Antiferroelectric phase transition in (Sr1-xCax)TiO3:II. X-ray diffraction studies

Abstract

We present here a detailed Rietveld analysis of the structure of paraelectricand antiferroelectric phases of Sr0.70Ca0.30TiO3using powder XRD data. It is shown that the observed antiferroelectricsuperlattice reflections can be explained by doubling of the Co parameterof the orthorhombic paraelectric phase. The space groups of the paraelectric(PE) and antiferroelectric (AFE) phases are shown to be Ibnm and Pbcmrespectively. The ionic displacements in the antiferroelectric phase revealthat the Ti4+and Sr2+/Ca2+ ions are displacedin the + [100] direction in a block of three layers followed by theirdisplacements in the -[100] direction in the next block of three layers,leading to the doubling of the cell parameter in the [001] direction. Theparaelectric to antiferroelectric phase transition in (Sr0.70Ca0.30)TiO3 and (Sr0.75Ca0.25)TiO3 isshown to be of first order type since (i) the AFE and PE phases coexist aroundthe phase transition temperature and (ii) the lattice parameters changediscontinuously at the transition temperature.