Low temperature X-ray diffraction study of the phase transitions in Sr_1-xCa_xTiO₃ (x=0.02,0.04): evidence for ferrielectric ordering

Abstract

X-ray diffraction studies on Sr_1-x Ca_x TiO₃ in the temperature range 12 to 300 K reveal cubic to tetragonal (I4/mcm) antiferrodistortive and tetragonal to orthorhombic (Ic2m) polar phase transitions at ~170 and 225, 32, 34 K, respectively, for x=0.02 and 0.04. It is shown that the polarization in the orthorhombic phase is along the <110> pseudocubic direction, in agreement with the experimentally reported easy polarization direction by Bednorz and Muller. However, in disagreement with them the moments are found to be ferrielectrically coupled in the neighboring (001) planes that is responsible for the anomalously low values of the spontaneous polarization in the polar phase.