A thermodynamic analysis of the V-H binary system has been performed by combining first-principles calculations with the CALPHAD approach. In order to represent the order-disorder transition between the monoclinic 1 and body-centered tetragonal 2 phases arising from the ordering of H atoms located at the octahedral interstitial sites, a (V) 1=2 (H,Va) 1=4 (H,Va) 1=4 -type three-sublattice model was applied. The formation energies of the -VH 2 hydride phase and solid solution phases were calculated in the ground state using the Full-potential Linearized Augmented Plane Wave method, and the estimated values were introduced into a CALPHAD-type thermodynamic analysis, along with some experimental information on the phase boundaries and the hydrogen isotherms. The calculated phase diagram and hydrogen isotherms were in good agreement with these experimental results.
A thermodynamic analysis of the Ti-Zr-H ternary system has been performed by combining
first-principles calculations with the CALPHAD approach. To enable the thermodynamic description
of the binary systems, the results from our previous evaluation were adopted for the Ti-H, Zr-H, and
Ti-Zr systems. The ternary compound, Ti2ZrH4, with an Fd3m-type crystal structure, exists over a
wide composition range, and the (Ti)2(Zr)1(H,Va)4-type three-sublattice model was applied to
describe its thermodynamic properties. Because of the lack of experimental information available, the
enthalpy of formation of the Ti2ZrH4 phase was evaluated using the Full Potential Linearized
Augmented Plane Wave method, and the estimated values were introduced into a CALPHAD-type
thermodynamic analysis with some other experimental information. The calculated phase diagrams
and the hydrogen isotherms were in good accordance with previous experimental results. Our
calculations revealed that the ternary compound decomposes into a bcc and gas phase in the vicinity
of 1270 K.
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