Title
Incommensurate modulation and luminescence in the <tex>$CaGd_{2(1-x)}Eu_{2x}(MoO_{4})_{4(1-y)}(WO)_{4y}$</tex> (<tex>$0\leq x\leq1$</tex>, <tex>$0\leq y\leq1$</tex>) red phosphorsIncommensurate modulation and luminescence in the <tex>$CaGd_{2(1-x)}Eu_{2x}(MoO_{4})_{4(1-y)}(WO)_{4y}$</tex> (<tex>$0\leq x\leq1$</tex>, <tex>$0\leq y\leq1$</tex>) red phosphors
Author
Faculty/Department
Faculty of Sciences. Physics
Research group
Electron microscopy for materials research (EMAT)
Publication type
article
Publication
Washington, D.C.,
Subject
Physics
Chemistry
Source (journal)
Chemistry of materials / American Chemical Society. - Washington, D.C.
Volume/pages
25(2013):21, p. 4387-4395
ISSN
0897-4756
ISI
000327045000030
Carrier
E
Target language
English (eng)
Full text (Publishers DOI)
Affiliation
University of Antwerp
Abstract
Scheelite related compounds (A',A '') [(B',B '')O-4], 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 I4(1)/a(alpha,beta,0)00(-beta,alpha,0)00, while the structures of all tungstates are (3 + 1)D incommensurately modulated with superspace group I2/b(alpha beta 0)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 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)(MoO4)(4(1-y))(WO4)(4y) phosphors emit intense red light dominated by the D-5(0)-F-7(2) transition at 612 nm, along with other transitions from the D-5(1) and D-5(0) excited states. The intensity of the 5D0-7F2 transition reaches a maximum at x = 0.5 for y = 0 and 1.
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