Incommensurate Modulation and Luminescence in the CaGd_2(1–x)Eu_2x(MoO₄)_4(1–y)(WO₄)_4y (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) Red Phosphors

Abstract

Scheelite related compounds (A′,A″) n [(B′,B″)O4] m with B′, B″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd2(1–x)Eu2x (MoO4)4(1–y)(WO4)4y (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)D incommensurately modulated structures with superspace group I41/a(α,β,0)00(−β,α,0)00, while the structures of all tungstates are (3 + 1)D incommensurately modulated with superspace group I2/b(αβ0)00. In both cases the modulation arises because of cation-vacancy ordering at the A site. The prominent structural motif is formed by columns of A-site vacancies running along the c-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd3+ by the larger Eu3+ at the A-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo6+ by W6+ switches the modulation from (3 + 2)D to (3 + 1)D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the A sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd2(1–x)Eu2x (MoO4)4(1–y)(WO4)4y phosphors emit intense red light dominated by the 5D0–7F2 transition at 612 nm, along with other transitions from the 5D1 and 5D0 excited states. The intensity of the 5D0–7F2 transition reaches a maximum at x = 0.5 for y = 0 and 1.

Preparation of PbLa2(MoO4)4:Er3+/Yb3+ particles via microwave sol–gel route and upconversion photoluminescence properties

“…This result accompanies the incommensurately modulated structure induced by the anion deficient MoO 4 À x group. This incommensurately modulated structures in the scheelite-type tetragonal phase were also observed in various molybdates and tungstates as a cation deficient structure [32][33][34]. Previously, our research group has reported the incommensurately modulated structure of CaGd 2 (WO 4 ) 4 :Er 3 þ /Yb 3 þ phosphors induced by the difference of effective ionic radii of Yb 3 þ , Er 3 þ and Gd 3 þ 4 Er 0.05 Yb 0.45 is higher than that of (c) PbLa 1.7 (MoO 4 ) 4 :Er 0.1 Yb 0.2 .…”

Effect of cation vacancies on the crystal structure and luminescent properties of Ca 0.85−1.5x Gd x Eu 0.1 □ 0.05+0.5x WO 4 (0 ≤ x ≤ 0.567) scheelite-based red phosphors

“…1). These reflections are attributed to cation-vacancy ordering and the formation of modulated structures [20,22,26,[28][29][30]. …”

“…Typically, occupational modulations in scheelite-based structures are confined to the ab planes, so that the ED taken along the [001] zone axis are the most informative [20,22,26,[28][29][30]. …”

“…Recently we investigated a CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) family of cation deficient scheelites [18,30,31]. …”

Engineering oxygen vacancies towards self-activated BaLuAlxZn4−xO7−(1−x)/2photoluminescent materials: an experimental and theoretical analysis

“…13,14 As we know, the structural disorder is a positive factor for the creation of wideemission-band phosphors in the doped systems. [15][16][17] More recently, the evaluation of the ''1147'' systems was continued and now the general formula can be written as LnBaM 4 O 7 (Ln = rare earth and Ca; M = Co, Fe, Mn, Zn, Ga, and Al). [18][19][20] Several rare-earth-containing phosphors were proposed from the LnBaM 4 O 7 -type hosts when M = Zn or Ga. [21][22][23] Up to now, however, self-activated luminescence remained unknown in this kind of complex oxide system.…”