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A perturbed yy angular correlation (PAC) study of the electric quadrupole intera c t i o n ( ~~) of"'CdprobesonsubstitutionalVsitesintheC15Lavesphase HW, hasrevealed a pronounced thermal hysteresis of the 01 and ule "'GI fraction on regular V sites: when samplesarecycledthrough700Kinvacuum,thequadrupolefrequencyvoonthewaydown is found to be systematically larger than that on thc 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 e 700 K, dissociating residual H 2 0 into its atomic components. HWt acts as a strong getter for the hydrogen, which is absorbed upon cooling below 700 K and again desorbed a1 T > 800K.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 the Q I in the hydride HfV,H, ,, for which the activation energy for the hydrogen diffusion has been determined to be E. = 0.070(25) eV.

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