Off-center displacements of Ti ions in oxide ferroelectrics and a gigantic photo-induced dielectric constant of quantum paraelectric perovskite oxides in the electron-lattice theory

Abstract

In this work we investigate the coupling of the F1u vibrations with the actual electronic states of BO6n− cluster in ABO3 ferrorelectric-oxides. This coupling leads to the dynamical covalency hybridization of B(Ti,Ta,Nb) and oxygen electronic states. It is shown that at fulfilment of definite criteria the free energy at T = 0, the adiabatic potential of BO6n− cluster have the following configurations: (1) one maximum at x0 = y0 = z0 = 0 (ferroelectric instability); (2) eight minima in the points |x0| = |y0| = |z0| = y0; (3) twelve saddle points at |p| = |q| ≠ 0, r = 0 (p, q, r = x, y, z) with a maximum in the r cross-section and minima along p and q. We show that the photo-induced changes of local ferroelectric distortions can take place. A gigantic enhancement of the dielectric constant by UV-light illumination is calculated in the electron-lattice theory in quantum paraelectrics of perovskite oxides, such as SrTiO3 and KTaO3, under a weak DC electric field. The temperature dependence of the gigantic real part of the dielectric constant ϵUVDC of SrTi16O3 under both UV-light and DC electric fields is calculated in satisfactory agreement with the experiment.