Thermal evolution of soddyite, (UO2)2SiO4(H2O)2 and structurally related Na2(UO2)2SiO4F2

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 compounds are 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 degrees, while in NAUSIF of 90 degrees. Upon increasing temperature, the acute inter-chain angles in soddyite increase due to hinge deformations, the overall symmetry approaching tetragonal. The mineral is stable below 325 +/- 25 degrees & Scy;; between 325 and 640 degrees & Scy;, 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 degrees & Scy;. 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 of the structure upon the temperature rise.