Cation Ordering and Flexibility of the BO₄^2– Tetrahedra in Incommensurately Modulated CaEu₂(BO₄)₄ (B = Mo, W) Scheelites

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 CaEu2(BO4)4 (B = Mo, W) red phosphors solved from high-resolution synchrotron powder X-ray diffraction data. Monoclinic CaEu2(WO4)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 CaEu2(MoO4)4 is (3 + 2)-dimensionally modulated [superspace group I4₁/a(αβ0)00(-βα0)00, a = 5.238 672(7) Å, c = 11.548 43(2) Å, q1 = 0.55331(8)a* + 0.82068(9)b*, q2 = -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 ABO4 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 MoO4(2-) and WO4(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.

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

“…are infinite possibilities to combine these species to produce the same total scattering density. To establish the distribution of the elements we used the procedure introduced by Abakumov et al [18]. First, the modulation of the total scattering density was refined using effective atomic species with the atomic number corresponding to the average atomic number of the cations occupying the A positions.…”

“…However, in stoichiometric scheelites (a = b in the formula), ordering between the A'/A'' cations is rare. Typically, it is realized when the ionic radius of A' is about 0.35 Å larger than that of A'', provided that the A' cation has a formal charge larger than that of A'' [18][19][20][21][22][23][24]. On the other hand, scheelite-based materials can accommodate a large amount of vacancies in the A sublattice (compounds with a < b) [25][26][27][28][29] In these structures, A'/A'' cation ordering is more common, because it is also coupled to the ordering between cations and vacancies; and can occur for the A'/A'' ionic radii differences as low as ∼ 0.1 Å.…”

“…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]. …”

KEu(MoO4)2: Polymorphism, Structures, and Luminescent Properties

“…The data in Table S4 of the Supporting Information reflect that two distortion modes with either stretching of the Mo−O bonds (1.92−1.70 Å) or bending of the O−Mo−O bond angles (∼132−93°) are active, similar to the situation observed in other incommensurately modulated cation-ordered scheelite-type structures. 46 The structure of β-KEu(MoO 4 ) 2 can, like other scheelitetype compounds, be described as columns along the c-axis consisting of A cations and MoO 4 ( Figure 5). Each column consists of either {KMoO 4 } or {EuMoO 4 } units.…”

Structural and spectroscopic properties of self-activated monoclinic molybdate BaSm2(MoO4)4

“…as well as the possibility of controlling the local environment of rare-earth ions by varying the nomenclature and content of additional ions [19][20][21][22]. The latter is of importance for the optical properties of rare-earth ions such as distribution of absorption cross-sections and luminescence intensities between different bands as well as the shapes of these bands themselves, due to strong influence of crystal field on the probabilities of f -f transitions and energy structure of crystal-fieldsplit sublevels.…”