A combined X-ray diffraction and Raman scattering study of the phase transitions in Sr 1−xCa xTiO 3 ( 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− x Ca x TiO 3 (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 a 0 b - b - tilt system and not I 4 / mcm with a 0 a 0 c - tilt system, as assumed by earlier workers. The lowering of the crystal symmetry from I 4 / 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 E g 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 TO 2 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 I 4 / mcm and Imma space groups. The implications of the change in the tilt system from a 0 a 0 c - to a 0 b - b - on the development of the polar order is also discussed.