Abstract
A perturbed gamma gamma angular correlation (PAC) study of the electric quadrupole interaction (QI) of 111Cd probes on substitutional V sites in the C15 Laves phase HfV2 has revealed a pronounced thermal hysteresis of the QI and the 111Cd fraction on regular V sites: when samples are cycled through 700 K in vacuum, the quadrupole frequency nu Q on the way down is found to be systematically larger than that on the way up at the same temperature. Simultaneously a sharp irreversible drop of the regular site fraction occurs upon cooling from 700 K. This behaviour indicates that HfV2 reacts with water vapour at T<or=700 K, dissociating residual H2O into its atomic components. HfV2 acts as a strong getter for the hydrogen, which is absorbed upon cooling below 700 K and again desorbed at T>800 K. The oxygen, however, is absorbed only in trace amounts. These conclusions are supported by a mass spectroscopic desorption measurement and a comparative PAC study of QI in the hydride HfV2H0.1, for which the activation energy for the hydrogen diffusion has been determined to be Ea=0.070(25) eV.
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