Thermal stabilities, electronic structures and optical properties of intrinsic defects and dopant cerium in Ca4F2Si2O7

The thermal stabilities, electronic structures and optical properties of intrinsic defects and dopant Ce3+ in Ca4F2Si2O7 host are studied by using density functional theory (DFT) calculations (with PBE and hybrid PBE0 functionals) and wave function-based embedded cluster ab-initio calculations (at the CASSCF/CASPT2/RASSI−SO level). The calculated formation energies reveal that anion vacancies (VO and VF) are always much more energetically favorable than cation vacancies (VCa and VSi) in Ca4F2Si2O7 host, which is generally prepared under reducing atmospheres. According to the thermodynamic transition energy levels of intrinsic defects readily generated in undoped Ca4F2Si2O7 (e.g. anion vacancies and antisite defects), we may identify the defect-induced host absorption and emission, whose exact origins are unclear previously. Moreover, on the basis of ab-initio calculated energies and relative oscillator strengths of the 4f→5d transitions of Ce3+ at calcium sites with charge-compensating defect OF in their local environments, the excitation bands in the experimental spectra of Ce3+-doped Ca4F2Si2O7 phosphors are also assigned. The main purpose of this work is to understand luminescence mechanisms of intrinsic defects and extrinsic dopants in the hosts for phosphors by using first-principles approaches.