•Thermal stabilities of defects in CFSO are analyzed by formation energy calculation.•Host absorption and emission in CFSO are ascribed to excitonic states related to VO.•4f→5d transitions of Ce3+ in CFSO are simulated by CASSCF/CASPT2/RASSI–SO methods.•Excitation bands in experimental spectra of CFSO: Ce3+ phosphors are assigned.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.

The formation energies of oxygen and calcium vacancies (VO and VCa) in β-Ca2SiO4 are derived from density functional theory (DFT) calculations performed on constructed supercells, revealing the thermodynamic stabilities of intrinsic defects under oxygen-poor conditions. On the basis of DFT calculations with HSE06 hybrid functional, defect states produced by vacancies (VO and VCa), dopants Eu (EuCa) and Eu-related defect complexes (EuCa+VO and EuCa+VCa) in the band gap of β-Ca2SiO4 host have been further distinguished. The calculated results indicate that the neutral and single negatively charged VCa (i.e., VCa× and VCa′) and the VO in positive charge states may act as trapping centers for electrons. When approaching to Eu2+ ions, VCa× and VCa′ may capture the electrons of Eu2+ ions and stabilize Eu3+ ions at Ca2+ sites in β-Ca2SiO4, but not the neutral and positively charged VO. This work demonstrates that electronic structure calculations combined with thermodynamic analyses can be utilized to study the interactions between europium ions and their neighboring defects, and further study the tuning mechanisms of the valence stabilities of europium ions in inorganic compounds.

In the present work, geometric structures, electronic properties, and 4f → 5d transitions of γ-Ca2SiO4:Ce3+ phosphors have been investigated by using first-principles calculations. Four categories of typical substitutions (i.e., the doping of the Ce3+ without the neighboring dopants/defects and with the neighboring VO••, AlSi′, and VCa″) are taken into account to simulate local environments of the Ce3+ located at two crystallographically different calcium sites in the γ-Ca2SiO4. Density functional theory (DFT) geometry optimization calculations are first performed on the constructed supercells to obtain the information about the local structures and preferred sites for the Ce3+. On the basis of the optimized crystal structures, the electronic properties of γ-Ca2SiO4:Ce3+ phosphors are calculated with the Heyd–Scuseria–Ernzerhof screened hybrid functional, and the energies and relative oscillator strengths of the 4f → 5d transitions of the Ce3+ are derived from the ab initio embedded cluster calculations at the CASSCF/CASPT2/RASSI-SO level. A satisfactory agreement with the available experimental results is thus achieved. Moreover, the relationships between the dopants/defects and the electronic as well as spectroscopic properties of γ-Ca2SiO4:Ce3+ phosphors have been explored. Such information is vital, not least for the design of Ce3+-based phosphors for the white light-emitting diodes (w-LEDs) with excellent performance.

The Ce3+ ions occupying the two crystallographically distinct Y3+ sites both with C1 point group symmetry in the X2-Y2SiO5 (X2-YSO) crystal are discriminated by their spectroscopic properties calculated with ab initio approaches and phenomenological model analyses. Density functional theory (DFT) calculations with the supercell approach are performed to obtain the local structures of Ce3+, based on which the wave function-based embedded cluster calculations at the CASSCF/CASPT2 level are carried out to derive the 4f → 5d transition energies. From the ab initio calculated energy levels and wave functions, the crystal-field parameters (CFPs) and the anisotropic g-factor tensors of Ce3+ are extracted. The theoretical results agree well with available experimental data. The structural and spectroscopic properties for the two types of Ce3+ ions in X2-YSO are thus distinguished in terms of the calculated local atomic structures, 4f → 5d transition energies, and spectral parameters.