Local configuration instability as an origin of relaxor-type properties of ferroelectric solid solutions SBN, SCT and KLTN

A model of phase transitions (PT) in concentrated and diluted ferroelectric oxide solid solutions with iso-charged doping is considered on examples of SBN (SrxBa1-xNb2O6), x = 0.61 for congruent composition, of SCT (Sr1-xCaxTiO3) for x much less than1, and of KLTN (K1-xLixTa1-yNbyO3) with weak (Nb5+) and strong (Li+) off-center impurities for x much less than1, ymuch less than1 cases. This nano-cluster model with taking into account in-cluster local configuration instability (LCI) can explain uncommon relaxor-type properties of these systems as well as the origin of active polar clusters. It is shown that cooperative mechanism of cluster polarization formation due to co-operative LCI can be important for SBN and for SCT. The model also explains strong suppressing of ferroelectric PT by Cr-impurities in SBN. While the SBN and the SCT are characterised by LCI which is induced by the chemical fluctuation field, KLTN consists in Nb-rich nano-clusters obey Li-impurities as cluster cores. Multi-mode soft dynamics with local in-cluster piezoeffect is responsible for the origin of long-range electric dipole-dipole interaction as well as for co-existence of ferroelectric and glass-like states with reentrant glass phenomenon. The former is a key effect in the phenomenon of small concentration impurity influence on the PT in KLTN. This model explains also an appearance of a new PT in KLTN with strong hysteresis and cusp-type susceptibility.