Phase transitions of Sm3NbO7, (Sm1-xLnx)3NbO7 (Ln = Nd, Eu) and Sm3TaO7 with fluorite-related structure

The phase transition of Sm3NbO7 with orthorhombic fluorite-related structure was investigated. Three kinds of solid solutions, Sm-3(Nb1-xTax)O-7, (Sm1-xNdx)(3)NbO7, and (Sm1-xEux)(3)NbO7 were prepared. Sm3NbO7 and Sm3TaO7 form complete solid solutions, and their structures are well described with the space group C222(1). The phase transition temperature for Sm-3(Nb1-xTax)O-7 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 (Sm1-xNdx)(3)NbO7 decreases with Nd concentration. On the other hand, that for (Sm1-xEux)(3)NbO7 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-x)Ln(x))(3)NbO7 (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. (C) 2018 The Ceramic Society of Japan. All rights reserved.