Displacive Phase Transitions in and Strain Analysis of Fe-Doped CaTiO 3 Perovskites at High Temperatures by Neutron Diffraction

Abstract

The influence of small amounts of Fe 3+ on the phase transitions of CaTiO 3 perovskite has been studied by means of in situ high-temperature neutron diffraction. The same sequence of phase transitions as observed in CaTiO 3 is shown by both CaTi 0.9Fe 0.1O 2.95 and CaTi 0.8Fe 0.2O 2.90 perovskites: from orthorhombic Pnma symmetry at room temperature (RT) to cubic Pm3 m at high temperature, with an intermediate I4/ mcm tetragonal phase which exists over a temperature range of about 100°C. The two phase boundaries in the temperature vs composition phase diagram of the system CaFe x Ti 1− x O 3− x/2 (0≤ x≤0.4) decrease in a quasi-linear manner with increasing Fe content up to x=0.2 and then they both drop abruptly to RT. The existence of a second orthorhombic phase ( Cmcm), which has been postulated for CaTiO 3, is ruled out in the Fe-doped CaTiO 3 perovskites in view of the behavior of specific diffraction peaks. Strain analysis shows first-order thermodynamic character for the Pnma→ I4/ mcm transition, while the character of the Pm3 m→ I4/ mcm transition could be second order or tricritical. Shear strains behave more or less classically, as described by order parameter coupling and shear strain/order parameter coupling models. The volume strain has an anomalous coupling with the order parameter components, which appears to be temperature-dependent.