A combined X-ray diffraction and Raman scattering study of the phase transitions in Sr_1-xCa_xTiO₃ (x=0.04, 0.06, and 0.12)

Abstract

Results of powder X-ray diffraction and Raman scattering studies on the phase transitions in Sr_1-xCa_xTiO₃ (SCT) are presented for x=0.04, 0.06 and 0.12 in the temperature range 8-473 K. It is proposed that the space group of SCT in the composition range 0.06≤x≤0.35 is Imma with a⁰b^-b^- tilt system and not I4/mcm with a⁰a⁰c^- tilt system, as assumed by earlier workers. The lowering of the crystal symmetry from I4/mcm to Imma is supported by the observation of additional Raman lines, in agreement with the factor group analysis for the Imma space group. The structural Eg mode, characteristics of the non-cubic phase, is shown to be present even in the cubic phases of x=0.06 and x=0.12 but not of x=0.04 indicating the change in the local structure of the cubic phase of SCT for x≥ 0.06. The presence of symmetry forbidden TO₂ mode in the Raman spectra of the cubic phase of SCT for x<0.06 and its absence for x≥0.06 provides yet another characteristic feature distinguishing the I4/mcm and Imma space groups. The implications of the change in the tilt system from a⁰a⁰c^- to a⁰b^-b^- on the development of the polar order is also discussed.