Site occupation and 4f → 5d transitions of Ce3+ ions at mixed Ca2+/Y3+ sites in CaYAlO4: Insights from first-principles calculations

Highlights: • The most stable configuration of unit cells of CaYAlO₄ is determined theoretically. • Formation energies of intrinsic defects and Ce³⁺ ions in CaYAlO₄ are calculated. • 4f.→5d transitions of Ce³⁺ at mixed Ca²⁺/Y³⁺ sites in CaYAlO₄ are simulated. • Excitation bands in experimental spectra of Ce³⁺-doped CaYAlO₄ are assigned. -- Abstract: The first-principles calculations in the combination of hybrid density functional theory (DFT) and multiconfigurational quantum-chemical methods are carried out to investigate geometric structures, electronic structures and 4f → 5d transitions of Ce³⁺ ions at mixed Ca²⁺/Y³⁺ sites in the CaYAlO₄ (CYAO). The most energetically stable unit cell among three nonequivalent configurations is firstly determined according to the total energies from DFT calculations. The calculated defect formation energies of lanthanide dopants and complexes in the host (with relatively stable configurations) then reveal the preferred substitution of Ce³⁺ ions in the host. Moreover, the energies and relative oscillator strengths of the 4f → 5d transitions of Ce³⁺ at both Ca²⁺ and Y³⁺ sites are derived from the embedded-cluster quantum-chemical calculations at the CASSCF/CASPT2/RASSI−SO level. By comparison, the excitation bands in the experimental spectra of Ce³⁺-doped CYAO phosphors are mainly attributed to 4f → 5d transitions of Ce³⁺ ions at Ca²⁺ sites, which is well consistent with the conclusions attained from the calculations on defect formation energies. The computational framework presented in this study is beneficial to identify the occupation sites of lanthanide ions and assign the excitation bands of the experimental spectra for the phosphors with mixed sites or solid-solution structures.