Cation Ordering and Flexibility of the BO 4 2- Tetrahedra in Incommensurately Modulated CaEu 2 (BO 4 ) 4 (B = Mo, W) Scheelites Artem M. Abakumov,* ,† Vladimir A. Morozov, †,‡ Alexander A. Tsirlin, § Johan Verbeeck, † and Joke Hadermann † † EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium ‡ Chemistry Department, Moscow State University, 119991 Moscow, Russia § National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia * S Supporting Information ABSTRACT: The factors mediating cation ordering in the scheelite-based molybdates and tungstates are discussed on the basis of the incommensurately modulated crystal structures of the CaEu 2 (BO 4 ) 4 (B = Mo, W) red phosphors solved from high-resolution synchrotron powder X-ray diffraction data. Monoclinic CaEu 2 (WO 4 ) 4 adopts a (3 + 1)-dimensionally modulated structure [superspace group I2/b(αβ0)00, a = 5.238 73(1)Å, b = 5.266 35(1) Å, c = 11.463 19(9) Å, γ = 91.1511(2)°, q = 0.56153(6)a* + 0.7708(9)b*, R F = 0.050, R P = 0.069], whereas tetragonal CaEu 2 (MoO 4 ) 4 is (3 + 2)-dimensionally modulated [superspace group I4 1 / a(αβ0)00(-βα0)00, a = 5.238 672(7) Å, c = 11.548 43(2) Å, q 1 = 0.55331(8)a* + 0.82068(9)b*, q 2 = -0.82068(9)a* + 0.55331(8)b*, R F = 0.061, R P = 0.082]. In both cases the modulation arises from the ordering of the Ca/Eu cations and the cation vacancies at the A-sublattice of the parent scheelite ABO 4 structure. The cation ordering is incomplete and better described with harmonic rather than with steplike occupational modulation functions. The structures respond to the variation of the effective charge and cation size at the A-position through the flexible geometry of the MoO 4 2- and WO 4 2- tetrahedra demonstrating an alternation of stretching the B-O bond lengths and bending the O-B-O bond angles. The tendency towards A-site cation ordering in scheelites is rationalized using the difference in ionic radii and concentration of the A-site vacancies as parameters and presented in the form of a structure map. 1. INTRODUCTION Scheelite (CaWO 4 ) related compounds (A′,A″) n (BO 4 ) m with B = W and/or Mo are promising materials for phosphors in pc-WLEDs (phosphor-converted white-light-emitting-diode) and solid-state lasers. 1-6 Scheelites can be prepared with a large concentration of vacancies in the A sublattice, yielding compositions characterized by an A:B ratio different from 1:1. The creation of cation vacancies in the scheelite-type framework and the ordering of the A-cations and vacancies are a new factor in controlling the scheelite-type structure and properties. 7,8 Very often the population of the A-sites in the scheelite-type structure by cations with different sizes and charges (such as alkali metal cations M + and rare-earth cations R 3+ ) and/or cation vacancies leads to modulated structures with a pronounced occupational modulation wave. 7,9-12 Recently, we have studied the luminescent properties of the CaGd 2(1-x) Eu 2x (MoO 4 ) 4(1-y) (WO 4 ) 4y (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) solid solutions with the scheelite-type structure. 13 These phosphors emit intense red light dominated by the 5 D 0 - 7 F 2 transition at 612 nm. Moreover, the temperature dependence of the ratio of the 5 D 1 to 5 D 0 emission intensity enables these materials to visualize temperature gradients with high accuracy and spatial resolution, making them excellent thermographic phosphors. 14 In that recent study, only a rough structure model has been constructed for (3 + 1)-dimensional [(3+1)D] incom- mensurately modulated monoclinic CaEu 2 (WO 4 ) 4 using precession electron diffraction (PED) data. We demonstrated that 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 W 6+ by Mo 6+ switches the modulation from the (3+1)D to the (3+2)D regime. In this contribution, we provide the structure solution for (3+1)D-modulated monoclinic CaEu 2 (WO 4 ) 4 and (3+2)D-modulated tetragonal CaEu 2 (MoO 4 ) 4 using high-resolution synchrotron powder X-ray di ffraction and discuss the behavior and role of the MoO 4 2- and WO 4 2- tetrahedral units in the A-site cation ordering. 2. EXPERIMENTAL SECTION The synthesis and preliminary characterization of the CaEu 2 (BO 4 ) 4 (B = Mo, W) samples have been described elsewhere. 13 High-resolution synchrotron X-ray powder diffraction (SXPD) data were collected at room temperature at the ID31 beamline of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using a constant wavelength of λ ≈ 0.4 Å and eight scintillation detectors, each preceded by a Si(111) analyzer crystal. The powder samples were placed in a thin- walled borosilicate glass capillary that was spun during the experiment. Received: June 29, 2014 Published: August 21, 2014 Article pubs.acs.org/IC © 2014 American Chemical Society 9407 dx.doi.org/10.1021/ic5015412 | Inorg. Chem. 2014, 53, 9407-9415