High-temperature heat capacity and thermal expansion of SrTiO3 and SrZrO3 perovskites

The heat capacity and thermal expansion of SrZrO3 and SrTiO3 have been determined by drop calorimetry and energy-dispersive x-ray diffraction from 300 up to 1800 K or more. For both compounds, thermal history has a slight influence on relative enthalpies in the range 900-1400 K but a satisfactory precision was obtained through systematic quenches from 1820 K before the calorimetric measurements. Strontium titanate is cubic at room temperature. Up to 1800 K, its heat capacity increases smoothly and its thermal-expansion coefficient remains almost independent of temperature with a value of 3.23(2) 10(-5) K-1. At room temperature, strontium zirconate is orthorhombic (space group Pbnm). The phase transitions from orthorhombic Pbnm to orthorhombic Cmcm at 995 K and then to tetragonal I4/mcm at 1105 K are revealed by symmetric, lambda-type variations of the heat capacity. A more diffuse thermal effect characterizes the I4/mcm to cubic Pm3m transition at 1440 K, above which the cubic phase shows an apparently low heat capacity at the highest temperatures. With the resolution of the x-ray technique, only the transition from orthorhombic Pbnm to orthorhombic Cmcm at 970 K was detected with a small volume change of 0.14%. Thermal expansion below 970 K is constant at 2.98(2) 10(-5) K-1. At higher temperatures, the apparent thermal expansion decreases smoothly from 2.78(2) to 2.43(2) 10(-5) K-1 between 1000 and 1800 K. With recent data on CaTiO3, the effects of (Ca,Sr) and (Ti,Zr) substitutions on thermal expansion and on the energetics of the phase transitions are discussed.