Charge transfer vibronic excitons as an origin of a new quantum paraelectric regime (Müller phase) and of polar microdomains in incipient perovskite-like ferroelectrics

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Abstract
The unified mechanism of a new quantum paraelectric regime (Müller phase) as well as of polar microdomain forming in incipient perovskite-like ferroelectrics is considered. Müller phase (MP) is formed by charge-transfer vibronic excitons (CTVE) with intrinsic elastic and electric dipole moments. CTVE form order-disorder clusters with co-operative lowering of energy due to charge transfer ordering. Such CTVE clusters are characterized by long life time and can be considered as proper defects. The transition to MP (T = TQ ≈ 37 K in SrTiO3) corresponds to local ferroelectric-type transition in such CTVE-clusters. This local ferroelectric transition is induced by interaction between lattice soft TO-mode and soft polarization mode of CTVE cluster. This is the mechanism of polar microdomain appearing. Simultaneously the model gives a new explanation of soft TO-mode stabilization as a result of repulsion from local polarization modes of CTVE clusters. The coexistence of order-disorder and displacive type behaviour in incipient perovskite-type ferroelectrics also find the explanation. The latter is based on the existence of order-disorder degrees of freedom of CTVE-clusters. The relaxationtype fluctuations of dipole moments of CTVE and CTVE-clusters also interact with TO- and E g soft lattice modes and create the central peak phenomena. The theoretical model is in agreement with experimental results.