Effects of vibronic interactions in polar microdomain formation in incipient ferroelectric KTaO3: Comparison analysis of Raman scattering and Second-Harmonic Generation

A Raman Scattering (RS) investigation of incipient perovskite-like ferroelectric KTaO3 (nominally pure and with off-center Li+ impurities) as well as a comparison analysis with Second Harmonic Generation (SHG) data on the same systems and theoretical model consideration are performed. It is shown that polar microdomains (PMD) have at least two possible origins. Namely there an dipole states of proper defects such as clusters of charge-transfer vibronic excitons with long life time which can be responsible for a SHG effect increasing in reduced samples, and dipole states induced by Lie impurities (including the case of unwanted Li+ off-center impurities in nominally pure KTaO3). The vibronic model of clusters of charge-transfer vibronic excitons which have polar and non-polar states is developed.