Effects of Si Codoping on Optical Properties of Ce-Doped Ca₆BaP₄O₁₇: Insights from First-Principles Calculations

It was recently reported that Ce-doped Ca₆BaP₄O₁₇ displayed blue-green emission under excitation in the near-ultraviolet (UV) region and that luminescence intensities can be greatly improved by codoping with Si. Here, a combination of hybrid density functional theory (DFT) and wave function-based CASSCF/CASPT2 calculations at the spin–orbit level has been performed on geometric and electronic structures of the material to gain insights into effects of Si codoping on its optical properties. It is found that the observed luminescence arises from 4f–5d transitions of Ce³⁺ occupying the two crystallograhically distinct Ca1 and Ca2 sites of the host compound with comparable probabilities, with the energy of the lowest 4f → 5d transition of Ce_Ca1 being slightly higher than that of Ce_ca2. The codopant Si prefers to substitute for the nearest-neighbor (NN) P1 atom over the NN P2 atom around Ce³⁺, and this preference induces a blueshift of the lowest-energy 4f → 5d transition, consistent with experimental observations. The blueshift originates from a reduction in 5d crystal field splitting of Ce³⁺ associated mainly with electronic effects of the NN Si_P1 substitution, while the contribution from the change in 5d centroid energy is negligible. On the basis of calculated results, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions on the Ca²⁺ sites of Ca₆BaP₄O₁₇ is constructed and discussed in connection with experimental findings