Аннотация:Thermal expansion of the mineral soddyite, (UO2)2SiO4(H2O)2, and structurally related synthetic compound Na2(UO2)2SiO4F2 (NAUSIF) has been studied by means of high-temperature single-crystal and powder X-ray diffraction. The mineral is orthorhombic, Fddd, while NAUSIF is tetragonal,I41/amd. The framework structures of both compoundsare comprised of either neutral [(UO2)2(SiO4)(H2O)2] or negatively charged [(UO2)2(SiO4)F2]2- chains of similar topology. In the structure of soddyite, the chains cross at the angle of 72°, while in NAUSIF of 90°. Upon increasing temperature, the acute inter-chain angles in soddyite increase due to hingedeformations, the overall symmetry approaching tetragonal. The mineral is stable below 325 ± 25 °С; between 325 and 640 °С, the decomposition products cannot be identified unambiguously and contain significant amount of amorphous phases; at higher temperatures, a mixture of U3O8 polymorphs is formed. NAUSIF is stable until its melting point of 625 ± 25 °С. The thermal expansion of both compounds is strongly anisotropic; for NAUSIF, it is due to difference in bond strength in the uranium and sodium polyhedra. Anisotropic thermal expansion of soddyite is controlled by shear deformations ofthe structure upon the temperature rise.
