Subtle Structural Distortions In Some Dielectric perovskites

Abstract

Dielectric perovskites exhibit a range of interesting phenomenon such as ferroelectricity, piezoelectricity, pyroelectricity, which have important technological implications. These materials show strong structure–property correlations. Majority of the perovskites exhibit crystal structures, which can be described in terms of distortions of the cubic prototype. In some cases the distortions are extremely weak and impossible to detect by x-ray diffraction (even at synchrotron sources). In such cases, use of complementary techniques, such as neutron powder diffraction and electron microdiffraction, are mandatory to reveal them. This paper reviews the work we have carried out over the past one decade on some dielectric and ferroelectric perovskites, with special emphasis on the subtle aspects of the structural distortions. Examples are taken from systems such as SrTiO₃–CaTiO₃, PbZrO₃–PbTiO₃, Na_0.5Ln_0.5TiO₃ (Ln = La, Pr and Nd), Na_0.5Nd_0.5TiO₃–SrTiO₃ and Pr-doped SrTiO₃. The first four cases have been chosen to highlight the global symmetry breaking due to the subtle distortions, some of them of very uncommon type. The last example emphasizes how analysis of the anomalous value of lattice parameter obtained by Rietveld refinement of the cubic structure is linked with the development of ferroelectric polar nano regions in the system.