Effect of cation vacancies on the crystal structure and luminescent properties of Ca0.85-1.5xGdxEu0.1□0.05+0.5xWO4(0 ≤ x ≤ 0.567) scheelite-based red phosphors

The Ca0.85-1.5xGdxEu0.1+0.05 square 0.5xWO4(0 <= x <= 0.567) series of cation-deficient scheelites is investigated to unveil the influence of the cation vacancies on the crystal structure and luminescent properties. The concentration of the vacancies is varied by the heterovalent substitution of Gd3+ for Ca2+, keeping the concentration of the Eu3+ luminescent centers constant in all compounds of the series. The crystal structure of the materials is studied using a combination of transmission electron microscopy and synchrotron X-ray powder diffraction. At low vacancy concentration (x - 0.1, 0.2), cations and cation vacancies are randomly distributed in the structure, and the materials preserve the I4(1)/alpha symmetry of the parent scheelite structure [x = 0.1: a = 5.25151(1) angstrom, c = 11.39479(2) angstrom; x = 0.2: a = 5.25042(1) angstrom, c = 11.41335(2) angstrom]. At higher concentration, the cation-vacancy ordering gives rise to incommensurately modulated structures. The x = 0.3 structure has a (3 + 2) D tetragonal symmetry [superspace group I4(1)/alpha(alpha,beta, 0)00(-beta,alpha,0)00, a = 5.24700(1) angstrom, c = 11.45514(3) angstrom, q(1) = 0.51637(14)a* + 0.80761(13)b*, q(2) = -0.80761a* + 0.51637b*]. At x = 0.4, the scheelite basic cell undergoes a monoclinic distortion with the formation of the (3 + 1) D structure [superspace group I-2/b(alpha,beta,0)00, a = 5.23757(1) angstrom, b = 5.25035(1) angstrom, c = 11.45750(2) angstrom, gamma = 90.5120(2) angstrom, q = 0.54206(8)a* + 0.79330(8)b*]. In both structures, the antiphase Ca and (Gd, Eu) occupancy modulations indicate that the ordering between the A cations and vacancies also induces partial Ca/(Gd, Eu) cation ordering. Further increase of the Gd3+ content up to x = 0.567 leads to the formation of a monoclinic phase (space group C2/c) with the Eu2/ 3WO4-type structure. Despite the difference in the cation-vacancy ordering patterns, all materials in the series demonstrate very similar quantum efficiency and luminescence decay lifetimes. However, the difference in the local coordination environment of the A cation species noticeably affects the line width and the multiplet splitting of the 4f(6)-4f(6) transitions. (C) 2017 Elsevier B.V. All rights reserved.