Studies on phase transitions of rare earth tantalates (Sm_1−xLn_x)₃TaO₇ (Ln = Nd, Eu) with fluorite-related structure

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The phase transition of Sm₃TaO₇ with orthorhombic fluorite-related structure was investigated. Solid solutions (Sm_1−xNd_x)₃TaO₇ and (Sm_1−xEu_x)₃TaO₇ were prepared, and their structures at room temperature are well described with the space group C222₁. The phase transition temperature for (Sm_1−xNd_x)₃TaO₇ decreases from 1340 K with increasing Nd concentration, while that for (Sm_1−xEu_x)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 (Sm_1−xLn_x)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 (Sm_1−xLn_x)₃TaO₇ (Ln = Nd, Eu) are well described with space group Pnma.