Novel Cr3+-doped double-perovskite Ca2MNbO6 (M = Ga, Al) phosphor: Synthesis, crystal structure, photoluminescence and thermoluminescence properties

Abstract A novel near-infrared (NIR) phosphor Ca2MNbO6: Cr3+ (CMN: Cr3+, M = Ga, Al) was successfully prepared by solid-state synthesis using Li2CO3 or LiF as the flux. The crystal structure of CMN: Cr3+ was characterized by the powder X-ray diffraction and refined with Rietveld methods, while the crystal structure of Ca2GaNbO6 (CGN) compound were reported for the first time. Diffuse reflection, excitation, emission, lifetime, and thermoluminescence spectra were measured to investigate the optical properties of Cr3+ ions in CMN phosphors. When excited at 316 nm, Ca2GaNbO6: Cr3+ (CGN: Cr3+) exhibits a sharp R-line at 739 nm attributed to 2E → 4A2 transition of Cr3+ and a broadband emission ascribed to 4T2 → 4A2 transition of Cr3+, and Ca2AlNbO6: Cr3+ (CAN: Cr3+) shows a narrow emission band centered at 745 nm corresponding to a spin-forbidden 2E → 4A2 transition. The concentration quenching of Cr3+ in the CGN and CAN host was determined to be 0.4 at. % and 0.3 at. %, respectively. The critical distance of the as-prepared phosphors at the quenching concentration were calculated to be 38.06 A for CGN: Cr3+ and 41.42 A for CAN: Cr3+, respectively. And the energy transfer mechanism of Cr3+ in CMN: Cr3+ phosphors was certified to be the dipole - dipole interaction. The crystal field strength and the Racah parameters were calculated to evaluate the crystal-field environment of Cr3+ in the host lattices. With increasing Cr3+ ion concentration in the range of 0.1–1.0 at. %, lifetimes of the phosphors decrease from 315 μs to 211 μs (λex = 316 nm, λem = 739 nm) and from 256 μs to 150 μs (λex = 316 nm, λem = 756 nm) for CGN: Cr3+ and 3.70 ms–1.43 ms for CAN: Cr3+, respectively. The fluorescence decay curves of CMN: Cr3+ phosphors were fitted based on Inokuti - Hirayama model, and the parameters such as energy transfer parameter Q and energy transfer probability (PSA) were calculated. Based on the thermoluminescence (TL) glow curves of the CAN: Cr3+ phosphors, the kinetic parameters have been determined by using Chen’s glow peak shape method and then the results are discussed in detail. The results show that Ca2MNbO6: Cr3+ (CMN: Cr3+, M = Ga, Al) is a new potential near-infrared phosphor for biological imaging due to its 660–840 nm emission regions.