Defect structures in the brannerite-type vanadates: VII. Preparation and study of Zn 1− xØ xV 2−2 xMo 2 xO 6 and Zn 1− x−yØ xLi yV 2−2 x−yMo 2 x+yO 6 solid solutions

Abstract

ZnV 2O 6 and LiVMoO 6, both of monoclinic brannerite-type structure, display miscibility in the whole range of composition and form the solid solution ZnLi = Zn 1− y Li y V 2− y Mo y O 6. ZnLi may be treated as a matrix able to incorporate an excess of Mo 6+ ions compensated by the equivalent number of cation vacancies Ø in the Zn 2+ sublattice, which results in the formation of ZnLiØ = Zn 1− x−y Ø x Li y V 2−2 x−y Mo 2 x+y O 6. At y = 0, x max = 0.15 and we deal with ZnØ = Zn 1− x Ø x V 2−2 x Mo 2 x O 6 (which belongs to the ZnV 2O 6MoO 3 system); at y = 0.25, x max ≈ 0.30, and at higher y, x max diminishes again. Over the border of the existence of ZnLiØ several areas may be distinguished in which such phases coexist as saturated ZnLiØ, MoO 3, V 2MoO 8, ZnMoO 4, and solid solution of MoO 3 in V 2O 5. Phase diagrams of ZnV 2O 6MoO 3 and ZnV 2O 6LiVMoO 6MoO 3 system are partly resolved on the basis of differential thermal analysis (DTA) and X-ray phase analysis. Lattice parameters of the above-mentioned solid solutions as a function of composition are determined. Comparative analysis of the data gathered for ZnLiØ and the formerly studied MnLiØ and CoØ leads to the following conclusions. The MeV 2O 6 lattices are amenable to substitution of Mo 6+ for V 5+ in a quantity exceeding 50%. The “solubility” of vacancies is strongly dependent on the kind of Me 2+ in the matrix. The synergetic effect linked with the simultaneous presence of Zn 2+ and Li +, manifested by negative deviations from Vegard's law, increases the stability of the matrix and its capacity to create vacancies (cf. x max at y = 0 and y = 0.25). Dopant ions of various sizes influence the values of lattice parameters. Parameters a and b are dependent on the Mo V ratio and practically insensitive to the occupation of the original Me 2+ site. Parameter c (or c sin β) is almost sensitive only to the size of Me 2+/Li +/ Ø. Cation vacancies behave as ions of a size smaller than the exhausted cations by at least 0.01 Å but rather close to 0.06 Å.