Vibronic Frenkel exciton in a soft-mode crystal

The pair electron-hole Jahn-Teller effect, which is realized on degenerate states of a vibronic Frenkel exciton (VFE) in soft-mode crystals, is considered. It is shown that the vibronic effect in a VFE can be significantly enhanced in crystals having soft lattice modes. The occurrence of strong local order-disorder fluctuations in this situation is predicted. The manifestations of VFEs localized at impurities in soft matrices (Cr³⁺ in SrTiO₃, Fe⁵⁺ in KTaO₃, and Mn⁵⁺ in TbVO₄ and TbPO₄) are considered. The possibility of fluctuation effects related to VFEs in nominally pure soft-mode crystals (SrTiO₃ and BaTiO₃) is discussed.