Construction of first-principles-based adiabatic and diabatic Hamiltonians for the TiO68− unit of the BaTiO3 crystal: photoemission spectra and ferroelectricity†
Abstract
The ferroelectric properties of the BaTiO3 crystal arise from the strong pseudo Jahn–Teller (PJT) interactions between the non-degenerate ground electronic state, 1A1g, and the degenerate 1T1u symmetry states through the nuclear distortions of t1u modes in the TiO68− unit. In the d0 electronic configuration of the Ti4+ ion, the PJT interaction leads to a stabilization effect, which has been explored using Beyond Born–Oppenheimer (BBO) theory. The 1T1u excited states form a three-state degeneracy, exhibiting feeble Jahn–Teller (JT) distortions over the t2g 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, TiO68−. 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 BaTiO3 crystal shows a close resemblance to the present work and other theoretical predictions.