Inhomogeneous instabilities in an illuminated crystal of doped BaTiO 3

Abstract

Homogeneous laser irradiation is performed on BaTiO₃: Fe³⁺, Co²⁺, Cu²⁺; the analysis of the data is done on the basis of a true Jahn-Teller effect at 3d ions, and of a pseudo J.T.E. at the K centres, introduced for compensation. We present results concerning the photoinduced phase transitions and photoferroelectric effect in crystals with an electron phonon coupling; two different Jahn-Tellei effects are introduced. First the pseudo J.T.E. (P.J.T.E.) where the coupling between electrons and phonons occur at different points of the reciprocal space. Second, the cooperative J.T.E. through localized centres where the electron-phonon coupling is restricted to particular lattice sites. We consider the problem in the ferroelectric perovskite BaTiO₃; the conduction process is determined by the lowest conduction bands for the direction of interest, these are Δ′₂ and Z ₄ for the longitudinal (L) and transversal (T) cases respectively. The optical transitions are determined by the critical unes Γ₁₅-Δ₅-χ′₅ and Γ₂₅-Δ′₂-χ₃.