Towards White Light Emission Through Metal‐to‐Metal Charge Transfer (MMCT) Effect in Bi3+ Activated Ca2YTi2‐mZrmGa3O12 (0≤m≤2) Garnets

Herein, we report the high-temperature solid-state synthesis and intriguing optical features of Bi³⁺/Ln³⁺ doped Ca₂YTi₂Ga₃O₁₂ (CYT). The optical properties of CYT were fine-tuned by judiciously substituting Zr⁴⁺ ions at Ti⁴⁺ sites and Bi³⁺, Ln³⁺ ions at Y³⁺ sites. All these compounds are crystallized in a cubic crystal system with an Ia-3d (no. 230) space group. The Bi³⁺ doped systems showed a broad photoluminescence (PL) band in the yellow-orange region centered at ~552 nm from the metal-to-metal charge transfer state (MMCT). The yellow-orange emissive CYT:Bi_x was co-doped with blue emissive Tm³⁺ to obtain an excitation wavelength-dependent color tuneable white light phosphor CYT:Tm_0.02Bi_0.012. Substituting Zr⁴⁺ ions in place of Ti⁴⁺ ions in CYT gave rise to CYTZ (Ca₂YTi_2-mZr_mGa₃O₁₂), which upon doping with Bi gave Ca₂YZr₂Ga₃O₁₂:Bi_x (CYZ:Bi_x) and they exhibit the ³P_0,1→¹S₀ emission in the violet-indigo region ( λ_em=395 nm; λ_ex=315 nm). On co-doping CYTZ:Bix with Sm³⁺ ions produced white light emissive Ca₂YTi_0.1Zr_1.9Ga₃O₁₂:Bi³⁺; Sm³⁺, (CYTZ:Bi³⁺; Sm³⁺). CYT:Bix showed slower (microsecond) PL decay than the CYZ:Bix (nanosecond). The crystal structure, steady state, and time-resolved optical studies, and semiempirical calculations were employed to rationalize the observed optical features of these compounds. The optical features of CYT:Bi_0.1, were explored for potential applications in secret writing.