Synthesis and Optical Properties of Single-Crystalline Phosphors Gd3In2Ga3O12:RE3+ (RE = Nd3+ and Ho3+) Grown via the Optical Float Zone Method

The continuous development of innovative optical materials with lanthanoid ions as activators has emerged as a modern sector of materials chemistry. The experience with the fabrication of single crystals with the optical float zone has motivated one to investigate the luminescence of Nd³⁺ and Ho³⁺ ions in the garnets (Gd_3−xRE_x)In₂Ga₃O₁₂ (RE = Nd and Ho, x = 0; 0.15–0.30). Upon usage of an Ar/O₂ (80:20 ratio) atmosphere and application of an auxiliary pressure (6 bar) to suppress In₂O₃ evaporation, single-crystalline domain sizes in the order of ≈6 × 6 × 1 mm³ are obtained. Structural analysis confirms the formation of a cubic garnet phase with space group $Ia\overline{3}d$, with the substituents incorporated in accordance with Vegard's law. Backscattered electron imaging and energy-dispersive X-ray spectroscopy are conducted, demonstrating a homogeneous elemental distribution within the crystals. Photoluminescence studies are carried out, revealing the characteristic narrow-line 4f ⁿ → 4f ⁿ transitions of Nd³⁺ and Ho³⁺, with decay times in the submillisecond range, suggesting non-negligible cross-relaxation effects are present. Despite this, the large nearest-neighbor Gd–Gd distance (3.88 Å) in Gd₃In₂Ga₃O₁₂ and the low phonon cutoff energy (≈700 cm⁻¹) are found to limit cross-relaxation pathways, preserving significant photoluminescence brightness. These results highlight the potential of Gd₃In₂Ga₃O₁₂:RE³⁺ single crystals as promising candidates for advanced optical applications.