‘A’ cation control of perovskite properties

The materials properties of many ABO₃ perovskites result primarily from the B cations, but are tuned by the cation(s) at the A sites. Notable examples are ferroelectric titanates, magnetoresistive manganites, and layered A₂BO₄ type superconducting cuprates. The A cation control of these properties can be described through a simple ionic approach by considering the sizes and charges of the A cations. The size control is approximated well by the mean A cation radius, equivalent to the traditional perovskite tolerance factor, and the size variance which describes the mismatch in ionic radii. Studies in recent years have shown that the latter term is extremely significant; ferroelectric, ferromagnetic, superconducting and structural transition temperatures all vary linearly with the size variance. A variance vs. size plot defines the ‘chemical window’ which enables property variations to be displayed usefully. The variance control of phase separation phenomena in manganite perovskites has also been demonstrated. In contrast, the A cation charge variance is shown to be unimportant in manganites and cuprates.