Phase transitions of Sm₃NbO₇, (Sm_1−xLn_x)₃NbO₇ (Ln = Nd, Eu) and Sm₃TaO₇ with fluorite-related structure

Abstract

The phase transition of Sm₃NbO₇ with orthorhombic fluorite-related structure was investigated. Three kinds of solid solutions, Sm₃(Nb_1−xTa_x)O₇, (Sm_1−xNd_x)₃NbO₇, and (Sm_1−xEu_x)₃NbO₇ were prepared. Sm₃NbO₇ and Sm₃TaO₇ form complete solid solutions, and their structures are well described with the space group C222₁. The phase transition temperature for Sm₃(Nb_1−xTa_x)O₇ increases from 1080 K with increasing Ta concentration. Sm₃TaO₇ undergoes the phase transition when the temperature is increased through ca. 1340 K and above the transition temperature, its structure is well described with space group Pnma. The phase transition temperature for (Sm_1−xNd_x)₃NbO₇ decreases with Nd concentration. On the other hand, that for (Sm_1−xEu_x)₃NbO₇ increases with the ratio of Eu concentration. However, both of these solid solutions show the same trend, i.e., with decreasing the average rare earth size, the phase transition temperature of (Sm_1−xLn_x)₃NbO₇ (Ln = Nd, Eu) increases. This trend is the same as that for Ln₃MO₇ (M = Mo, Ru, Re, Os, or Ir). That is, the phase transition occurs with lattice contraction.