Dynamical evidence of hydrogen sublattice melting in metal-hydrogen systems

Journal of the Less-Common

279

Metals, 129 (1987) 279 - 285

DYNAMICAL EVIDENCE OF HYDROGEN SUBLATTICE MELTING IN METAL-HYDROGEN SYSTEMS* R. G. BARNESa, F. BORSAa*, M. JEROSCH-HEROLDa, J.-W. HANa, M. BELHOULa, J. SHINARa, D. R. TORGESONa, D. T. PETERSONa, G. A. STYLESb and E. F. W. SEYMOURb aAmes Laboratory, IA 50011 (U.S.A.) bPkysics

department,

U.S. Department Unive~ity

of Energy, Iowa State University,

of Warwick,

Coventry

Ames,

CV4 7AL (U.K.)

summary Measurements of the temperature dependence of the proton and deuteron spin-lattice relaxation time T1 in cubic fluorite (CaF2) structure dihydrides and dideuterides (MH, and MD2) show a second high temperature turndown in T1 in addition to the usual minimum associated with the independent hopping motion of H(D) among tetrahedral (T) interstitial sites at lower temperatures. The close analogy to the motion of anions in fluorite structure superionics suggests that the high temperature minimum indicates the onset of strongly correlated hydrogen motion, possibly accompanied by the occurrence of long-lived hydrogen clusters.

1. Introduction The transition to a state of unusually fast ion diffusion at high temperatures is well-known in so-called superionic conductors. In the fluorides of the CaF, structure (PbF2, BaF, etc.), for example, the transition is “diffuse” and occurs without an accomp~y~g structural phase transition. Recent neutron scattering studies indicate that the term “sublattice melting”, although descriptively appealing, is not literally correct [ 1 J. Rather, the sharp increase in F- ion diffusion is a consequence of the rapid growth of anion Frenkel pairs at high temperatures, leading to significant dynamic disorder on the anion sublattice, which allows the normally sited anions to diffuse more rapidly and possibly in a highly correlated manner. Transition metal dihydrides MHz (TiH2, ScH2 etc.) also form in the CaF, structure and show hydrogen diffusion rates at moderate temperatures *Paper presented at the International Symposium on the Properties and Applications of Metal Hydrides V, Maubui~on, France, May 25 - 30,1986. TPermanent address: Dipartimento di Fisica, Universita di Pavia, Pavia, Italy. 0022-5088/87/$3.50

@ Elsevier Sequoia/Printed in The Netherlands

entirely comparable with those of F- in the ionic fluorides. On this basis alone, it might be anticipated that metaf hydrides would exhibit some characteristics of superionic materials at high temperatures. In fact, the possibility that hydrogen diffusion in metals enters a highly correlated “fluid-like” regime at high temperatures has been discussed theoretically [ 21, and experimental indications of such behavior based on neutron quasi-elastic scattering have been reported for NbH e.r6 [ 31. In the case of fast-ion conductors, anomalous behavior of the nuclear spin relaxation times of both diffusing and stationary ion species has been found in a number of cases and associated with the transition to the superionic state 141. Accordingly, we have extended our nuclear magnetic resonance (NMR) studies of hydrogen diffusion in metal hydrides [ 5] to si~ifi~~tly higher temperatures, seeking evidence of such a transition, and we report here the initial results of this effort.

2. Experimental results We have measured the proton (‘H) spin-lattice relaxation time T1 at several resonance frequencies at temperatures up to 1330 K in the dihydride phases of scandium, yttrium, lanthanum, titanium, and zirconium. (This temperature was limited by the high hydrogen pressure generated in the quartz sample containers which caused several samples to explode.) Representative results for §cHi.ss and YHr,ss are shown in Fig. 1. The sharp decrease of Ti that occurs at high temperatures in all the systems studied should be noted. Denoting the temperature at which the turndown in 2’1 occurs by T,, it is seen in Fig. 1 that Tc for YHi.ss is less than Tc for ScH,.ss. In fact, in all cases investigated, Tc is the temperature at which the hydrogen hopping rate extrapolated from the low temperature diffusion-induced T1 minimum reaches a value vd * 10” s-‘, which is also the case in PbFz [4]. Up to the vicinity of T,,the behavior of the relaxation rate RI = (T1)-' results from contributions due to conduction electrons (R le = f !I’,,)-’ ) and the normal diffusional modulation of the ‘II-‘H dipolar interaction (R& which is responsible for the deep T1 minimum at iower temperatures. A weak contribution R tp owing to residual paramagnetic impurities may also occur on the low ~mperat~e side of the deep T1 minimum. As seen in Fig. 1, the conduction electron contribution RI, is not responsible for the high temperature Tt behavior. In addition, whereas the low temperature minimum shows the expected dependence on resonance frequency oO, T1 behavior in the vicinity of !l’c depends only weakly on wO. Entirely similar behavior is found for the deuteron T, . The temperature dependence of the “D T1 in ScD,.s, and YDr,ss measured at resonance frequencies of 12.2 and 7 MHz respectively are shown in Fig. 2. Comparison of the SeD 1.s2 data with the proton data for ScH1.s3 in Fig. 1 shows that the entire sD curve is shifted to lower temperatures with respect to the ‘H data. (The same holds for the YD1.ss data; we have not shown ‘H data for the same composition.) For the normal d~fu~on-induct flow temperature)

281 TBHPEKATURE

(K)

r

n

.

n

TC' 1000 K

n m

I

0

n -80 %

0

8 v

no

.YH1.98 .(at 40 ma

'L

1.0 RECIPROCAL

0

Y"1.98 0 (at 12.2 J4Rr

2.0 TEMPERATURE

3.0 (103/K)

Fig. 1. Temperature dependence of the proton spin-lattice relaxation time in ScH1.s~ (at 12.2 MHz) and YHr.as (at 12.2 and 40 MHz). The respective turndown temperatures Tc are indicated by the arrows. The data for YHr.ss show clearly the normal frequency dependence of the proton T1 at the low temperature minimum and the weak frequency dependence at Tc. The solid curve shows the temperature dependence of the conduction electron contribution T1, for YHr.as, extrapolated from low temperatures. T1 minimum: this reflects the fact that *D relaxation is primarily quadrupolar in origin and strongly sensitive to both deuteron and vacancy motion, whereas ‘H relaxation is dipolar and sensitive only to proton motion. Near the monovacancy limit, rP-’ = c,T,-~ where c, is the vacancy concentration, and we have T, < T, so that the condition w,r = 1 (which holds at the diffusion-induced minimum in T,) is satisfied at a lower temperature for vacancy

282 TEHPE~TURE

(K)

0 loTC -

675

K

(at

12.2 MHz)

! w

5.0-

950 K 1

0

v

i

0 l

T

***e

0

T.88 7 fimf

(at 0 l

0 0 0

0.6

~

1

I

t

1.0

2.0

3.0

RECIPROCAL

TEMPERATURE

4.0

(103/K1

Fig. 2. Temperature dependence of the deuteron spin-lattice relaxation time in ScD1.s~ and YH1.as at resonance frequencies of 12.2 and 7 MHz respectively. The respective turndown temperatures 2’~ are indicated by the arrows. The solid curve shows the temperature dependence of the conduction electron contribution T1, for YDl.ss, extrapolated from low temperatures. That for ScD1.s~ is similar.

motion than for proton motion. A treatment for arbitrary c, [6] leads to a qualitatively similar conclusion. Moreover, in YH, and YD, the hopping rates are greater than in the scandium systems at the same x value and also increase markedly with increasing x value. This, combined with the lower resonance frequency (7 MHz) for YD1.ss, causes the latter to be strongly shifted to lower temperatures with respect to that for ScDl.sz. The anomalous T, behavior at high temperatures is clearly delineated, and the turndown in T1 again occurs well before the conduction electron contribution T1, is reached,

3. Discussion Before proceeding to discuss possible interpretations of the data in terms of a transition to a superionic regime, we consider other mechanisms which might be effective. Both for superionics and Mets-hydrogen systems,

283

low levels of paramagnetic impurities can produce very signific~t Ti changes, especially at temperatures at which anion diffusion is rapid [ 5, 7 ] . We have therefore made careful checks on samples containing controlled levels of gadolinium up to 100 ppm in ScH,.,s. The ‘H T1 develops the expected secondary minimum [ 51 on the low temperature side of the normal diffusioninduced minimum, but the high temperature turndown of T1 is present in all samples, essentially unaffected by the gadolinium. At the highest temperatures d~fus~on of metal ions, or conceivably of oxygen or nitrogen impurities, may occur, but the superposition of such relatively slow motions on the fast hydrogen motion can make very little difference to the overall dipolar or quad~pol~ fluctuations. Any subst~ti~ change in Tie, associated with an electronic structure transition, can be ruled out, at least for ScH,, since no corresponding change in the 4sSc Knight shift is observed [S] . It is possible that at these temperatures some fraction of the hydrogen ions transfer from T sites to octahedral (0) sites. Indeed, there is evidence for some of the dihydrides (e.g. YHz) [9] but not for others (e.g. ScH,), that a small fraction of 0 sites are occupied at room temperature, However, this fraction does not seem ta be an increasing function of temperature, a phenomenon which has been attributed to increasing difficulty of 0 site occupation in the face of increasing hydrogen vibrational amplitude in neighboring T sites [lo]. There are no relevant data for our highest temperatures, so that the effect of a rapidly increasing fraction of 0 site hydrogen ions around 7’;: should be explored. A suitable model for discussion of this situation in the case of dipolar interactions is that for the 19F relaxation in LaFs [ 111. This involves the introduction of three temperature dependent dwell times for jumps between sites: TTT, 7oo and 7-To.Since the dipolar interactions are not changed by orders of magnitude in occupation of 0 sites, such a model would require at least one of the dwell times (presumably 7oo) to be long compared with 7~ above Tc. To this must be added the temperature dependence of no/~, the relative numbers of ions on 0 and T sites. Although by suitable choice of all these parameters it has been found possible to reproduce the general behavior trend observed, including the high temperature turndown of T1 (as part of a second T1(T) minimum), and freedom from non-exponenti~ spy-lattice relaxation behavior, we have not found it possible to reproduce the experimental data qu~titatively, particularly the relative insensitivity of T1 to frequency near Tc accompanied by normal frequency dependence at the low temperature minimum. Although we have not carried out the calculations, similar considerations must apply to the quadrupolar relaxation of 2D. A further reason why this model appears not to be completely satisfactory is that the phenomenon occurs equally in dihydrides where, at normal temperatures, some 0 sites are occupied and in others where 0 sites have been shown not to be occupied (e.g. ScH,), and in systems which do and others which do not form trihydrides (where, of course, 0 sites become fuliy occupied). We return, therefore, to the discussion of correlated hydrogen motion. For simplicity we consider first a simple model in which a column of neigh-

284

boring hydrogen ions all move together among T sites, maintaining fixed mutual separations both in magnitude and direction. In this case the dipolar and quadrupolar interactions between pairs of ions within the column would not be affected. In more detail, jumps of successive ions along the column would be expected to occur at intervals of a few vibrational periods (approximately lo-i4 s), but such delays are of no significance since they are short compared with the Larmor period of ‘H or 2D. Interactions between pairs of nuclei, one of which is a member of the correlated column and the other not, will fluctuate at the usual rate related to the individual jump time. Near Tc the observed relaxation rate will be given by [4] T1-’ = (1- f)Tld-’

+ fTl,-’

(1)

where f is a fraction closely associated with the fraction of ions taking part in the cooperative motion, Tl,-’ is the relaxation rate in that state, and Tldpl the rate in an uncorrelated environment. Well above T,, f= i if we use as a model superionic PbF2, in which it has been found [ 1, 121 that approximately half the fluorine ions jump rapidly between regular lattice sites, and half are situated interstitially and do not take part in the correlated motion. Since we have a rather weak dependence of TIC on frequency, we must have W,T, <, 1 where 7, is the correlation time for the correlated state and may be regarded as the lifetime of a correlated group, and must therefore expect, roughly, Tl,-’ a T,. In a similar way Tldpl 0: Td, approximately. Hence, Tl,/Tld = rd/r,. From the data in Figs. 1 and 2, for example, we find that at the highest temperature reached (1250 K) the lifetime of a correlated group is between 5 (for ScH1.s3) and 200 (for YDl.ss) times the individual T - T jump time. In the case of superionics such as PbF2 the rapid correlated flow of a fraction of the anions between regular anion sites is made possible by withdrawal of the remaining anions to form interstitial clusters. The cluster lifetime is approximately lo-l2 s. Motion of cluster ions on this timescale cannot contribute appreciably to relaxation. However, carrying over this model to the hydrides, it has been inferred [8] from the T1 behavior of 45Sc in ScH, that analogous hydrogen clusters may exist with a much longer lifetime, more nearly comparable with (but still shorter than) the Larmor period c&‘. This suggests that, at least in the case of ScH, and ScD,, the enhanced ‘H and 2D relaxation rates may be partially associated with a long correlation time for interactions within clusters. One formulation which might be suitable for dealing with this could be based on the approach of Fedders [ 131. From our present data we have no way to separate the correlated motion and cluster contributions. In conclusion, we find from our new measurements that fluorite structure dihydrides exhibit surprisingly long correlation times for proton dipolar and deuteron quadrupolar interactions at high temperatures. We suggest this may be a general phenomenon among such dihydrides, and indeed hydrides with other structures. We interpret these observations as indicating the onset of strongly correlated hydrogen motions, analogous to the motion of anions

285

in fluorite structure superionics, possibly accompanied by the occurrence of long-lived hydrogen clusters. An alternative interpretation in terms of the inhibition of T-O-T jumps in favor of slower direct T-T jumps has been suggested by Richards [ 141. Further experiments designed to distinguish between the contrasting possibilities (i.e. reduced or enhanced hydrogen diffusion) of this picture and that of superionic behavior are planned. Additional supporting NMR evidence for these conclusions concerning the existence of metallic superionics has been given elsewhere [ 81.

Acknowledgments The authors are indebted to B. J. Beaudry and A. Johnson for their careful preparation of the hydride samples, and to M. Newton for the calculations of Ti in the two-sublattice model [lo]. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract W-7405-Eng-82. This work was supported by the Director for Energy Research, Office of Basic Energy Sciences. The work at the University of Warwick was supported by the United Kingdom Science and Engineering Research Council. Travel support provided by a grant from the National Science Foundation (Grant INT-8403045) is gratefully acknowledged by the Ames Laboratory group.

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