Complex structural phase transitions in slightly Ca modified Na_0.5Bi_0.5TiO₃

Abstract

A temperature dependent neutron powder diffraction study, in conjunction with dielectric and ferroelectric characterization, of slightly Ca modified Na_0.5Bi_0.5TiO₃ (NBT) revealed an instability with regard to a non-polar orthorhombic (Pbnm) distortion above room temperature. This intermediate orthorhombic phase has earlier been reported for unmodified NBT by electron diffraction studies, but has never been captured by global (x-ray/neutron) diffraction techniques. Calcium substitution seems to amplify the magnitude of this intermediate orthorhombic distortion thereby making the corresponding superlattice reflections become visible in the neutron diffraction pattern. The study revealed the following sequence of very complex structural evolution with temperature: Cc → Cc + Pbnm → Pbnm + P4/mbm → P4/mbm → PmN3m.