Perovskites. Chemical chameleons

The range of perovskite type metal oxides and of compounds containing perovskitic structural fragments appears to be as variable as the colours of a chameleon. The combination of the metal cations not only determines the symmetry of the structure but also the physical properties and the thermochemical behaviour in inert, oxidising or reducing atmospheres. Moreover, substitution of the A or B cations increases the potential of modifying the properties of such compounds, including their ability to incorporate different structural elements, as demonstrated by the composite superconducting copper oxides. In many cases the oxygen stoichiometry differs from the ideal value. Quite often in the oxygen-deficient phases the oxygen vacancies are ordered and lead to superstructures. As the formal oxidation states of the metal cations and consequently the electronic and magnetic structures depend directly on the oxygen stoichiometry, the related physical properties can be influenced by reduction or re-oxidation treatments.