Crystal structure-physical property relationships in perovskites

The crystal structures of seven compounds with the perovskite structure are analysed in terms of their cation coordination polyhedra. The analysis shows that the occurrence of ferroelectricity in a perovskite of composition ABO3 requires the ratio of cuboctahedral A-ion volume to octahedral B-ion volume to be exactly five, and that there is a minimum octahedral B-ion volume associated with ferroelectric properties. The analysis also suggests that each ion may be assigned a characteristic range of polyhedral volumes which is transferable to other crystal structures. The methodology is applied to two perovskite ceramic systems, lead zirconate titanate (PZT) and lead zinc niobate (PZN). The phase diagram of the PZT system is rationalized in terms of the coordination polyhedra of the lead, zirconium and titanium ions, and a critical lead cuboctahedral volume is defined at the morphotropic phase boundary. The effect of the lanthanum ion in stabilizing the tetragonal phase to a higher Zr/Ti ratio and in reducing the Curie temperature is also discussed. In the PZN system the polyhedral analysis permits a rationalization of the behaviour of seven different additives in inhibiting the formation of a pyrochlore phase, Pb3Nb4O13, during the fabrication of PZN ceramic. The potential of the technique as a predictive structural tool is also assessed. (orig.)