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Origin of Ferroelectricity in Highly-Polar Oxides: Small Changes of Chemical Bonding Enhanced by Local Electric Field

The infrared reflection spectra of strontium titanate are fitted with a new dielectric function model which takes explicitely into account the couplings of the soft ferroelectric mode with the other polar modes. The origin of ferroelectricity in oxidic perovskites is understood as a consequence of a weak softening of the mode internal to the oxygen octahedra, induced by small changes of the chemical bonding of the pairs formed by oxygen and a transition metal ion, and considerably enhanced by energy level repulsion phenomena.