The various phases of the system Sr_1-xCa_xTiO₃ - a Raman scattering study

Abstract

This paper deals with the structural Raman-active and the disorder-induced TO and LO Raman modes in Sr_1-xCaxTiO₃ (SCT) ceramics. Spectra were recorded at ambient pressure in a wide range of temperatures from 8 K up to ≈500 K. The chemical composition (x) was chosen so as to investigate the phase boundaries which have been previously determined in the x-T phase diagram. An eigenvector analysis is given for all the known phases. The Raman data confirm the occurrence of a double-tilt system of Imma space group lying between the tetragonal I4/mcm and the orthorhombic Pbnm structures for x ≥ 0.06. Higher-order transformation of the antiferroelectric phase (0.20 ≤ x ≤ 0.40) at low temperature is also suggested. The main interesting disorder-induced effects originate from the temperature behaviour of the hard polar modes TO₂ and TO₄, which appear in the Raman spectra of the paraelectric domain. This behaviour allows one to discuss the precursor range of ferroelectricity in the SCT system in terms of ferroelectric- or antiferroelectric-type dipolar interactions.