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

Abstract

A pentavalent uranium germanate, Cs₃UGe₇O₁₈, was synthesized under high-temperature, high-pressure hydrothermal conditions at 585 °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 ^[6]GeO₆ octahedron is bonded to six three-membered single-ring ^[4]Ge₃O₉^6– units to form germanate triple layers in the <i>ab</i> 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 ⟨11̅0⟩ 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, <i>P</i>3̅<i>c</i>1 (No. 165), <i>a</i> = 12.5582(4) Å, <i>c</i> = 19.7870(6) Å, <i>V</i> = 2702.50(15) ų, <i>Z</i> = 6, <i>D</i>_calc = 5.283 g·cm^–3, μ­(Mo Kα) = 26.528 mm^–1, <i>R</i>₁ = 0.0204, <i>wR</i>₂ = 0.0519 for 1958 reflections with <i>I</i> > 2σ­(<i>I</i>). GooF = 1.040, ρ_max,min = 1.018, and −1.823 e·Å^–3