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Ab initio thermodynamics of fcc H-Zr and formation of hydrides.

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Title: Ab initio thermodynamics of fcc H-Zr and formation of hydrides.
Authors: Besson, R.1, Candela, R.1
Source: Computational Materials Science. Mar2016, Vol. 114, p254-263. 10p.
Subjects: Ab initio quantum chemistry methods, Face centered cubic structure, Zirconium compounds, Hydrides, Chemical stability, Free energy (Thermodynamics), Low temperatures, Phase diagrams
Abstract: 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 γ-ZrH compound: (i) at low temperature, γ is metastable with respect to α-Zr + ε-ZrH 2− x , (ii) there exists a temperature domain around 500 K in which γ becomes stable, in agreement with recent experiments, (iii) contrasting with the ill-defined off-stoichiometry of the stable δ-ZrH y ( y ∼ 1.5) and ε-ZrH 2− x hydrides, γ is a line-compound undergoing a γ → δ order/disorder transition at moderate temperature (∼800 K), (iv) the γ 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 δ 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 δ + ε 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 γ. Our work therefore indicates that including phonons into CEs should be a promising direction for high-quality modelling of Zr hydrides. [ABSTRACT FROM AUTHOR]
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Abstract: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 γ-ZrH compound: (i) at low temperature, γ is metastable with respect to α-Zr + ε-ZrH 2− x , (ii) there exists a temperature domain around 500 K in which γ becomes stable, in agreement with recent experiments, (iii) contrasting with the ill-defined off-stoichiometry of the stable δ-ZrH y ( y ∼ 1.5) and ε-ZrH 2− x hydrides, γ is a line-compound undergoing a γ → δ order/disorder transition at moderate temperature (∼800 K), (iv) the γ 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 δ 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 δ + ε 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 γ. Our work therefore indicates that including phonons into CEs should be a promising direction for high-quality modelling of Zr hydrides. [ABSTRACT FROM AUTHOR]
ISSN:09270256
DOI:10.1016/j.commatsci.2015.12.043