First-principles study of hydrogen ordering in β-YH2+x

The phase stability is studied for the β-phase ${\mathrm{YH}}_{2+\mathit{x}}$ system based on first-principles total energy calculations. Our study predicts that the ${\mathrm{D}0\mathrm{}}_{22}$, ‘‘40’’, and ${\mathrm{D}1\mathrm{}}_{\mathit{a}}$ structures are stable near x=0.25, 0.5, and 0.8, respectively. Using the effective cluster interactions obtained from the first-principles total-energy data, the phase diagram for the ${\mathrm{D}0\mathrm{}}_{22}$ and ‘‘40’’ ordered phases is calculated by the cluster variational method. The calculated order-disorder transition temperature at x=0.1 for the ${\mathrm{D}0\mathrm{}}_{22}$ structure is around 280 K, which is consistent with the recent observation of the metal-semiconductor transition near 230–280 K and resistivity anomalies near 200–250 K for the system with x near 0.1 [Daou and Vajda, Phys. Rev. B 45, 10 907 (1992)].