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Pulsed proton nuclear magnetic resonance measurements of Korringa relaxation in ZrV2Hx and HfV2Hx hydrides
Journal of the Less-Common
Metals,
104 (1984)
125
125 - 130
PULSED PROTON NUCLEAR MAGNETIC RESONANCE MEASUREMENTS OF KORRINGA RELAXATION IN ZrV,H, HfV2H, HYDRIDES*
AND
J. SHINAR
Solid State Institute Haifa ,320OO (Israel) (Received
and Physics
Department-
Technion,
Israel Institute
of Technology,
April 11,1984)
Summary The temperature dependence of the proton spin-lattice relaxation rate l/T, in HfV*H, and ZrV2H, (1 < x < 4) was measured in the temperature range 77 - 450 K at several frequencies. In some cases, l/T, was also measured in the range 2 - 77 K at 51.95 MHz. Thus, in HfVzH3.s, l/T1 is linear in 2’ from 2 to 150 K with a slope of 0.011 s-l K-‘. In ZrV2H, (x = 3.5, 4), l/T1 is linear in T from 77 to 180 K, but the slope is 0.019 s-l K -‘. In ZrV2H, the slope changes from 0.008 f 0.004 s-l K-’ below 45 K to 0.019 s-l K-’ above 65 K. At lower hydrogen concentrations the upper bounds on the Korringa relaxation rate l/T,, indicate a lower slope. These results are briefly discussed in relation to band structure calculations for ZrV,, other measured properties of the ZrV2H, and HfV2H, systems and the electronic structure of the transition metal hydrides.
1. Introduction The thermodynamic and various physical properties of intermetallic compound hydrides have been extensively investigated over the last decade and a half. However, relatively few experimental or theoretical studies of the electronic structures of these systems have been published [ 1 - 31. In contrast, detailed calculations and nuclear magnetic resonance (NMR) studies of the band structures of various transition metal hydrides have appeared [ 4 - 71. In this preliminary work the proton electronic spin-lattice relaxation rates for various concentrations of hydrogen in HfV*H, and ZrV,H, are reported. These ternary hydrides are among the most attractive for electronic structure investigations owing to the following features. They are continuous, i.e. at room temperature and above they maintain the *Paper presented at the International Symposium of Metal Hydrides IV, Eilat, Israel, April 9 - 13, 1984.
0022-5088/84/$3.00
on the Properties
0 Elsevier Sequoia/Printed
and Applications
in The Netherlands
126
hydrogen-free Cl5 Laves phase structure, and the lattice constant increases linearly with x [8,9]. A recent NMR study of the proton diffusion in these systems has revealed striking concentration-dependent features [ 91. The hydrogen-free compounds are the highest T, superconductors among the Laves phase intermetallics [lo - 121 and have been extensively investigated using various experimental techniques. Thus, resistivity [ 131, heat capacity [12, 141, susceptibility [12] and X-ray measurements [13,15] of the Hf,Zri_,V, and Ta,Hf,_,V, systems have revealed a transition from a cubic to a lower symmetry phase at a temperature 70 K < T, S 120 K which is closely (and inversely) related to T,. This behaviour is in agreement with the relation between lattice instabilities and T, in the f.c.c. Al5 intermetallic compounds [ 161. The lattice instabilities and high values of T, are related to the high d band densities of states N&?&) at the Fermi level. Recent band structure calculations for ZrVz do indeed indicate a sizable peak in N(E,), which is due mainly to the vanadium d band states and to a lesser extent to the Zr d band states [17]. Indeed, in qualitative agreement with these calculations and a rigid band picture, the superconducting transition temperature T, of HfV2H, and Zro.sHfO.sVzH, generally decreases with increasing x from T,= 8 K at x = 0 to T,G 2 K at 3t = 1.5 [lo, 111, indicating that N(E,) decreases with 3t in that region. The results of this work, which are described in Section 3 (following a brief account of the experimental procedure in Section 2), indicate that N(E,) increases with x in hydrides with rx > 2. However, only a few theoretical or experimental studies of such hydrides have been reported [18 - 231. The discussion in Section 4 is therefore limited to a brief comparison with these previous studies. 2. Experimental
procedure
Samples of HfVz and ZrVz were prepared by arc melting hafnium clippings (purity, 99.9%) with vanadium wire (purity, 99.7%) in an argon atmosphere. The resulting ZrVz buttons were sealed in quartz tubes in argon at a pressure of about 100 mmHg and were annealed for 24 h at 1448 K. Annealing of HfVz was unnecessary. X-ray diffraction measurements revealed 97% - 99% cubic Laves phase. The powdered samples (about 0.5 g; grain size, 100 pm) were then placed in a Pyrex reactor, degassed at 648 K for 10 - 30 min and hydrogenated to the desired concentration. The concentration was determined by the decrease in the pressure in the reactor chamber. The hydrided samples were cooled to 77 K, the pressure in the reactor was decreased to several millimetres of mercury and the Pyrex ampoule was sealed off with a gas flame. This ensured that the nominal hydrogen concentration was maintained at all times. To ensure the phase homogeneity of the samples with low hydrogen contents, these were also annealed at about 573 K following hydrogenation. During the NMR measurements the temperature of the samples was varied and controlled using a home-built cryostat operating in a helium gas
flow mode at temperatures above about 10 K. For measurements below 4.2 K the sample and r.f. coil were immersed in liquid helium which could be pumped down to 2 K. The conduction electron contribution to the proton spin relaxation rate l/T1, was determined from free induction decay (FID) measurements of Ti at 51.95 MHz using a home-built spectrometer. In several cases the FID could not be fully saturated and consequently the resulting recovery was non-exponential. This behaviour was assumed to result from the presence of small amounts of paramagnetic impurities [24] around which the relaxation of neighbouring protons would be much more rapid than that of the distant protons. Therefore in these cases the long T, tail of the FID was assumed to be representative of the “true” T, which would be observed in an ideal paramagnetic-impurity-free hydride.
3. Experimental
results
Plots of l/T1 uersus the temperature T in ZrV2H, (3~= 1, 2, 3, 3.5, 4) and HfV2H3.s are shown in Figs. 1 - 3. The striking features are as follows. (a) The low temperature behaviour of l/T, in ZrVZHsS5 is almost identical with that in ZrV2H4 (Fig. l), the slope Cx being about 0.019 s-l K-’ (see Table 1). (b) Although the absolute values of l/T1 are larger in ZrV2H3, Cx (down to 65 K) is equal to that in ZrV2H3.5 and ZrV2H4 (Table 1). (c) In ZrV2H, the slope Cx decreases from 0.019 s-l K-’ above 60 K to about 0.008 t 0.004 s-l K-’ below 50 K. (d) Although the results of the measurements in ZrV2Hl and ZrV2H2 (Fig. 3) do not enable Cx to be extracted, the values of l/T1 T set upper I
I
I
I
I
I
I
I
I
3-
T
Fig. 1. Low temperature proton ZrVZH3.5 (0) and ZrVZH4 (a).
(K) spin-lattice
relaxation
rates
l/T1
in HfVzH3.5
(*),
T (K) Fig. 2. Low temperature proton spin-lattice relaxation rates l/T1 temperature slope is equal to that of ZrV 2H 3.s and ZrV2H4 while slope is roughly equal to that of HfV2Hs.s.
50
0
100
in ZrV2Hs.
The
high
the low temperature
150
T (K) Fig. 3. Low temperature proton spin-lattice relaxation rates l/Z’1 in ZrVzHl (0) and ZrVzHz (0). Despite the observed scatter in the data points, the observed values are significantly lower than in Figs. 1 and 2. TABLE
1
Summary
Sample
HfVzH3.5 ZrVzHl ZrV2H2 ZrV2H3
ZrVd-b.5 ZrV2H4
of experimental
results
Temperature
lITleT (s-l K-‘) 0.011 < 0.005 < 0.009
(K)
f 0.001
0.008 f 0.004
2 - 125 77 - 115 4.2 - 20
0.019
4.2 - 45 65 - 130
0.019 0.018
77 - 175
77 - 185
range
129
bounds on it. These upper bounds are considerably lower than the values at higher concentrations (Table 1). (e) In HfVzHs.s (Fig. l), l/T, is linear in T from 2 to 130 K with no change in the slope as occurs in ZrV2H3 (Fig. 2). The slope CK in HfV2H3.s is about one-half of that of ZrV,H,., and ZrV2H4 (Table 1).
4. Discussion
l/Ti,
and concluding
The conduction electron is given by [6]
remarks contribution
to the spin-lattice
relaxation
rate
where a,, ad and q, are the electron-proton hyperfine coupling constants of the polarization of the core s electrons by the d band electrons at the Fermi level and the d band orbital angular momentum respectively. Although generally CY,is much larger than od or o. [25, 261, in most transition metal systems Nd(EF) is much larger than N,(E,). Indeed, a band structure calculation reported recently [17] indicates that in hydrogen-free ZrV2 the overwhelming contribution to N(,?#?,) comes from the d electrons (Nd(Er) = is less than 5 states 200 states rydberg-’ (unit cell))’ whereas N,(E,) rydberg-’ (unit cell)-‘). This also appears to be the case in many hydrided systems since the s band contributed by the hydrogen usually lies several electronvolts below the Fermi level, as calculated for several systems [4, 5, 271 and observed using photoelectron spectroscopy [ 281. In addition, NMR studies of titanium and zirconium hydrides [7, 25, 291 and Ti-V and Ti-Nb alloy hydrides [2, 30, 311 and previous 51V NMR studies of Zr(Vi_,Co,), [ 321 and ZrV,H, [ 181 all indicate that the dominant contribution to l/TJ’ comes from the core polarization term. It therefore appears that the relatively low values of l/TI,T observed in ZrV2Hl and ZrV2H2 (see Table 1) reflect relatively low densities of Fermi level d band states. However, the increased values of l/T1,T in ZrV2H, (X = 3, 3.5 and 4) are not necessarily due to an increase in Nd(Er). The s states contributed by the excess hydrogen in these relatively unstable concentrations may also lie several electronvolts below the Fermi level. In any case, however, the increase in l/TI,T as the hydrogen concentration x increases from 2 or less to 3 or more probably reflects an increase in the total N(E,). Finally, HfV2D4 and ZrV2D3.6 are known to undergo a structural orderto-disorder phase transition at 278 K and 325 K respectively [ 22, 231. This transition, which is very similar in the two systems, transforms the high temperature cubic Laves phase deuteride (in which the deuterium sublattice is disordered) to a low temperature face-centred tetragonal structure which orders the deuterium sublattice. The observed relaxation rates of ZrV2H3.5 and ZrV2H4 (and possibly of HfV2HSa5 also) are therefore probably those of the protons in the lower symmetry phase. If this phase transition is due to a
130
Jahn-Teller distortion (as in, for example, TiH, and ZrH, [2, 7, 25]), the total IV(&) in the cubic phase is expected to be even larger. A band structure calculation of these hydrided systems would obviously be helpful in clarifying the observed Korringa relaxation and may also disclose the nature of the structural transition in the concentrated hydrides.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
29 30 31 32
R. C. Bowman, Jr., A. J. Maeland and W.-K. Rhim, Phys. Rev. B, 26 (1982) 6362. R. C. Bowman, Jr., and W.-K. Rhim, Phys. Rev. B, 24 (1981) 2232. B. M. Klein and W. E. Pickett, J. Less-Common Met., 88 (1982) 231. A. C. Switendick, Solid State Commun., 8 (1970) 1463. A. C. Switendick, 2. Phys. Chem. N.F., 117 (1979) 447; in G. Alefeld and J. Vijlki (eds.), Hydrogen in Metals Z, Top. Appl. Phys., 28 (1978) 101. R. M. Cotts, in G. Alefeld and J. Volkl (eds.), Hydrogen in Metals I, Top. Appl. f’hys., 28 (1978) 227, and references cited therein. C. Korn, Phys. Rev. B, 28 (1983) 95. A. Pebler and E. A. Gulbransen, Trans. Metall. Sot. AIME, 239 (1967) 1593. J. Shinar, D. Davidov and D. Shaltiel, Phys. Reu. B, in the press. P. Duffer, D. M. Gualtieri and V. U. S. Rao, Phys. Rev. Lett., 37 (1976) 1410. V. U. S. Rao, D. M. Gualtieri, S. Krishnamurthy, A. Patkin and P. Duffer,Phys. Lett. A, 67 (1978) 223. V. A. Marchenko and V. M. Polovov, Sou. Phys. - JETP, 51 (1980) 535. A. C. Lawson and W. H. Zachariasen,Phys. Lett. A, 38 (1972) 1. 0. Rapp and L. J. Vieland, Phys. Lett. A, 36 (1971) 369. D. E. Moncton, Solid State Commun., 13 (1973) 1779. I. B. Goldberg and M. Weger, Solid State Phys., 28 (1973) 1. B. M. Klein, W. E. Pickett, D. A. Papaconstantopoulos and L. L. Boyer, Phys. Rev. B, 27 (1983) 6721. M. Peretz, J. Barak, D. Zamir and J. Shinar,Phys. Rev. B, 23 (1981) 1031. J. J. Didisheim, K. Yvon, D. Shaltiel, P. Fischer, P. Bucjard and E. Walker, Solid State Commun., 32 (1979) 1087. D. P. Shoemaker and C. B. Shoemaker, J. Less-Common Met., 68 (1979) 43. J. J. Didisheim, K. Yvon, P. Fischer and D. Shaltiel, J. Less-Common Met., 73 (1980) 355. J. J. Didisheim, K. Yvon, P. Fischer and P. Tissot, Solid State Commun., 38 (1981) 637. -4. V. Irodova, V. P. Glazkov, V. A. Somenkov and S. Sh. Shilstein, J. Less-Common Met., 77 (1981) 89. A. Abragam, Principles of Nuclear Magnetism, Clarendon, Oxford, 1961, p. 379. R. C. Bowman, Jr., E. L. Venturini, B. D. Craft, A. Attalla and D. B. Sullenger, Phys. Rev. B, 27 (1983) 1474. G. C. Carter, L. H. Bennett and D. J. Kahan, Metallic Shifts in NMR, Pergamon, Oxford, 1977. M. Gupta, Solid State Commun., 29 (1979) 47. M. Gupta and J. P. Burger, Phys. Rev. B, 24 (1981) 7099. B. W. Veal, D. J. Lam and D. G. Westlake,Phys. Rev. B, 19 (1979) 2856. J. H. Weaver, D. J. Peterman, D. T. Peterson and A. Franciosi, Phys. Rev. B, 23 (1981) 1692. C. Korn,Phys. Rev. B, 17 (1978) 1707. B;Nowak, N. Pislewski and W. Leszczynski, Phys. Status Solidi A, 37 (1976) 669. B. Nowak, 0. J. Zogal and M. Minier, J. Phys. C, 12 (1979) 4591. M. Peretz, D. Zamir, D. Shaltiel and J. Shinar, 2. Phys. Chem. N.F., 117 (1979) 221.
Metals,
104 (1984)
125
125 - 130
PULSED PROTON NUCLEAR MAGNETIC RESONANCE MEASUREMENTS OF KORRINGA RELAXATION IN ZrV,H, HfV2H, HYDRIDES*
AND
J. SHINAR
Solid State Institute Haifa ,320OO (Israel) (Received
and Physics
Department-
Technion,
Israel Institute
of Technology,
April 11,1984)
Summary The temperature dependence of the proton spin-lattice relaxation rate l/T, in HfV*H, and ZrV2H, (1 < x < 4) was measured in the temperature range 77 - 450 K at several frequencies. In some cases, l/T, was also measured in the range 2 - 77 K at 51.95 MHz. Thus, in HfVzH3.s, l/T1 is linear in 2’ from 2 to 150 K with a slope of 0.011 s-l K-‘. In ZrV2H, (x = 3.5, 4), l/T1 is linear in T from 77 to 180 K, but the slope is 0.019 s-l K -‘. In ZrV2H, the slope changes from 0.008 f 0.004 s-l K-’ below 45 K to 0.019 s-l K-’ above 65 K. At lower hydrogen concentrations the upper bounds on the Korringa relaxation rate l/T,, indicate a lower slope. These results are briefly discussed in relation to band structure calculations for ZrV,, other measured properties of the ZrV2H, and HfV2H, systems and the electronic structure of the transition metal hydrides.
1. Introduction The thermodynamic and various physical properties of intermetallic compound hydrides have been extensively investigated over the last decade and a half. However, relatively few experimental or theoretical studies of the electronic structures of these systems have been published [ 1 - 31. In contrast, detailed calculations and nuclear magnetic resonance (NMR) studies of the band structures of various transition metal hydrides have appeared [ 4 - 71. In this preliminary work the proton electronic spin-lattice relaxation rates for various concentrations of hydrogen in HfV*H, and ZrV,H, are reported. These ternary hydrides are among the most attractive for electronic structure investigations owing to the following features. They are continuous, i.e. at room temperature and above they maintain the *Paper presented at the International Symposium of Metal Hydrides IV, Eilat, Israel, April 9 - 13, 1984.
0022-5088/84/$3.00
on the Properties
0 Elsevier Sequoia/Printed
and Applications
in The Netherlands
126
hydrogen-free Cl5 Laves phase structure, and the lattice constant increases linearly with x [8,9]. A recent NMR study of the proton diffusion in these systems has revealed striking concentration-dependent features [ 91. The hydrogen-free compounds are the highest T, superconductors among the Laves phase intermetallics [lo - 121 and have been extensively investigated using various experimental techniques. Thus, resistivity [ 131, heat capacity [12, 141, susceptibility [12] and X-ray measurements [13,15] of the Hf,Zri_,V, and Ta,Hf,_,V, systems have revealed a transition from a cubic to a lower symmetry phase at a temperature 70 K < T, S 120 K which is closely (and inversely) related to T,. This behaviour is in agreement with the relation between lattice instabilities and T, in the f.c.c. Al5 intermetallic compounds [ 161. The lattice instabilities and high values of T, are related to the high d band densities of states N&?&) at the Fermi level. Recent band structure calculations for ZrVz do indeed indicate a sizable peak in N(E,), which is due mainly to the vanadium d band states and to a lesser extent to the Zr d band states [17]. Indeed, in qualitative agreement with these calculations and a rigid band picture, the superconducting transition temperature T, of HfV2H, and Zro.sHfO.sVzH, generally decreases with increasing x from T,= 8 K at x = 0 to T,G 2 K at 3t = 1.5 [lo, 111, indicating that N(E,) decreases with 3t in that region. The results of this work, which are described in Section 3 (following a brief account of the experimental procedure in Section 2), indicate that N(E,) increases with x in hydrides with rx > 2. However, only a few theoretical or experimental studies of such hydrides have been reported [18 - 231. The discussion in Section 4 is therefore limited to a brief comparison with these previous studies. 2. Experimental
procedure
Samples of HfVz and ZrVz were prepared by arc melting hafnium clippings (purity, 99.9%) with vanadium wire (purity, 99.7%) in an argon atmosphere. The resulting ZrVz buttons were sealed in quartz tubes in argon at a pressure of about 100 mmHg and were annealed for 24 h at 1448 K. Annealing of HfVz was unnecessary. X-ray diffraction measurements revealed 97% - 99% cubic Laves phase. The powdered samples (about 0.5 g; grain size, 100 pm) were then placed in a Pyrex reactor, degassed at 648 K for 10 - 30 min and hydrogenated to the desired concentration. The concentration was determined by the decrease in the pressure in the reactor chamber. The hydrided samples were cooled to 77 K, the pressure in the reactor was decreased to several millimetres of mercury and the Pyrex ampoule was sealed off with a gas flame. This ensured that the nominal hydrogen concentration was maintained at all times. To ensure the phase homogeneity of the samples with low hydrogen contents, these were also annealed at about 573 K following hydrogenation. During the NMR measurements the temperature of the samples was varied and controlled using a home-built cryostat operating in a helium gas
flow mode at temperatures above about 10 K. For measurements below 4.2 K the sample and r.f. coil were immersed in liquid helium which could be pumped down to 2 K. The conduction electron contribution to the proton spin relaxation rate l/T1, was determined from free induction decay (FID) measurements of Ti at 51.95 MHz using a home-built spectrometer. In several cases the FID could not be fully saturated and consequently the resulting recovery was non-exponential. This behaviour was assumed to result from the presence of small amounts of paramagnetic impurities [24] around which the relaxation of neighbouring protons would be much more rapid than that of the distant protons. Therefore in these cases the long T, tail of the FID was assumed to be representative of the “true” T, which would be observed in an ideal paramagnetic-impurity-free hydride.
3. Experimental
results
Plots of l/T1 uersus the temperature T in ZrV2H, (3~= 1, 2, 3, 3.5, 4) and HfV2H3.s are shown in Figs. 1 - 3. The striking features are as follows. (a) The low temperature behaviour of l/T, in ZrVZHsS5 is almost identical with that in ZrV2H4 (Fig. l), the slope Cx being about 0.019 s-l K-’ (see Table 1). (b) Although the absolute values of l/T1 are larger in ZrV2H3, Cx (down to 65 K) is equal to that in ZrV2H3.5 and ZrV2H4 (Table 1). (c) In ZrV2H, the slope Cx decreases from 0.019 s-l K-’ above 60 K to about 0.008 t 0.004 s-l K-’ below 50 K. (d) Although the results of the measurements in ZrV2Hl and ZrV2H2 (Fig. 3) do not enable Cx to be extracted, the values of l/T1 T set upper I
I
I
I
I
I
I
I
I
3-
T
Fig. 1. Low temperature proton ZrVZH3.5 (0) and ZrVZH4 (a).
(K) spin-lattice
relaxation
rates
l/T1
in HfVzH3.5
(*),
T (K) Fig. 2. Low temperature proton spin-lattice relaxation rates l/T1 temperature slope is equal to that of ZrV 2H 3.s and ZrV2H4 while slope is roughly equal to that of HfV2Hs.s.
50
0
100
in ZrV2Hs.
The
high
the low temperature
150
T (K) Fig. 3. Low temperature proton spin-lattice relaxation rates l/Z’1 in ZrVzHl (0) and ZrVzHz (0). Despite the observed scatter in the data points, the observed values are significantly lower than in Figs. 1 and 2. TABLE
1
Summary
Sample
HfVzH3.5 ZrVzHl ZrV2H2 ZrV2H3
ZrVd-b.5 ZrV2H4
of experimental
results
Temperature
lITleT (s-l K-‘) 0.011 < 0.005 < 0.009
(K)
f 0.001
0.008 f 0.004
2 - 125 77 - 115 4.2 - 20
0.019
4.2 - 45 65 - 130
0.019 0.018
77 - 175
77 - 185
range
129
bounds on it. These upper bounds are considerably lower than the values at higher concentrations (Table 1). (e) In HfVzHs.s (Fig. l), l/T, is linear in T from 2 to 130 K with no change in the slope as occurs in ZrV2H3 (Fig. 2). The slope CK in HfV2H3.s is about one-half of that of ZrV,H,., and ZrV2H4 (Table 1).
4. Discussion
l/Ti,
and concluding
The conduction electron is given by [6]
remarks contribution
to the spin-lattice
relaxation
rate
where a,, ad and q, are the electron-proton hyperfine coupling constants of the polarization of the core s electrons by the d band electrons at the Fermi level and the d band orbital angular momentum respectively. Although generally CY,is much larger than od or o. [25, 261, in most transition metal systems Nd(EF) is much larger than N,(E,). Indeed, a band structure calculation reported recently [17] indicates that in hydrogen-free ZrV2 the overwhelming contribution to N(,?#?,) comes from the d electrons (Nd(Er) = is less than 5 states 200 states rydberg-’ (unit cell))’ whereas N,(E,) rydberg-’ (unit cell)-‘). This also appears to be the case in many hydrided systems since the s band contributed by the hydrogen usually lies several electronvolts below the Fermi level, as calculated for several systems [4, 5, 271 and observed using photoelectron spectroscopy [ 281. In addition, NMR studies of titanium and zirconium hydrides [7, 25, 291 and Ti-V and Ti-Nb alloy hydrides [2, 30, 311 and previous 51V NMR studies of Zr(Vi_,Co,), [ 321 and ZrV,H, [ 181 all indicate that the dominant contribution to l/TJ’ comes from the core polarization term. It therefore appears that the relatively low values of l/TI,T observed in ZrV2Hl and ZrV2H2 (see Table 1) reflect relatively low densities of Fermi level d band states. However, the increased values of l/T1,T in ZrV2H, (X = 3, 3.5 and 4) are not necessarily due to an increase in Nd(Er). The s states contributed by the excess hydrogen in these relatively unstable concentrations may also lie several electronvolts below the Fermi level. In any case, however, the increase in l/TI,T as the hydrogen concentration x increases from 2 or less to 3 or more probably reflects an increase in the total N(E,). Finally, HfV2D4 and ZrV2D3.6 are known to undergo a structural orderto-disorder phase transition at 278 K and 325 K respectively [ 22, 231. This transition, which is very similar in the two systems, transforms the high temperature cubic Laves phase deuteride (in which the deuterium sublattice is disordered) to a low temperature face-centred tetragonal structure which orders the deuterium sublattice. The observed relaxation rates of ZrV2H3.5 and ZrV2H4 (and possibly of HfV2HSa5 also) are therefore probably those of the protons in the lower symmetry phase. If this phase transition is due to a
130
Jahn-Teller distortion (as in, for example, TiH, and ZrH, [2, 7, 25]), the total IV(&) in the cubic phase is expected to be even larger. A band structure calculation of these hydrided systems would obviously be helpful in clarifying the observed Korringa relaxation and may also disclose the nature of the structural transition in the concentrated hydrides.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
29 30 31 32
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