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 les s 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 perovski tes undergo at least two displacive phase transitions with increasing Fe co ntent: 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 th at the orthorhombic to tetragonal transition is first order in character wh ile the tetragonal to cubic one is second order. An in situ high temperatur e XRD study on compositions with n = 0.109 and x = 0.188 shows that both sa mples undergo the same set of phase transitions at high temperature as foun d in the CaFexTi1-xO3-x/2 samples with increasing Fe content at room temper ature. The transition temperatures indicate that the phase boundaries in th e temperature-composition phase diagram may be non-linear.