K 5 Yb 1− x Eu x (MoO 4 ) 4 phosphors: aperiodic structures and luminescence properties

The effects of composition and preparation conditions of palmierite-type K 5 Yb 1− x Eu x (MoO 4 ) 4 (KYEMO) compounds on their structural and luminescence properties were studied in this work. The solid solutions were obtained in three ways: 1) annealing at 893 ± 10 K (low-temperature (LT) phases); 2) slow cooling to room temperature from 1123 ± 10 K (intermediate (IM) phases); 3) quenching in liquid nitrogen from 1123 ± 10 K (high temperature (HT) phases). As a result, three modifications, α-phase (space group (SG) R 3̄ m ), β-phase (superspace group (SSG) C 2/ m (0β0)00 or X 2/ m (0β0)00), and γ-phase (SG C 2/ c ) were obtained and characterized. The regions of existence of various modifications of the solid solutions were established. α-KYEMO (0 ≤ x ≤ 1 solid solutions, HT-) and LT- and IM-phases with 0.8 ≤ x ≤ 1 have trigonal symmetry (SG R 3̄ m ). The LT- and IM-phases with 0.3 ≤ x < 0.8 have monoclinic symmetry (SG C 2/ m ). The incommensurately modulated structure of β-K 5 Yb 0.3 Eu 0.7 (MoO 4 ) 4 was refined from synchrotron XRD data by the Rietveld method in the superspace group C 2/ m (0β0). The luminescence characteristics of the series of KYEMO solid solutions were studied. The influence of the Eu 3+ concentration on the PL spectra of α-KYEMO under excitation at the 7 F 0 → 5 L 6 Eu 3+ transition ( λ ex = 395 nm) and CTB ( λ ex = 260 nm) and the influence of synthesis conditions on the luminescence intensities of Eu 3+ and Yb 3+ emission were investigated. Increasing the concentration of Eu 3+ shifts the CIE coordinates from an orange to an orange-red region. The synthesis method, composition and structure affect the color change from light orange to orange in the order HT → LT → IM for x = 0.5 and LT → HT → IM for x = 0.8. Periodic (γ-phase) and aperiodic (β-phase) ordering of K1 and Yb in M1 positions in the structure is more preferable for the emission of 2 F 5/2 → 2 F 7/2 transition of Yb 3+ ions than a random distribution of the cations in the α-phase structure. The 5 D 0 → 7 F 0 and 5 D 0 → 7 F 2 Eu 3+ transition intensities and the Eu 3+ /Yb 3+ emission ratio depend on the excitation. From the PLE spectra for β-KYMO, the optical band gap was determined to be 4.13 eV. The quantum yield measured for α-K 5 Yb 0.3 Eu 0.7 (MoO 4 ) 4 has a very high value of 62%. The intensities of Eu 3+ and Yb 3+ emission depend on the Eu 3+ /Yb 3+ ratio as well as the excitation wavelength. The structural and optoelectronic characteristics make the K 5 Yb 1