VIBRONIC CHARGE-TRANSFER EXCITONS - POSSIBLE NATURE OF THE UNUSUAL PROPERTIES OF VIRTUAL PEROVSKITELIKE FERROELECTRICS

A vibronic charge-transfer exciton, which is a pair of Jahn-Teller ele ctron and hole polarons, is considered as a possible cause of the appe arance of the Muller phase in the virtual ferroelectric SrTiO3 and the ''green'' luminescence in the virtual ferroelectric KTaO3. The two '' green'' luminescence bands can be associated with emission from two st ates of a typical intrinsic defect, viz., a vibronic charge-transfer e xciton trapped by an oxygen vacancy and an isolated vibronic charge-tr ansfer exciton. In both cases the ''green'' luminescence corresponds t o the recombination of the electron and the hole in the vibronic charg e-transfer exciton, which is accompanied by the emission of light. The properties of the Muller phase can be attributed to mixing of the nor mal state and states of the vibronic charge-transfer exciton phase whe n they interact with polarization in the soft SrTiO3 matrix under the conditions of a pseudo-Jahn-Teller (pseudo-JT) effect on a soft TO mod e of the displacement type. In this case the vibronic charge-transfer exciton phase forming the low-lying excited states has ''order-disorde r'' degrees of freedom and exists at temperatures significantly below the point of the order-disorder ferroelectric transition in SrTiO3 at T=T-Q approximate to 37 K. The corresponding lowering of the symmetry of the vibronic charge-transfer exciton phase to polar symmetry leads to the possibility of a long-period incommensurate phase in such excit ed states, which arises as a result of the appearance of a Lifshitz in variant. The valence-band state making the largest contribution of the pseudo-JT effect corresponds to a wave vector equal to the critical w ave vector of the incommensurate vibronic charge-transfer exciton phas e. When the temperature is lowered, the pseudo-JT distortion increases down to similar to T-Q and subsequently saturates in accordance with the saturation of the dielectric constant. The basic assumption in the model is that the temperature T=T-Q corresponds to the narrow tempera ture range for the transition from an intermediate to a strong pseudo- JT effect under the conditions for the realization of polarization tun neling states. The appearance of a significant admixture of states of the modulated ferroelectric vibronic charge-transfer exciton phase to the ground state under the conditions for the realization of polarizat ion tunneling states at low temperatures provides an explanation for t he principal properties of the Muller phase. (C) 1998 American Institu te of Physics.