CHARGE-TRANSFER VIBRONIC EXCITONS AS AN ORIGIN OF A MULLER PHASE AND OF POLAR MICRODOMAINS IN INCIPIENT PEROVSKITE-LIKE FERROELECTRICS

The unified mechanism of a new quantum paraelectric regime (Muller pha se) as well as of polar microdomain forming in incipient perovskite-li ke ferroelectrics is considered. Muller phase (MP) is formed by charge -transfer vibronic excitons (CTVE) with intrinsic elastic and electric -dipole moments. CTVE are created due to pair Jahn-Teller effect on th e soft lattice mode. The CTVE under consideration form order-disorder clusters with co-operative lowering of energy due to charge-transfer o rdering. Such CTVE clusters are characterized by long life time and ca n be considered as proper defects. The transition to MP (T = T-Q appro ximate to 37 K in SrTiO3) corresponds to a local ferroelectric-type tr ansition in such CTVE-clusters. This local ferroelectric transition is induced by the interaction between the lattice soft TO-mode and the s oft polarization mode of CTVE cluster. This is the mechanism of polar microdomain appearing. Simultaneously the model gives a new explanatio n of soft TO-mode stabilization as a result of repulsion from local po larization modes of the CTVE clusters. The theoretical model is in agr eement with experimental results.