First principles studies of the structural stability and lattice dynamics for rare earth hydridies REHx (RE = Sm, Gd, Tb, Dy, Ho, Tm, x = 2, 3) under pressure

Abstract

In the paper, the structures, pressure-induced structural transitions (from cubic to hexagonal), lattice dynamics and the possible transition mechanisms for rare earth dihydrides REH2 and trihydrides REH3 (RE = Sm, Gd, Tb, Dy, Ho, and Tm) are investigated comparatively and systematically using first-principles calculations. The transition pressures from the cubic CaF2-type structure to the hexagonal REB2-type structure for REH2 (RE = Sm, Gd, Tb, Dy, Ho, Tm) and the transition pressures from the cubic BiF3-type structure to the hexagonal LaF3-type structure for REH3 (RE = Sm, Gd, Tb, Dy, Tm) are predicted reasonably. It is concluded that the two kinds of structural phase transitions are not sensitive to RE 4f electrons. In the two kinds of structural phase transitions under pressure, we note that the ‘‘2.1 Å rule’’ (the purposed minimum HH distance for the stable metal hydirde at 0 K and 0 GPa) doesn't work. While lattice dynamics show that the dynamical stable pressure ranges for the cubic REH2 (RE = Sm Gd, Tb, Dy, Ho, Tm) are about in 0–52, 0–60, 0–70,0-87, 0–94 and 0–108 GPa, respectively. Ones of the cubic REH3 (RE = Sm, Gd, Tb, Dy, Tm) are around 17–147, 31–182, 13–200, 16–224 and 14–280 GPa, respectively. In contrast to the corresponding structural transition pressure as well as the lattice dynamics of the hexagonal structure under high pressure, the phonon modes softening behaviors under pressure might be responsible for the pressure-induced structural phase transition in the cubic REH2, but not in the cubic REH3.