Proton NMR study of diffusion in continuous, nonstoichiometric metal-hydrogen systems: HfV2Hx and ZrV2Hx

J. Shinar*, D. Davidov, D. Shaltiel

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

69 Scopus citations

Abstract

The proton spin-relaxation times T1 and T2 in the continuous, nonstoichiometric, single-phase hydrides ZrV2Hx and HfV2Hx (0.25x4.2) were measured in the temperature range 100 KT500 K at 10.7, 33.0, and 51.95 MHz. T1 exhibits a minimum versus 1T as well as asymmetry about this minimum. This asymmetry manifests itself mainly at low hydrogen concentrations (x2.0) but is noticeable up to x=3.5. Although there are some differences in the behavior of the HfV2Hx and ZrV2Hx systems at low hydrogen concentrations (x2.0), at higher ones (x>2.0) the behavior is similar. The temperature and frequency dependence of T1and the asymmetry around the minimum in particularare analyzed in terms of fluctuations of the dipolar interaction at the thermally activated hopping rate. This hopping rate is assumed to be governed by a distribution of activation energies. Good agreement with the experimental results is obtained. The "average" activation energy Ea0 generally increases with x, from about 1000 K at x=0 to about 5000 K at x=4. The relative width of the distribution EaEa0 decreases with x above x=2. The logarithm of the "average" preexponential factor, ln 0, is found to be linear in Ea0, ln 0=ln 00+Ea. The intercept is 00=1.66×109 sec-1 and the slope =3.09×10-3 K-1. According to the model of Wert and Zener (WZ), is the temperature derivative of the relative shear modulus d(0)dT. However, the present value is larger than the value of d(0)dT observed for interstitial diffusion of various ions in transition metals. This point is discussed. In addition, according to the WZ model, 00 is the hopping coordination number times the proton vibration frequency. The present value, 1.66 × 109 sec-1, is obviously too low to be interpreted as such. The solution of this problem, which has appeared in many disordered systems, is believed to be related to local concentration gradients and/or strain fields. These inhomogeneities apparently decrease (consequently causing the rapid rise of 0 and Ea0) as the concentration increases towards the quasistoichiometric limit x=4.

Original languageEnglish
Pages (from-to)6331-6341
Number of pages11
JournalPhysical Review B
Volume30
Issue number11
DOIs
StatePublished - 1984

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