Ag1/8Pr5/8MoO4: An incommensurately modulated scheelite-type structure

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]. …”

“…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. Typically this results in the formation of incommensurately modulated structures with pronounced occupational modulation [27,29].…”

Structural perfection of (Na0.5Gd0.5)MoO4:Yb laser crystals

“…However, depending on the growth conditions and the initial compositions of charge, a change in both symmetry (in particular, the loss of a center of symmetry) and unit cell size may occur in the local regions of double molybdates and tungstates [9,13,14]. In addition, formation of commensurately and incommensurately modulated structures was found for scheelite-like crystals of defective compositions: Ag 1/8 Pr 5/8 MoO 4 [15], Na x Eu 3+ (2 À x)/3 MoO 4 with x -0.5 and 0.286 [16], Na 2/7 Gd 4/7 MoO 4 [17], CaGd 2(1 À x) Eu 2x (MoO 4 ) 4(1 À y) (WO 4 ) 4y (0 6 x 6 1, 0 6 y 6 1) [18]. These phenomena are accompanied by the appearance of additional, specific for modulated crystals, satellite reflections in the low-angle part of the X-ray diffraction (XRD) patterns.…”

“…Incommensurate modulation in a scheelite-like structure was first established for a crystal of initial defective composition Ag 1/8 Pr 5/8 MoO 4 crystallized in the superspace group B2/b(ab0)00 (3 + 1)D (according to transmission electron microscopy) or I2/b(ab0)00 (according to the X-ray powder diffraction data) [15].…”