Studies on phase transitions of rare earth tantalates (Sm1-xLnx)3TaO7 (Ln = Nd, Eu) with fluorite-related structure

The phase transition of Sm3TaO7 with orthorhombic fluorite-related structure was investigated. Solid solutions (Sm1-xNdx)(3)TaO7 and (Sm1-xEux)(3)TaO7 were prepared, and their structures at room temperature are well described with the space group C222(1). The phase transition temperature for (Sm1-xNdx)(3)TaO7 decreases from 1340 K with increasing Nd concentration, while that for (Sm1-xEux)(3)TaO7 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-x)Ln(x))(3)TaO7 (Ln = Nd, Eu) increases. This trend is the same as that observed for Ln(3)MO(7) (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-x)Ln(x))(3)TaO7 (Ln = Nd, Eu) are well described with space group Pnma. (C) 2019 The Ceramic Society of Japan. All rights reserved.