Perovskite: A Solid-State Chemistry Chameleon, Illustrating the Elements, Their Properties and Location in the Periodic Table

Abstract

Oxides with the perovskite structure are able to accommodate cations of almost all elements of the periodic table in sites that ideally are either octahedral or 12-coordinate. This allows many observations of periodic trends in coordination environment and structural distortions that depend on ion size and electronic configuration. Consideration of radius ratio effects provides a useful starting point to understand relations between ion size and crystal structure and, for instance, to rationalise the effect of applied pressure on the polymorphism of silicate structures. Physical properties such as polarizability and permittivity depend greatly on the proximity in energy space of different crystal structures that have similar free energies and thermodynamic stability; this can lead to novel emergent phenomena and properties that are not shown by either crystal structure in isolation. The very high permittivity of barium titanate, BaTiO₃, arises at the crossover between the undistorted cubic perovskite structure and a closely related tetragonally distorted perovskite structure. The high permittivity is associated with a small displacement of Ti from the centre of an octahedral site that is slightly too large and depends on whether such displacements are randomised at the transition between cubic and tetragonal structures or are correlated in the low-temperature tetragonal structure. The ferroic properties of piezo-, pyro- and ferroelectricity, and their many associated applications, depend on such displacements, how they are modified in response to the external variables of pressure, temperature and electric field and how they may be fine-tuned by compositional changes or doping. These, and many other, correlations between structure, composition and properties are discussed in this overview of solid-state inorganic chemistry and illustrate the central role of the periodic table in enabling chemists to both rationalise and predict the properties of oxides.