EFFECT OF Ca SUBSTITUTION ON THE ROOM TEMPERATURE STRUCTURE OF SrTiO₃ : A POWDER NEUTRON DIFFRACTION STUDY. Rajeev Ranjan and Dhananjai Pandey, School of Materials Science and Technology, Banaras Hindu University, Varanasi-221005, India.

Superlattice reflections observed for the first time in the room temperature powder neutron diffraction patterns of (Sr_1-xCa_x)TiO₃, x >0.12 are shown to arise due to an orthorhombic distortion of the perovskite structure. A Rietveld analysis of the neutron powder diffraction profiles shows that the structure consists of tilted TiO₆ octahedra and off-centred A-site cations. These novel structural features have important bearing on the quantum ferroelectric behavior of this system. Our work confirms the suggestion of Kleeman and Schremmer [Phys. Rev.B 40 (1989) 7428] that Ca induced electric dipoles responsible for quantum ferroelectricity in (Sr,Ca)TiO₃ results from off-centre occupancy of Ca along <110> of the perovskite unit cell. We show that the presence of tilted TiO₆ octahedra in the structure forces A-site cations to occupy off-centre positions for steric reasons. We do not find any evidence for the presence of Ca at the Ti site, postulated by Bednorz and Muller [Phys. Rev. Lett.52 (1984) 2289], as the source of Ca induced electric dipoles. Neutron scattering was ideally suited for settling this issue since the neutron scattering lengths of Ca and Ti are of opposite sign.