Phase transitions of Sm₃NbO₇, (Sm₁₋<i>_xLn_x</i>)₃NbO₇ (<i>Ln</i> = Nd, Eu) and Sm₃TaO₇ with fluorite-related structure

Abstract

The phase transition of Sm 3 NbO 7 with orthorhombic fluorite-related structure was investigated. Three kinds of solid solutions, Sm 3 (Nb 1¹x Ta x)O 7 , (Sm 1¹x Nd x) 3 NbO 7 , and (Sm 1¹x Eu x) 3 NbO 7 were prepared. Sm 3 NbO 7 and Sm 3 TaO 7 form complete solid solutions, and their structures are well described with the space group C222 1. The phase transition temperature for Sm 3 (Nb 1¹x Ta x)O 7 increases from 1080 K with increasing Ta concentration. Sm 3 TaO 7 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 1¹x Nd x) 3 NbO 7 decreases with Nd concentration. On the other hand, that for (Sm 1¹x Eu x) 3 NbO 7 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 1¹x Ln x) 3 NbO 7 (Ln = Nd, Eu) increases. This trend is the same as that for Ln 3 MO 7 (M = Mo, Ru, Re, Os, or Ir). That is, the phase transition occurs with lattice contraction.