Construction of first-principles-based adiabatic and diabatic Hamiltonians for the TiO unit of the BaTiO crystal: photoemission spectra and ferroelectricity.

The ferroelectric properties of the BaTiO crystal arise from the strong pseudo Jahn-Teller (PJT) interactions between the non-degenerate ground electronic state, A, and the degenerate T symmetry states through the nuclear distortions of t modes in the TiO unit. In the d electronic configuration of the Ti ion, the PJT interaction leads to a stabilization effect, which has been explored using Beyond Born-Oppenheimer (BBO) theory. The T excited states form a three-state degeneracy, exhibiting feeble Jahn-Teller (JT) distortions over the t planes. For the first time, we compute adiabatic potential energy surfaces (PESs) and non-adiabatic coupling terms (NACTs), and, thereafter, diabatic PESs and couplings for the perovskite unit, TiO. Using a time-dependent discrete variable representation (TDDVR) approach, the theoretical photoemission spectra exhibit good agreement with the experimental results. Moreover, the experimental observation on the order parameter associated with the ferroelectric properties of the BaTiO crystal shows a close resemblance to the present work and other theoretical predictions.