Orthorhombic to trigonal phase transition of perovskite-type (Ndx,Sm1-x)AlO3

Phase transition of solid solution phases of (Nd-x,Sm1-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,Sm1-x)AlO3 with the relation of T-c(degrees 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,Sm1-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+. (C) 1998 Elsevier Science S.A.