Displacive Phase Transitions in and Strain Analysis of Fe-Doped CaTiO3 Perovskites at High Temperatures by Neutron Diffraction

Abstract

The influence of small amounts of Fe3+ on the phase transitions of CaTiO3 perovskite has been studied by means of in situ high-temperature neutron diffraction. The same sequence of phase transitions as observed in CaTiO3 is shown by both CaTi0.9Fe0.1O2.95 and CaTi0.8Fe0.2O2.90 perovskites: from orthorhombic Pnma symmetry at room temperature (RT) to cubic Pm3m 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 CaFexTi1−xO3−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 CaTiO3, is ruled out in the Fe-doped CaTiO3 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 Pm3m→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.