Studies on phase transitions of rare earth tantalates (Sm11xLnx)3TaO7 (Ln = Nd, Eu) with fluorite-related structure

Abstract

The phase transition of Sm3TaO7 with orthorhombic fluorite-related structure was investigated. Solid solutions (Sm11xNdx)3TaO7 and (Sm11xEux)3TaO7 were prepared, and their structures at room temperature are well described with the space group C2221. The phase transition temperature for (Sm11xNdx)3TaO7 decreases from 1340 K with increasing Nd concentration, while that for (Sm11xEux)3TaO7 increases with Eu concentration. However, both of these solid solutions show the same trend, i.e., with decreasing average rare earth radii, the phase transition temperature of (Sm11xLnx)3TaO7 (Ln = Nd, Eu) increases. This trend is the same as that observed for Ln3MO7 (M = Mo, Ru, Re, Os, or Ir). That is, the phase transition occurs with lattice contraction. Above the transition temperature, crystal structures for (Sm11xLnx)3TaO7 (Ln = Nd, Eu) are well described with space group Pnma.