First Mixed Alkaline Uranyl Molybdates: Synthesis and Crystal Structures of CsNa 3 [(UO 2 ) 4 O 4 (Mo 2 O 8 )] and Cs 2 Na 8 [(UO 2 ) 8 O 8 (Mo 5 O 20 )]

Two new mixed alkaline uranyl molybdates CsNa 3 [(UO 2 ) 4 O 4 Mo 2 O 8 ] ( 1 ) and Cs 2 Na 8 [(UO 2 ) 8 O 8 (Mo 5 O 20 )] ( 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) Å 3 , R 1 = 0.06 for 2865 unique reflections with | F o | ≥ 4σ F ; Compound 2 : orthorhombic, Ibam , a = 6.8460(2), b = 23.3855(7), c = 12.3373(3) Å, V = 1975.2(1) Å 3 , R 1 = 0.049 for 2120 unique reflections with | F o | ≥ 4σ F . The structure of 1 contains complex sheets of Ur O 5 pentagonal bipyramids and molybdenum polyhedra. The sheets have [(UO 2 ) 2 O 2 (MoO 5 )] 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 Mo 2 O 9 groups and force the molybdenum polyhedra to rotate relative to the [(UO 2 ) 2 O 2 (MoO 5 )] sheets. Such rotation is impossible for U 6+ polyhedra due to their rigid edge‐sharing complexes. The distance between the U 6+ 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 [(UO 2 ) 2 O 2 (MoO 5 )] sheets parallel to (010). The sheets are linked into a 3‐D framework by sharing vertices with the Mo(2)O 4 tetrahedra, located between the sheets. Each MoO 4 tetrahedron shares two of its corners with two MoO 6 octahedra in the sheet above, and the other two with MoO 6 octahedra of the sheet below. Thus four MoO 6 octahedra and one MoO 4 tetrahedron form chains of composition Mo 5 O 18 . The resulting framework has a system of channels occupied by the Cs + and Na + ions.