Effect of Chemical Composition on the Optical Properties of Cr³⁺ Impurity in <i>A</i>₃<i>B</i>₅O₁₂ Garnets (<i>A</i> = Lu, Y, Gd, La; <i>B</i> = 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+–O2– bond lengths. Longer Cr3+–O2– 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+–O2– 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.