Hydrides of Alloys and Intermetallic Compounds

In alloys and intermetallic compounds the sublattices of interstitial sites usually have rather complex structures. Furthermore, hydrogen atoms dissolved in alloys and intermetallics can occupy a number of inequivalent types of interstitial sites. 

These features may give rise to a coexistence of several types of H motion with different characteristic jump rates. Here we shall discuss the experimental results on hydrogen jump diffusion mechanisms in a number of representative interme-tallic compounds. A comprehensive review of the experimental studies in this field before 1992 can be found in Ref [11]. 

The results of QENS measurements for TaV2H [76] are consistent with this microscopic picture of H motion. First, on the frequency scale of tf the measured QENS spectra S(Q, co) are well described by the sum of a narrow elastic line and a broader quasielastic line having Q-dependent intensity, but Q-independent width. These features are typical of the case of spatially-confined (localized) motion [14]. 

Second, the Q-dependence of the measured elastic incoherent structure factor (EISF) appears to be in excellent agreement with the predictions of the model of localized atomic motion over a hexagon (Eq. (26.13)) with the distance between the nearest-neighbor sites equal to the experimental value. As an example of these results, Eig. 26.5 shows the behavior of the EISE for TaV2Hj j as a function of Q at several temperatures. The solid curves represent the fits of the six-site model to the data. In these fits the distance between the nearest-neighbor sites has been fixed to its value resulting from the structure, = 0.99 A, so that the 

A similar microscopic picture of hydrogen motion has been observed for other cubic Laves-phase hydrides with exclusive g-site occupation and r2/rj 1 ZrCt2H  

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