Cs3UGe7O18: A Pentavalent Uranium Germanate Containing Four- and Six-Coordinate Germanium

A pentavalent uranium germanate, Cs3UGe7O18, was synthesized under high-temperature, high-pressure hydrothermal conditions at 585 degrees C and 160 MPa and structurally characterized by single-crystal X-ray diffraction and infrared spectroscopy. The valence state of uranium was confirmed by X-ray photoelectron spectroscopy and electron paramagnetic resonance. The room-temperature EPR spectrum can be simulated with two components using an axial model that are consistent with two distinct sites of uranium(V). In the structure of the title compound, each (GeO6)-Ge-[6] octahedron is bonded to six three-membered single-ring (Ge3O96-)-Ge-[4] units to form germanate triple layers in the ab plane. Each layer contains nine-ring windows; however, these windows are blocked by adjacent layers. The triple layers are further connected by UO6 octahedra to form a three-dimensional framework with intersecting six-ring channels along the < 1 (1) over bar0 > directions. The Cs+ cation sites are fully occupied, ordered, and located in the cavities of the framework. Pentavalent uranium germanates or silicates are very rare, and only two uranium silicates and one germanate analogue have been published. However, all of them are iso-structural with those of the Nb or Ta analogues. In contrast, the title compound adopts a new structural type and contains both four- and six-coordinate germanium. Crystal data of Cs3UGe7O18: trigonal, P (3) over bar c1 (No. 165), a = 12.5582(4) angstrom, c = 19.7870(6) angstrom, V = 2702.50(15) angstrom(3), Z = 6, D-calc = 5.283 g.cm(-3), mu(Mo K alpha) = 26.528 mm(-1), R-1 = 0.0204, wR(2) = 0.0519 for 1958 reflections with I > 2 sigma(I). GooF = 1.040, rho(max,min) = 1.018, and -1.823 e.angstrom(-3).