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

摘要

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. CaGd_(2(1-x))Eu_(2x)(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 I4_1/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 [110] direction of the scheelite subcell. The replacement of the smaller Gd~(3+) by the larger Eu~(3+) at the A-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo~(6+) by W~(6+) 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. CaGd_(2(1-x))Eu_(2x)(MoO4)_(4(1-y))(WO4)_(1y) phosphors emit intense red light dominated by the ~5D0-~7F2 transition at 612 nm, along with other transitions from the ~5B1 and ~5D0 excited states. The intensity of the ~5D0-~7F2 transition reaches a maximum at x = 0.5 for y = 0 and 1.