K5Yb1−xEux(MoO4)4 phosphors: aperiodic structures and luminescence properties

Abstract

The effects of composition and preparation conditions of palmierite-type K₅Yb_1−xEu_x(MoO₄)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) Rm), β-phase (superspace group (SSG) C2/m(0β0)00 or X2/m(0β0)00), and γ-phase (SG C2/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 Rm). The LT- and IM-phases with 0.3 ≤ x < 0.8 have monoclinic symmetry (SG C2/m). The incommensurately modulated structure of β-K₅Yb_0.3Eu_0.7(MoO₄)₄ was refined from synchrotron XRD data by the Rietveld method in the superspace group C2/m(0β0). The luminescence characteristics of the series of KYEMO solid solutions were studied. The influence of the Eu³⁺ concentration on the PL spectra of α-KYEMO under excitation at the ⁷F₀ → ⁵L₆ Eu³⁺ transition (λ_ex = 395 nm) and CTB (λ_ex = 260 nm) and the influence of synthesis conditions on the luminescence intensities of Eu³⁺ and Yb³⁺ emission were investigated. Increasing the concentration of Eu³⁺ 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 ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions than a random distribution of the cations in the α-phase structure. The ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₂ Eu³⁺ transition intensities and the Eu³⁺/Yb³⁺ 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-K₅Yb_0.3Eu_0.7(MoO₄)₄ has a very high value of 62%. The intensities of Eu³⁺ and Yb³⁺ emission depend on the Eu³⁺/Yb³⁺ ratio as well as the excitation wavelength. The structural and optoelectronic characteristics make the K₅Yb_1−xEu_x(MoO₄)₄ phosphors strong contenders for near UV-WLEDs.