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.     

Structure of heterogeneous states and magnetic properties of CoPd nanocrystalline films

Thin films of a CoPd equiatomic alloy have been obtained by condensation at a high substrate temperature. In their initial states, they are characterized by a microdiffraction pattern in the form of a diffuse halo. This state is a polymorphic martensite-type transformation, which occurs for the martensite transition from the low-temperature hexagonal close-packed phase to the high-temperature face-centered cubic one. Crystalline metastable structures formed in films of this alloy have been identified, and their magnetic characteristics have been determined.     

The impurity ion off-center behavior induced by a localized vibronic Frenkel exciton in a soft-mode crystal

The 3d-4p Frenkel exciton localized at a Mn⁴⁺ impurity ion in BaTiO₃ is considered. The electron and hole of this exciton are correlated, degenerate, and involved in the pair Jahn-Teller effect. Such an exciton is characterized by the significant contribution of indirect electron-hole attraction through lattice (in particular, soft) vibrations to the coupling energy, which is comparable with the Coulomb attraction. Thus, the exciton under consideration is both Coulomb and vibronic excitation. The vibronic contribution increases when approaching the point of soft phonon condensation. It is shown that a vibronic Frenkel exciton allows the off-center behavior of an impurity ion, which is significantly enhanced when approaching the point of soft phonon condensation. As a result, the vibronic Frenkel exciton is a low-lying excitation.

     

Mechanisms of off-center ion behavior in oxygen-octahedral perovskite-type ferroelectrics

A mechanism of the off-center impurity-ion behavior induced by the quadratic Jahn-Teller effect on polar distortions and (or) joint action of the local electrostriction and linear Jahn-Teller effect on nonpolar distortions is proposed. The off-center behavior of Ni⁵⁺ and Fe⁴⁺ ions in the incipient ferroelectric KTaO₃ is substantiated. It is shown that, in the limit of neglecting the correlation between off-center Li⁺ ions, their displacements from KNbO₃ lattice sites decrease, whereas, under the conditions of strong correlation, a critical increase in their displacements from lattice sites occurs with approaching the Curie point.