High temperature phase transitions of CaTiO3 and (Ca0.85Nd0.15)TiO3 by X-ray diffractometry and differential thermal analysis

The two phase transitions of CaTiO3 and (Ca0.85Nd0.15)TiO3 were studied by high temperature X-ray diffraction and differential thermal analysis in the temperature range 300-1723 K. The presence of the two kinds of phase transitions in CaTiO3 and (Ca0.85Nd0.15)TiO3 at about 1390 K and about 1530 K recently found by the heat capacity measurement in our laboratory was confirmed. From the X-ray diffraction patterns of CaTiO3 and (Ca0.85Nd0.15)TiO3, the first phase transition at lower temperature was due to the change from the orthorhombic Pbnm to the orthorhombic Cmcm structure, and the second one at higher temperature was due to change from Cmcm to the cubic Pm3m structure, originating from the sequential rotations of the TiO6 octahedra about one of the axes. Since no clear peak was seen at the phase transition around 1390 K in the DTA curve in contrast with the presence of the endothermic peak at the transition around 1530 K, it is thermodynamically considered that the former and the latter transition were second- and first-order phase transition, respectively. (orig.)