Displacive phase transitions and spontaneous strains in oxygen deficient CaFexTi1-xO3-x/2 perovskites (0≤x≤0.40)

The symmetry of CaFexTi1-xO3-x/2 perovskites (0 less than or equal to n less than or equal to 0.40) has been studied by means of room temperature and high temperature x-ray powder diffraction. At room temperature the perovskites undergo at least two displacive phase transitions with increasing Fe content: from the CaTiO3 Pbnm structure to the tetragonal I4/mcm polymorph at x = 0.205 +/- 0.017 followed by transformation to the cubic Pm3m structure at I = 0.251 +/- 0.029. A strain analysis of the unit cell demonstrates that the orthorhombic to tetragonal transition is first order in character while the tetragonal to cubic one is second order. An in situ high temperature XRD study on compositions with n = 0.109 and x = 0.188 shows that both samples undergo the same set of phase transitions at high temperature as found in the CaFexTi1-xO3-x/2 samples with increasing Fe content at room temperature. The transition temperatures indicate that the phase boundaries in the temperature-composition phase diagram may be non-linear.