Understanding near-infrared luminescence in Cr-doped La-gallogermanates through first-principles calculations and ligand-field theory

Near-infrared phosphors based on ${\mathrm{Cr}}^{3+}$ have a wide range of important applications. The design and optimization of these phosphors require accurate determination of site occupancy and prediction of the luminescence mechanism of ${\mathrm{Cr}}^{3+}$ in solids. The former can typically be achieved through first-principles calculations of formation energies, while the latter can be accomplished through first-principles calculations aided by ligand field (LF) analyses. This study shows that it is possible to obtain all the information needed to construct adiabatic potential energy surfaces, including LF parameters and orbitals, the energies and equilibrium structures of certain states, and electron-phonon parameters, through calculation. The reliability of these results is confirmed by comparing them with experimental data. This improves the accuracy of the predictions for excitation, emission, and Stokes shift energies in the photoluminescence of ${\mathrm{Cr}}^{3+}$ ions. These calculations are then used to interpret the multipeak luminescence observed in Cr-doped La-gallogermanate systems. The results show that the ${\mathrm{Cr}}^{3+}$ ions located at a typical distorted-octahedral site in the host and another octahedral site similar to that in $\ensuremath{\beta}$--${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ are responsible for producing the low and high bands in the luminescence spectra of various La-gallogermanates, respectively. This study demonstrates that combining first-principles calculations with LF analysis is an effective approach for obtaining the low excited states of $3d$ transition metal activators, which has important implications for the design and optimization of luminescent materials.