Energetic, Optical, and Electronic Properties of Intrinsic Electron-Trapping Defects in YAlO₃: A Hybrid DFT Study

The formation energies of cation antisite defects (Y_Al and Al_Y), oxygen vacancies (V_O), and nearest-neighbor defect complexes (Y_Al–Al_Y and Y_Al–V_O) in various charge states in the YAlO₃ crystal are calculated using density functional theory (DFT) with a modified PBE0 hybrid functional containing 32% Hartree–Fock (HF) exchange. It is found that the formation of Y_Al is more energetically favorable than Al_Y under oxygen-poor condition, consistent with the fact that the latter was not observed in experiments. On the basis of calculated optical transition energies associated with the excitons trapped at Y_Al, V_O, and Y_Al–V_O, the two emission bands observed under excitonic excitation at low temperature are identified. Electronic properties of Y_Al–V_O complexes in the neutral and singly negative charge states are finally investigated. It shows that the extra electron added into the negative charge state is mainly localized at 4d orbitals of Y_Al with a two-component feature of its density distribution extending axially along the Y_Al–V_O direction

Electronic Properties of Ce³⁺-Doped Sr₃Al₂O₅Cl₂: A Combined Spectroscopic and Theoretical Study

Photoluminescence properties of Ce-doped Sr₃Al₂O₅Cl₂ crystals prepared by a solid-state reaction method are first investigated with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range. Six bands are observed in the excitation spectrum of the Ce³⁺ 5d → 4f emission at 15 K. The highest energy band is attributed to the host excitonic absorption, from which the band gap energy of the host is estimated to be around 7.2 eV. The four lowest energy bands are assigned to the 4f₁ → 5d_1–4 transitions of Ce³⁺ located on the three distinct Sr²⁺ sites in Sr₃Al₂O₅Cl₂ with almost equal probability, based on a comparison between excitation band maxima energies and 4f → 5d transition energies obtained from wave-function-based CASSCF/CASPT2 calculations with spin–orbit coupling on Ce-centered embedded clusters. The 4f₁ → 5d₅ transition, not observed in the low-temperature excitation spectrum, is found to be overshadowed by a nearby defect-related excitonic absorption. On the basis of present experimental and calculated results for Ce-doped Sr₃Al₂O₅Cl₂, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions on the Sr²⁺ sites of Sr₃Al₂O₅Cl₂ is constructed and discussed in association with experimental findings