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 studie d by high temperature X-ray diffraction and differential thermal analy sis 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 o ur laboratory was confirmed. From the X-ray diffraction patterns of Ca TiO3 and (Ca0.85Nd0.15)TiO3, the first phase transition at lower tempe rature was due to the change from the orthorhombic Pbnm to the orthorh ombic 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. Sin ce 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 th e transition around 1530 K, it is thermodynamically considered that th e former and the latter transition were secund-and first-order phase t ransition,respectively. (C) 1997 Elsevier Science B.V.