A covalency model of ferroic phase transitions in perovskites

Oxygen perovskites show remarkable diversity in chemical composition. Ferroelectric and ferrodistortive effects are observed extraordinarily often. The chemical bonds represent an intermediate state between ionic (heteropolar) and covalent (homopolar) bond formation. The electronegativities defined by Pauling serve here as useful criterion. Predominantly ionic bonds form cubic perovskites; greater covalent bonding leads to element-typical coordination structures. The lattice undergoes deformation if the cubic lattice spacings determined by dense ion packing do not coincide in absolute value and bond angle with the covalent bonds. The order parameters of the accepted physical models of critical phase transitions, i.e., polarization, rotation and deformation, appear in this valency-chemical model as the result of relatively weak covalent bonds in the ionic lattice.