1ST-PRINCIPLES STUDY OF HYDROGEN ORDERING IN BETA-YH2+X

The phase stability is studied for the beta-phase YH2+x system based o n first-principles total energy calculations. Our study predicts that the D0(22), ''40'', and D1a structures are stable near x = 0. 25, 0.5, and 0.8, respectively. Using the effective cluster interactions obtai ned from the first-principles total-energy data, the phase diagram for the D0(22) and ''40'' ordered phases is calculated by the cluster var iational method. The calculated order-disorder transition temperature at x = 0.1 for the D0(22) structure is around 280 K, which is consiste nt with the recent observation of the metal-semiconductor transition n ear 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)].