Photoluminescence of the undoped and Bi3+ - Doped Ca3Ga2Ge3O12 garnets

Highlights: • Ca₃Ga₂Ge₃O₁₂ and Ca₃Ga₂Ge₃O₁₂:Bi³⁺ powders are synthesized and analyzed by XRD. • Photoluminescence of Ca₃Ga₂Ge₃O₁₂ and Ca₃Ga₂Ge₃O₁₂:Bi³⁺ is investigated. • Ultraviolet emissions arise from excited state of Bi³⁺ centers of different types. • A visible emission band arises from an exciton localized around a Bi³⁺ ion. • Mechanisms of processes taking place in Bi³⁺-related excited states are clarified. Photoluminescence characteristics of the undoped and Bi³⁺ - doped Ca₃Ga₂Ge₃O₁₂ garnets are investigated in the 4.2–500 K temperature range by the methods of steady-state and time-resolved spectroscopy. The Bi³⁺ ions substituting for Ca²⁺ ions in the garnet Ca₃Ga₂Ge₃O₁₂ and parasitic Ca₅Ge₃O₁₁ phases are revealed by the X-ray diffraction method. Under excitation in the Bi³⁺ - related absorption band, the ultraviolet and visible emissions are observed in Ca₃Ga₂Ge₃O₁₂:Bi. The ultraviolet emission bands are ascribed to the radiative decay of the triplet relaxed excited state of Bi³⁺ ions having different nearest surroundings. A visible emission is found to arise from an exciton localized around a Bi³⁺ ion. The suggestion is made that the lowest-energy excited level of Bi³⁺ is located close to the bottom of the conduction band of Ca₃Ga₂Ge₃O₁₂. The mechanisms of the processes, resulting in the appearance of luminescence of the undoped and Bi³⁺ - doped Ca₃Ga₂Ge₃O₁₂ garnets, are discussed.