Rich Non-centrosymmetry in a Na-U-Te Oxo-System Achieved under Extreme Conditions

Two new sodium uranyl tellurites and two new sodium uranyl tellurates have been synthesized from high-temperature/high-pressure conditions and structurally characterized. We demonstrated that crystalline phases, forming in a Na-U-Te system under extreme conditions, appear to favorably have non-centrosymmetric structures. Three out of four novel uranyl tellurium compounds, Na[(UO2)Te(IV)2O5(OH)], Na2[(UO2)(Te(VI)2O8)], and Na2[(UO2)Te(VI)O5], crystallize in non-centrosymmetric space groups. The crystal structure of Na[(UO2)Te(IV)2O5(OH)] is based on two-dimensional [UO2Te2O5(OH)](-) corrugated sheets, which are charge balanced by guest Na(+) cations. The structure of Na2[(UO2)Te(VI)2O8] is constructed from [(UO2)2Te2O8](2+) anionic layers composed of UO7 pentagonal bipyramids and TeO6 octahedra. Na2[(UO2)(Te(VI)O5)] is a new type of three-dimensional anionic open framework built from the interconnection of UO7 pentagonal bipyramids and TeO6 octahedra with different types of interlacing channels within the U-Te anionic framework. Na[(UO2)Te(IV)6O13(OH)], as the only centrosymmetric compound isolated in the Na-U-Te family, is crystallized in space group Pa3̅, and its structure is highly related to that of cliffordite (UO2(Te3O7)), which is composed from UO8 hexagonal bipyramids and TeO5 square pyramids. The vibrational modes associated with U-O, Te(IV)-O, and Te(VI)-O bonds are discussed, and the Raman spectra of the four compounds are characterized for signature bands.