First Mixed Alkaline Uranyl Molybdates: Synthesis and Crystal Structures of CsNa3[(UO2)4O4(Mo2O8)] and Cs2Na8[(UO2)8O8(Mo5O20)

Abstract

AbstractTwo new mixed alkaline uranyl molybdates CsNa3[(UO2)4O4Mo2O8] (1) and Cs2Na8[(UO2)8O8(Mo5O20)] (2) have been obtained by high‐temperature solid state reactions. Their crystal structures have been solved by direct methods: Compound 1: triclinic, P$\bar{1}$, a = 6.46(1), b = 6.90(1), c = 11.381(2) Å, α = 84.3(1), β = 91.91(1), γ = 80.23(1)°, V = 488.6(2) Å3, R1 = 0.06 for 2865 unique reflections with |Fo| ≥ 4σF; Compound 2: orthorhombic, Ibam, a = 6.8460(2), b = 23.3855(7), c = 12.3373(3) Å, V = 1975.2(1) Å3, R1 = 0.049 for 2120 unique reflections with |Fo| ≥ 4σF. The structure of 1 contains complex sheets of UrO5 pentagonal bipyramids and molybdenum polyhedra. The sheets have [(UO2)2O2(MoO5)] composition. Natrium and cesium atoms are located in the interlayer space. Cesium atoms are situated between the molybdenum clusters, whereas natrium atoms are segregated between the uranyl complexes. The large Cs+ ions are localized between the Mo2O9 groups and force the molybdenum polyhedra to rotate relative to the [(UO2)2O2(MoO5)] sheets. Such rotation is impossible for U6+ polyhedra due to their rigid edge‐sharing complexes. The distance between the U6+ polyhedra vertices of neighboring layers is 3.8 Å, that allows the Na+ ion to be positioned between the uranyl groups. The crystal structure of 2 is based upon a framework consisting of [(UO2)2O2(MoO5)] sheets parallel to (010). The sheets are linked into a 3‐D framework by sharing vertices with the Mo(2)O4 tetrahedra, located between the sheets. Each MoO4 tetrahedron shares two of its corners with two MoO6 octahedra in the sheet above, and the other two with MoO6 octahedra of the sheet below. Thus four MoO6 octahedra and one MoO4 tetrahedron form chains of composition Mo5O18. The resulting framework has a system of channels occupied by the Cs+ and Na+ ions.