Construction of First Principle Based Adiabatic and Diabatic Hamiltonian for TiO68- unit of BaTiO3 Crystal: Photoemission Spectra and Ferroelectricity

The ferroelectric property of BaTiO₃ crystal arises from the strong Pseudo Jahn-Teller (PJT) interactions between the non-degenerate ground electronic state, ¹A_1g and the degenerate ¹T_1u symmetry states through the nuclear distortions of t_1u modes in TiO₆^8- unit. In a d⁰ electronic configuration of Ti⁴⁺ ion, the PJT interaction leads to a stabilization effect, which has been explored using Beyond Born-Oppenheimer (BBO) theory. The ¹T_1u excited states form a three-state degeneracy, exhibiting feeble Jahn-Teller (JT) distortions over the t_2g planes. For the first time, we compute ab initio adiabatic potential energy surfaces (PESs) and non-adiabatic coupling terms (NACTs), and thereafter, diabatic PESs and couplings for the perovskite unit, TiO₆^8-. Using a Time-Dependent Discrete Variable Representation (TDDVR) approach, the theoretical photoemission spectra exhibit good agreement with the experimental ones. Moreover, the experimental observation on order parameter associated with ferroelectric properties of BaTiO₃ crystal show close resemblance with present and other theoretical predictions.