Orthorhombic to trigonal phase transition of perovskite-type (Nd_x,Sm_{1 - X})AlO₃

Phase transition of solid solution phases of (Nd_x,Sm_{1 - x})AlO₃ was investigated by powder X-ray diffraction technique. The transition temperature T_c from orthorhombic to trigonal structure is linearly related to x in (Nd_x,Sm_{1 - x})AlO₃ with the relation of T_c(°C) = -1043.4x+785.8 as an approximation at ambient pressure. This transition is reversible against the change of temperature. At room temperature, the structural change from orthorhombic to trigonal system takes place at around x=0.73 when x varies from 0.0 to 1.0 in (Nd_x,Sm_{1 - x})AlO₃. Thus, the average ionic radius of R³⁺ plays an important role to decide the structure of RAlO₃, and an appropriate selection of ionic radius r_R for R will function so as to control temperature and/or pressure for the structural change. As a result, structural diagram of RAlO₃ given by temperature condition and atomic number of R, which implies a phase diagram under pressure and temperature conditions, was proposed. Change of molar volume of a series of RAlO₃ was also discussed based on both effects of temperature and ionic radius R³⁺.