sure that it has a low value. bCi, fraction of lslTa nuclei in site i. ‘Calculated with (1 - ym) = 62 for Ta 5+ [lo] and +3 (-1) charges at the dysprosium (hydrogen, when present) lattice sites. d lslTa nuclei in non-regular h.c.p. lattice sites (see text). eWeighted mean value of (1 - yw)Vzz = 0.845.
Dy(Hf)Hz.,
1 2d
Dy(Hf)
4
Site i
Sample
Room temperature quadrupole interaction parameters at rslTa impurities in Dy(Hf), calculated lattice or vacancies EFGs
TABLE 1
160
,(
I:! I Oi___/__, I
I
Fig. 2. Crystallographic unit cell for a rare earth dihydride: (0) rare earth, (0) interstitial tetrahedral sites, (X) interstitial octahedral sites.
a=
VZ, (1 - %)vzzlat
In the idealized f.c.c. structure of rare earth dihydrides, hydrogen atoms occupy the centre of rare earth tetrahedra. In fact, there is experimental evidence that this configuration of hydrogen atoms will never be reached [ 151. As the ratio of hydrogen to rare earth atoms approaches 2.0, hydrogen atoms begin to fill octahedral sites located at the centre of the cube and at the midpoints of the cube edges before the full occupancy of the tetrahedral sites (Fig. 2). Recent neutron diffraction studies on terbium hydrides indicate that the octahedral sites begin to be filled for a hydrogen to terbium ratio greater than 1.95 [16]. We have studied the dysprosium hydride with a hydrogen to dysprosium ratio of 1.83, a value sufficiently far from 1.95 to assume that only the tetrahedral sites are occupied. However, in an hypothetical hydride of composition DyH,.,, there is statistically one hydrogen vacancy per unit cell. Four types of hydrogen vacancies Vi(X, y, z) may be distinguished relatively to a dysprosium (hafnium) atom at (O,O, 0): VI (i, a, $); V2 (z, i, a); V3 (i, 4, i) and V, (:, a, $ ), the proportions of which are in the ratio 1:3: 3:l. We suppose that there is no correlation between vacancies. We have calculated the EFG created at the metal lattice site by the four different vacancies (because of its rp3 dependence, the EFG due to vacancies farther away than V, is almost zero). These values are listed in Table 1. In the hypostoichiometric hydride DyH i,ss +a-a3the unit cell contains an average of 0.68 + 0.12 vacancies. Thus 32 f 12% of the unit cells have a perfect cubic symmetry. Thus, to a first approximation, five EFG may act at a metal lattice site: (i) one EFG of zero resulting from the cubic symmetry and (ii) four EFG arising from the four types of vacancies. The two larger frequencies (site 1 and site 2) in the PAC results are probably due to VI and V2 respectively. The third frequency (site 3) with the highest fraction (63%) is attributed to the combined effect of V3, V, and more distant vacancies. Finally, the zero frequency (site 4) corresponds to lBITa nuclei in cubic symmetry. The experimental proportion of this latter site (18 + 2%) is lower than expected (32 f 12%). This may be related to. the non-vanishing influence of vacancies which are more than one lattice
161
parameter distant. For sites 1 and 2, good agreement is found between experimental and calculated values of the EFG (cu = 1). Obviously conduction electrons which have the same cubic symmetry as the lattice do not contribute to the EFG. They only play a screening part which increases with the distance, related to the decreasing trend of CYfrom site 1 to site 3. Dysprosium (hafnium) and Dy(Hf)H2_9s both have the h.c.p. crystal structure. In the virgin compound, the quasi-zero value of the measured asymmetry parameter 7 (corresponding to an axially symmetric EFG) is expected for islTa impurities on substitutional sites in a hexagonal metallic lattice. Furthermore, the EFG value (V,, = 6.41 X lo*’ V mw2) is in good agreement with that measured by Forker and Steinborn: V,, = 6 50 X lO*i V m-* [ll]. A satisfactory agreement is also found between our’o ratio and that obtained by the same authors (a = 4.22). Thus the electronic EFG contribution is about three times larger than the lattice contribution. Atomic positions of hydrogen in DyH, are as yet unknown. Nevertheless, we may reasonably assume that they do not differ drastically from the atomic positions of deuterium in HOD, [17]. However, many arguments are in favour of the anionic character of hydrogen in rare earth trihydrides, where the electrons come from the conduction band [ 11. On these bases, we have calculated the lattice EFG contribution by setting charges of -1 at hydrogen sites, +3 charges being located at dysprosium sites as in the calculation for the virgin compound (the lattice parameters are given in ref. 17). Obviously the (1 - ym ) VzzCalvalue thus obtained is to be related to the experimental V,, value of the main fraction (site 1) from which it differs by less than 15% (see Table 1). Accordingly the salt-like (hydridic) character of DyH, - and more generally of REHs first suggested by Pleber and Wallace [l] is confirmed. Miissbauer experiments performed on isostructural GdHs and TmH, [5,18] lead to lattice EFG values of (+) 5.1(5) X lo*’ V m-* and +6.6(1.8)X 1021 V rnA2 respectively. Our experimental value of (k) 5.21(17)1021 V m-* is in good agreement with these last two values if we take into account the small differences in lattice parameters and in Sternheimer factors (1 - ym) (Gd3+:62; Tm3+:60; Ta5+:62). Another important aspect of our results concerns the symmetry of the EFG. All attempts to fit our data with 17 larger than about 0.05 have failed. On the other hand, the EFG tensor calculated within the previously described model has a quasi-axial symmetry (q = 3 X 10d3). These two results are completely inconsistent with the conclusions of Stewart and Wortmann [19] who obtained temperature-dependent 7 values larger than 0.5 using Mossbauer experiments on the same hydride. A detailed discussion of the origin of this discrepancy will be published elsewhere. It should be noted that (i) their sample has been hydrogenated at a lower temperature (230 “C) than ours has (570 “C), (ii) PAC experiments with “‘Ta as a probe nucleus permit a more direct determination of the EFG tensor than does the 61Dy Mijssbauer technique because of strong paramagnetic relaxation combined
162
with the additional EFG contribution of the incomplete 4f shell of dysprosium, (iii) the Mossbauer data have been fitted with only one site. However, the influence of hydrogen vacancies must not be ruled out. That is clearly demonstrated by the necessity of two additional sites to fit out data better (site 2 and site 3). The quasi-zero frequency of site 3 is to be related to a rest of cubic or almost cubic DyHz phase in the slightly defective trihydride we have studied. In conclusion, the high sensibility and selectivity of the PAC technique make it well adapted to investigate hydrogen-induced crystallographic and electronic changes in rare earths. Studies of other rare earth hydrides are in progress or planned in our laboratories.
References 1 A. Pleber and W. E. Wallace, J. Phys. Chem., 66 (1962) 148. 2 R. R. Arons, Landolt-Bb’rnstein, Group 111, Magnetic and other properties of oxides and related compounds, Vol. 126, Springer-Verlag, Berlin, 1982, p. 372. 3 G. Alefed and J. VSlkl, Hydrogen in Metals Z/II, Top. Appl. Phys., 28/29 (1978). 4 J. M. Friedt, G. K. Shenoy, B. D. Dunlap, D. G. Westlake and A. T. Alred, Phys. Rev. B, 20 (1979) 251. 5 G. A. Stewart, G. Kaikowski and G. Wortmann, J. Less-Common Met., 73 (1980) 291. 6 E. Bodenstedt, Hyperfine Interact., 24 - 26 (1985) 521. 7 M. Forker, Hyperfine Interact., 24 - 26 (1985) 907. 8 H. Franenfelder and R. M. Stefen, in K. Siegbahn (ed.), Alpha-, Beta- and Gamma Ray Spectroscopy, Vol. 2, North-Holland, Amsterdam, 1968, Chapter XIXA. 9 G. Netz and E. Bodenstedt, Nucl. Phys. A, 208 (1973) 503. 10 F. D. Feiock and W. R. Johnson,Phys. Rev., 187 (1969) 39. 11 M. Forker and W. Steinborn, Phys. Rev. B, 20 (1979) 1. 12 B. Perczuk and M. Forker, Solid State Commun., 21 (1977) 1131. 13 S. Legvold, in E. P. Wohlfarth (ed.), Ferromagnetic Materials, Vol. 1, North-Holland, Amsterdam, 1980, p. 184. 14 R. Vianden, Hyperfine Interact., 15 - 16 (1983) 189. 15 P. Vorderwish and S. Hautecler, Phys. Status Solidi B, 66 (1974) 595. 16 R. R. Arons, W. Shafer and J. Schweizer, J. Appl. Phys., 53 (1982) 2631. 17 M. Mansmann and W. E. Wallace, J. Phys. (Paris), 25 (1964) 454. 18 S. J. Lyle, P. T. Walsh, A. D. Witts and J. W. Ross, J. Chem. Sot., Dalton Trans., (1975) 1406. 19 G. A. Stewart and G. Wortmann, Phys. Lett., 85 A (1981) 185.
130 (1987)
155 - 162
‘y-y PERTURBED ANGULAR CORRELATION DYSPROSIUM HYDRIDES* L. P. FERREIRAa, G. TEISSERONb, P. VULLIETb, A. P. DE LIMABand N. AYRES DE CAMPOSa
aDepartamento
155
STUDIES
OF
J. M. GILa, P. J. MENDESa,
de Fisica, Universidade de Coimbra, 3000 Coimbra (Portugal)
bCentre d%tudes Nucle’aires, DRF/Service de Physique/Magnetisme et Diffusion par Interactions Hyperfines, 85 X, 38041 Grenoble Cedex, and Universite’ Scientifique, Technologique et Medicale de Grenoble, Grenoble (France) (Received
May 9,1986)
Summary Perturbed angular correlation measurements have been performed on two dysprosium hydrides (DyH, with x = 2 and x = 3) using the lslTa nucleus as a substitutional probe. When x = 2, the observed electric field gradients result from hydrogen vacancies in a cubic lattice. The contribution of the conduction electron density to the electric field gradient is discussed. In the trihydride the electric field gradient has an axial symmetry.
1. Introduction Rare earth hydrides have long been known to exhibit interesting electronic and magnetic properties. In the heavy rare earths, addition of hydrogen induces crystallographic changes: rare earth metals and rare earth trihydrides have an h.c.p. structure, whereas rare earth dihydrides show an f.c.c. structure [ 11. As well as these structural differences, the density of conduction electrons varies with the hydrogen concentration: rare earth and rare earth dihydrides are good conductors, rare earth trihydrides are semiconductors or insulators [ 2, 31. Among the numerous experimental methods which can give information on structural or electronic changes, hyperfine techniques such as the Mijssbauer effect and y-y perturbed angular correlation (PAC) are of special interest. They allow measurements of local electric and/or magnetic fields
*Paper presented at the International of Metal Hydrides V, Maubuisson, France,
Symposium on the Properties May 25 - 30,1986.
0022-5033/87/$3.50
@ Elsevier
Sequoia/Printed
and Applications
in The Netherlands
156
and thus are a priori sensitive to any change in the neighbourhood of the probe atom. Although the Mijssbauer effect has been widely used in RE hydrides studies (see refs. 3 - 5), we were unable to find any paper in the literature in which the PAC method has been used. However, it has been successfully employed to determine electric field gradients (EFG) in metals and particularly in rare earth metals [ 6,7]. In this paper, we report preliminary results on dysprosium hydrides using “lTa as the PAC probe nucleus.
2. Experimental
details
lslHf labelled dysprosium ingots are prepared by irradiating about 3 mg of natural hafnium in a neutron flux of 4 X 1013 neutrons cme2 s-l for 3 days and then by melting this radioactive material with dysprosium(4N) in an induction furnace under a very pure argon stream. The final hafnium concentration in dysprosium is about 700 ppm. When necessary, thermal annealing of the radioactive samples is made in a furnace in a vacuum of 2 X lO-‘j Torr. Hydrogenation is performed in a quartz vessel. A vacuum of lO-‘j Ton is achieved before heating the sample. Hydrides which have a hydrogen to dysprosium ratio of about two are obtained by introducing the appropriate number of moles of hydrogen into the reacting cell (volume, about 1 1). The temperature of the ingot is maintained at 520 “C for 4 h and then slowly decreased to room temperature in half an hour. To prepare hydrogen saturated samples (which have a hydrogen to dysprosium ratio of about 3), hydrogen is admitted at a pressure of one atmosphere. The temperature of the ingot is stabilized at 570 “C for 2 h and then gradually decreased to room temperature in 4 h. The DyH, compound is very sensitive to oxygen and water, so it is immediately transferred to sealed quartz tubes. The uncertainty in the hydrogen content is 0.03. The experimental 7-y perturbed angular correlation set-up is a conventional one with three detectors which has time resolution of 2 ns. The -y-y cascade of the “iTa nucleus follows the P_ disintegration of the parent lelHf nucleus (mean lifetime of 44 days). For the I = 5/2 intermediate nuclear level of islTa, the theoretical perturbation factor G,(t) for a randomly oriented electric quadrupole interaction is given by [8]
Szn and fn(r)) are tabulated coefficients. r) and o. are the asymmetry parameter of the EFG and the quadrupole frequency respectively. The exponential factor takes into account a possible distribution of the EFG which has a relative width 6. o. is related to the main component of the EFG V,, by: o. = 6eQVz,/4J(21l)fi, where Q is the quadrupole moment of the I = 5/2
157
lslTa level (Q = 2.51 b [9]). It should be noted that conventional PAC measurements do not permit the sign of V,, to be determined. If the i81Ta nuclei experience more than one hyperfine interaction, the following expression is fitted to the data: G,(t) = CCiG,‘(t) i
where Ci is the fraction
of the ith hyperfine
interaction.
3. Results The time dependence of the perturbation factor G2(t) of i8iTa in dysprosium which has been annealed at 500 “C for 4 h is shown in Fig. l(a). The observed spectrum is best fitted with two hyperfine interactions: a welldefined one with a quasi-axial symmetry (11= 0), whose proportion Ci amounts to 89% of the total spectrum and a non-perturbed one (wO(*) = 0
l.W
0.75
0.50
0.25
0.00
0.
10.
0.
10.
20.
30.
20.
IO.
LO.
50.
II I nr
LO.
3.
Ill ns
I
I
I
I
I
I
0.
10.
20.
30.
40.
xl.
I
ItI nc
l.W
0.75
0.50
0.25
0.00
Fig.
1. Room
temperature
PAC spectra
of (a) Dy(Hf),
(b) Dy(Hf)Hl_sx,
(c) Dy(Hf)Hz.ps.
158
Mrad s-l) with Cz = 11% (see Table 1). Another sample annealed at the same temperature for 16 h gives the same results. Either without annealing or with annealing at 750 “C for 4 h, higher values of Cz are observed (30% and 40% respectively). The presence of a second interaction has been observed in similar experiments and has been attributed to hafnium impurities on nonregular lattice sites only to the low solubility of hafnium in dysprosium [ 11, 121. In the present case, the value oot2) = 0 results from ‘*lTa nuclei in cubic symmetry. It is not known if interstitial sites available for hafnium impurities (with a lower atomic volume than the host) offer the proper symmetry. This special symmetry may come from the cubic high temperature phase of RE [13], locally quenched during the very rapid cooling process of the melted ingot. Taking into account the observed evolution of a fraction of non-regular lattice sites with temperature and the time of annealing, we assume that this fraction might be reduced to a vanishing value by carefully choosing the annealing conditions. Dysprosium(hafnium) hydrides have been prepared from ingots annealed at 500 “C for 4 h. The room temperature G,(t) spectrum of Dy(‘81Ta)H,.,s is shown in Fig. l(b). The best fit is obtained if we suppose the presence of four different axially symmetric quadrupole interactions, i.e. four types of tantalum nuclei experiencing different axially symmetric EFGs, the parameter values of which are listed in Table 1. The high frequency of about 1000 Mrad s-’ is responsible for the rapid decrease of the G,(t) factor in the first nanosecond. In Fig. l(c), the PAC spectrum of Dy(181Ta)H2.s5 at room temperature is shown. A good fit to the data is obtained with three quadrupole interactions with axial symmetry. The main fraction (about 80%) has a broad frequency distribution (6 = 15%), indicating that there are many more strains and much more local disorder in the trihydride than in the dihydride. 4. Discussion The total EFG V,, at nuclear site in non-cubic conducting materials is usually considered to arise from: (i) a lattice EFG Vzzlat due to the positively charged lattice ions, multiplied by the Stemheimer enhancement factor (1- ym) and (ii) an electronic contribution Vzzel caused by the non-spherical charge density of the conducting electrons v,, = (1 - y,)V/
+ vztei
Although Vrzht can be computed easily from a point-charge model, the electronic EF.G contribution is far from fully understood. In all pure h.c.p. rare earths as well as at substitutional impurity sites in h.c.p. rare earth hosts, it has been shown that Vzzht and Vzzel have the same positive sign (see ref. 14 and the references therein). This means that the conduction electron density must be concentrated closer to the hexagonal plane than between the hexagonal planes. A measurement of the electronic contribution relatively to the lattice contribution is given by
1 2 3
Dy(Hf)Hr.ss
1 2 3
,,Ci)
0 0 0 0
298(10) 123(8) O(2)
0 0 0
0.04(l) 0
va
O(2)
1007(100) 138(E) 32(5)
367(4) O(d)
(Mrad s-l)
15(l) 5(5) 0
0
5(2) 4(2) 7(2)
l(O.2) 0
6a (%)
b
78(3) 7(2) 15(2)
18(2)
9(3) 10(5) 63(3)
89(2) ll(2)
f&)
5.21(17) 2.15(14) O(O.04)
O(O.04)
17.6(1.8) 2.41(14) 0.56(9)
6.41(7) 0
IV ?
(1tP V m-2)
5.90
15.7 (V,) 2.26 (V,) 0.94 (Va) 0.56 (V,)
1.60
(1 -YmNzz cdc
(102’ V mea)
0.88(3)
1.12(11) 1.06(6) 0.66(11)e
4.00(4)
I VZLil
Cry= (1 - r,)Vzzca’
Dy(Hf)H r.ss and Dy(Hf)Ha.as and corresponding
%hen no error is given, the parameter has been fixed in the fitting procedure to make sure that it has a low value. bCi, fraction of lslTa nuclei in site i. ‘Calculated with (1 - ym) = 62 for Ta 5+ [lo] and +3 (-1) charges at the dysprosium (hydrogen, when present) lattice sites. d lslTa nuclei in non-regular h.c.p. lattice sites (see text). eWeighted mean value of (1 - yw)Vzz = 0.845.
Dy(Hf)Hz.,
1 2d
Dy(Hf)
4
Site i
Sample
Room temperature quadrupole interaction parameters at rslTa impurities in Dy(Hf), calculated lattice or vacancies EFGs
TABLE 1
160
,(
I:! I Oi___/__, I
I
Fig. 2. Crystallographic unit cell for a rare earth dihydride: (0) rare earth, (0) interstitial tetrahedral sites, (X) interstitial octahedral sites.
a=
VZ, (1 - %)vzzlat
In the idealized f.c.c. structure of rare earth dihydrides, hydrogen atoms occupy the centre of rare earth tetrahedra. In fact, there is experimental evidence that this configuration of hydrogen atoms will never be reached [ 151. As the ratio of hydrogen to rare earth atoms approaches 2.0, hydrogen atoms begin to fill octahedral sites located at the centre of the cube and at the midpoints of the cube edges before the full occupancy of the tetrahedral sites (Fig. 2). Recent neutron diffraction studies on terbium hydrides indicate that the octahedral sites begin to be filled for a hydrogen to terbium ratio greater than 1.95 [16]. We have studied the dysprosium hydride with a hydrogen to dysprosium ratio of 1.83, a value sufficiently far from 1.95 to assume that only the tetrahedral sites are occupied. However, in an hypothetical hydride of composition DyH,.,, there is statistically one hydrogen vacancy per unit cell. Four types of hydrogen vacancies Vi(X, y, z) may be distinguished relatively to a dysprosium (hafnium) atom at (O,O, 0): VI (i, a, $); V2 (z, i, a); V3 (i, 4, i) and V, (:, a, $ ), the proportions of which are in the ratio 1:3: 3:l. We suppose that there is no correlation between vacancies. We have calculated the EFG created at the metal lattice site by the four different vacancies (because of its rp3 dependence, the EFG due to vacancies farther away than V, is almost zero). These values are listed in Table 1. In the hypostoichiometric hydride DyH i,ss +a-a3the unit cell contains an average of 0.68 + 0.12 vacancies. Thus 32 f 12% of the unit cells have a perfect cubic symmetry. Thus, to a first approximation, five EFG may act at a metal lattice site: (i) one EFG of zero resulting from the cubic symmetry and (ii) four EFG arising from the four types of vacancies. The two larger frequencies (site 1 and site 2) in the PAC results are probably due to VI and V2 respectively. The third frequency (site 3) with the highest fraction (63%) is attributed to the combined effect of V3, V, and more distant vacancies. Finally, the zero frequency (site 4) corresponds to lBITa nuclei in cubic symmetry. The experimental proportion of this latter site (18 + 2%) is lower than expected (32 f 12%). This may be related to. the non-vanishing influence of vacancies which are more than one lattice
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parameter distant. For sites 1 and 2, good agreement is found between experimental and calculated values of the EFG (cu = 1). Obviously conduction electrons which have the same cubic symmetry as the lattice do not contribute to the EFG. They only play a screening part which increases with the distance, related to the decreasing trend of CYfrom site 1 to site 3. Dysprosium (hafnium) and Dy(Hf)H2_9s both have the h.c.p. crystal structure. In the virgin compound, the quasi-zero value of the measured asymmetry parameter 7 (corresponding to an axially symmetric EFG) is expected for islTa impurities on substitutional sites in a hexagonal metallic lattice. Furthermore, the EFG value (V,, = 6.41 X lo*’ V mw2) is in good agreement with that measured by Forker and Steinborn: V,, = 6 50 X lO*i V m-* [ll]. A satisfactory agreement is also found between our’o ratio and that obtained by the same authors (a = 4.22). Thus the electronic EFG contribution is about three times larger than the lattice contribution. Atomic positions of hydrogen in DyH, are as yet unknown. Nevertheless, we may reasonably assume that they do not differ drastically from the atomic positions of deuterium in HOD, [17]. However, many arguments are in favour of the anionic character of hydrogen in rare earth trihydrides, where the electrons come from the conduction band [ 11. On these bases, we have calculated the lattice EFG contribution by setting charges of -1 at hydrogen sites, +3 charges being located at dysprosium sites as in the calculation for the virgin compound (the lattice parameters are given in ref. 17). Obviously the (1 - ym ) VzzCalvalue thus obtained is to be related to the experimental V,, value of the main fraction (site 1) from which it differs by less than 15% (see Table 1). Accordingly the salt-like (hydridic) character of DyH, - and more generally of REHs first suggested by Pleber and Wallace [l] is confirmed. Miissbauer experiments performed on isostructural GdHs and TmH, [5,18] lead to lattice EFG values of (+) 5.1(5) X lo*’ V m-* and +6.6(1.8)X 1021 V rnA2 respectively. Our experimental value of (k) 5.21(17)1021 V m-* is in good agreement with these last two values if we take into account the small differences in lattice parameters and in Sternheimer factors (1 - ym) (Gd3+:62; Tm3+:60; Ta5+:62). Another important aspect of our results concerns the symmetry of the EFG. All attempts to fit our data with 17 larger than about 0.05 have failed. On the other hand, the EFG tensor calculated within the previously described model has a quasi-axial symmetry (q = 3 X 10d3). These two results are completely inconsistent with the conclusions of Stewart and Wortmann [19] who obtained temperature-dependent 7 values larger than 0.5 using Mossbauer experiments on the same hydride. A detailed discussion of the origin of this discrepancy will be published elsewhere. It should be noted that (i) their sample has been hydrogenated at a lower temperature (230 “C) than ours has (570 “C), (ii) PAC experiments with “‘Ta as a probe nucleus permit a more direct determination of the EFG tensor than does the 61Dy Mijssbauer technique because of strong paramagnetic relaxation combined
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with the additional EFG contribution of the incomplete 4f shell of dysprosium, (iii) the Mossbauer data have been fitted with only one site. However, the influence of hydrogen vacancies must not be ruled out. That is clearly demonstrated by the necessity of two additional sites to fit out data better (site 2 and site 3). The quasi-zero frequency of site 3 is to be related to a rest of cubic or almost cubic DyHz phase in the slightly defective trihydride we have studied. In conclusion, the high sensibility and selectivity of the PAC technique make it well adapted to investigate hydrogen-induced crystallographic and electronic changes in rare earths. Studies of other rare earth hydrides are in progress or planned in our laboratories.
References 1 A. Pleber and W. E. Wallace, J. Phys. Chem., 66 (1962) 148. 2 R. R. Arons, Landolt-Bb’rnstein, Group 111, Magnetic and other properties of oxides and related compounds, Vol. 126, Springer-Verlag, Berlin, 1982, p. 372. 3 G. Alefed and J. VSlkl, Hydrogen in Metals Z/II, Top. Appl. Phys., 28/29 (1978). 4 J. M. Friedt, G. K. Shenoy, B. D. Dunlap, D. G. Westlake and A. T. Alred, Phys. Rev. B, 20 (1979) 251. 5 G. A. Stewart, G. Kaikowski and G. Wortmann, J. Less-Common Met., 73 (1980) 291. 6 E. Bodenstedt, Hyperfine Interact., 24 - 26 (1985) 521. 7 M. Forker, Hyperfine Interact., 24 - 26 (1985) 907. 8 H. Franenfelder and R. M. Stefen, in K. Siegbahn (ed.), Alpha-, Beta- and Gamma Ray Spectroscopy, Vol. 2, North-Holland, Amsterdam, 1968, Chapter XIXA. 9 G. Netz and E. Bodenstedt, Nucl. Phys. A, 208 (1973) 503. 10 F. D. Feiock and W. R. Johnson,Phys. Rev., 187 (1969) 39. 11 M. Forker and W. Steinborn, Phys. Rev. B, 20 (1979) 1. 12 B. Perczuk and M. Forker, Solid State Commun., 21 (1977) 1131. 13 S. Legvold, in E. P. Wohlfarth (ed.), Ferromagnetic Materials, Vol. 1, North-Holland, Amsterdam, 1980, p. 184. 14 R. Vianden, Hyperfine Interact., 15 - 16 (1983) 189. 15 P. Vorderwish and S. Hautecler, Phys. Status Solidi B, 66 (1974) 595. 16 R. R. Arons, W. Shafer and J. Schweizer, J. Appl. Phys., 53 (1982) 2631. 17 M. Mansmann and W. E. Wallace, J. Phys. (Paris), 25 (1964) 454. 18 S. J. Lyle, P. T. Walsh, A. D. Witts and J. W. Ross, J. Chem. Sot., Dalton Trans., (1975) 1406. 19 G. A. Stewart and G. Wortmann, Phys. Lett., 85 A (1981) 185.