Effect of Chemical Composition on the Optical Properties of Cr³⁺ Impurity in A₃B₅O₁₂ Garnets (A = Lu, Y, Gd, La; B = Al, Ga, Sc)

We present a comprehensive first-principles study on a series of isostructural Cr³⁺-doped garnets with varying chemical compositions. The study aims to explore the effect of host properties and local coordination environments on the optical properties of the Cr³⁺ impurity ion. Specifically, we calculated the energies of the excitation and emission band maxima, as well as the zero-phonon line energies, for the ⁴A₂-²E and ⁴A₂-⁴T₂ optical transitions of Cr³⁺ ions in A₃B₅O₁₂ garnets (A = Lu, Y, Gd, La; B = Al, Ga, Sc). The calculated optical transition energies are in good agreement with experimental measurements. Our results reveal that the position of the ⁴T₂ energy level, governed by the crystal-field (CF) strength, is primarily determined by variations in the Cr³⁺-O^2- bond lengths. Longer Cr³⁺-O^2- bond lengths reduce the CF strength, placing the ⁴T₂ energy level below the ²E energy level, which results in the broadband ⁴T₂ → ⁴A₂ emission. In contrast, shorter Cr³⁺-O^2- bond lengths increase the CF strength, raising the ⁴T₂ energy level above the ²E energy level and producing only the R-line (²E → ⁴A₂) and its vibronic sideband in the Cr³⁺ emission spectrum at room temperature. In garnet compounds where the ⁴T₂ and ²E energy levels are close in energy, the emission spectrum is composed of the R-line superimposed on the broadband ⁴T₂ → ⁴A₂ emission. These findings enhance our understanding of the relationship between chemical composition and optical properties in Cr³⁺-doped garnets. Moreover, they hold significant scientific and technological importance, paving the way for the discovery of efficient phosphors for near-infrared phosphor-converted light-emitting diode devices using high-throughput design methodologies.