High-temperature, high-pressure hydrothermal synthesis, characterization, and structural relationships of mixed-alkali metals uranyl silicates

Abstract Three mixed-alkali metals uranyl silicates, Na 3 K 3 [(UO 2 ) 3 (Si 2 O 7 ) 2 ]·2H 2 O ( 1 ), Na 3 Rb 3 [(UO 2 ) 3 (Si 2 O 7 ) 2 ] ( 2 ), and Na 6 Rb 4 [(UO 2 ) 4 Si 12 O 33 ] ( 3 ), have been synthesized by high-temperature, high-pressure hydrothermal reactions at 550 °C and 1440 bar, and characterized by single-crystal X-ray diffraction, photoluminescence, and thermogravimetric analysis. Compound 1 and 2 are isostructural and contain layers of uranyl disilicate. The smaller cation, Na + , is located in the intralayer channels, whereas the larger cations, K + and Rb + , and water molecule are located in the interlayer region. The absence of lattice water in 2 can be understood according to the valence-matching principle. The structure is related to that of a previously reported mixed-valence uranium(V,VI) silicate. Compound 3 adopts a 3D framework structure and contains a unique unbranched dreier fourfold silicate chain with the structural formula {uB,4 1 ∞ }[ 3 Si 12 O 33 ] formed of Q 2 , Q 3 , and Q 4 Si. The connectivity of the Si atoms in the Si 12 O 33 18− anion can be interpreted on the basis of Zintl–Klemm concept. Crystal data for compound 1 : triclinic, P -1, a =5.7981(2) A, b =7.5875(3) A, c =12.8068(5) A, α =103.593(2)°, β =102.879(2)°, γ =90.064(2)°, V =533.00(3) A 3 , Z =1, R 1=0.0278; compound 2 : triclinic, P -1, a =5.7993(3) A, b =7.5745(3) A, c =12.9369(6) A, α =78.265(2)°, β =79.137(2)°, γ =89.936(2)°, V =546.02(4) A 3 , Z =1, R 1=0.0287; compound 3 : monoclinic, C 2/ m , a =23.748(1) A, b =7.3301(3) A, c =15.2556(7) A, β =129.116(2)°, V =2060.4(2) A 3 , Z =2, R 1=0.0304.