Impurity trapping in the Laves phase HfV₂detected by perturbed angular correlations

Abstract

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.

A Proposal for a Composite with Temperature-Independent Thermophysical Properties: HfV2–HfV2O7

“…The obtained increase in lattice parameter may suggest continuous interstitial incorporation of impurities into the HfV2 lattice, which consequently also explains reported property changes in (Hf,Zr)V2 bulk samples after a one-year storage period [81]. HfV2 has been reported to act as a strong (weak) getter for hydrogen (oxygen) by dissociation of water [82]. In the as-deposited state, referring to a minimized air-exposure time of approximately 15 min, the peak positions coincide well with literature data for cubic HfV 2 [78].…”

Temperature and Impurity Induced Stabilization of Cubic HfV2 Laves Phase

“…Deviations to the experimentally obtained formation range of cubic HfV 2 (T ≥ 112 K) may originate from other factors influencing the low-temperature stability. A change in physical properties with time was reported for HfV 2 when exposed to air, which is attributed to severe absorption of hydrogen into the cubic structure of HfV 2 [12,22,30]. Furthermore, due to the high affinity for oxygen of the elemental constituents [18][19][20][21], the effect of O and H impurities on the stability of cubic C15 HfV 2 was investigated.…”

“…In addition to the pronounced effect of temperature on the stability of cubic C15 HfV 2 , real intermetallic compounds may exhibit H and O impurities due to their availability in atmosphere, and hence, presence in most production processes, in combination with the high hydrogen storage capacity of HfV 2 [7][8][9][10] and the affinity for oxygen of the elemental constituents [18][19][20][21]. Indeed, intermetallic HfV 2 was found to dissociate H 2 O at ambient temperatures and subsequently absorb the atomic constituents [22]. In general, point defects may lead to structural stabilization as found for cubic VN, which is stabilized by vacancies [23], as well as for cubic ZrO 2 , which can be stabilized by impurities [24].…”