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摘要
In order to elucidate the currently debated stability of zirconium hydrides, the energetics of fcc H-Zr was investigated by means of ab initio-based cluster expansions (CEs) including the effect of long-range interactions, the latter being possibly important due to the interstitial H site occupancy in fcc-Zr. Allowing a detailed temperature-and composition-dependent evaluation of the configuration part of free energies in fcc H-Zr, our modelling provides new insights into the controversial properties of the gamma-ZrH compound: (i) at low temperature, gamma is metastable with respect to alpha-Zr + epsilon-ZrH2-x, (ii) there exists a temperature domain around 500 K in which gamma becomes stable, in agreement with recent experiments, (iii) contrasting with the ill-defined off-stoichiometry of the stable delta-ZrHy (y similar to 1.5) and epsilon-ZrH2-x hydrides, gamma is a line-compound undergoing a gamma -> delta order/disorder transition at moderate temperature (similar to 800 K), (iv) the gamma composition domain corresponds to a strong failure of the usual random approximation for the configurational entropy. Relying on a large set of ab initio data, our results also reveal an intricate energetic behaviour of fcc H-Zr around the delta domain. This complex behaviour could not be not reliably captured by CE modelling, which implies that, contrary to experiments, our proposed theoretical H-Zr phase diagram contains no delta + epsilon domain. Moreover, the coupling between phonons and chemical order, much more significant at ZrH composition than for higher H contents, may be responsible for a lowering of the order-disorder transition temperature of gamma. Our work therefore indicates that including phonons into CEs should be a promising direction for high-quality modelling of Zr hydrides.
- 出版日期2016-3
