Speaker
Ms
Kseniya Chaplieva
(Aleksandrovna)
Description
A new method of synthesis of schoepite [(UO2)8O2(OH)12](H2O)12(UO3•2.25H2O) has been proposed in this work. Dehydration process of the synthesized compound has been studied, the state and role of H2O in the structure has been established by mean of high-temperature X-ray diffraction, IR spectroscopy and thermography. The transformation processes of schoepite in other forms of the hydrated oxide U(VI) have been investigated.
The individual crystalline phases of schoepite were synthesized by the two stages process. Earlier UO4•2H2O was prepared and decomposed into UO3 at 200°С. Thereafter this freshly obtained uranium (VI) oxide was hydrated by water vapor at 100°С.
An IR spectroscopic study was carried out in order to evaluate the functional group composition, the state of H2O molecules and the state hydroxide groups in the structure of schoepite. The IR spectrum of UO3•2.25H2O contains three groups of absorption bands. Among these bands there are vibrations of H2O molecules, UO-H group and uranyl bonds. In schoepite all H2O molecules forms two types of H-bonds. First type of bonds is formed by the interaction of H2O molecules with each other. Interlayer H2O molecules and hydroxide groups, which are included in the structure of layer, interact and form other type of H-bond. The average intensity pick of ν (UO-H) appears in the spectrums at 3356 см-1. Corresponding bands of δ (U-O-H) vibrations are intensive ones at 997 см-1.
For detailed determination of the schoepite structure and its resistance to high temperatures, a thermographic investigation has been carried out. Two Н2О molecules is removed [(UO2)8O2(OH)12](H2O)12 in one stage according to the first endotherm effect at 80–116°С. A new crystalline phase of meta-schoepite [(UO2)8O2(OH)12]•(H2O)10 is formed as a result of these processes. This compound is stable at room temperature. The process of the meta-schoepite thermal decomposition occurs in one stage according to the second endotherm effect in the temperature range of 118–156°С. The dehydration product ([(UO2)8O2•(OH)12](H2O)2) could not be obtained as the individual phase, because two Н2О molecules per formula unit of the compound are insufficiently for binding of the layers in the stable formation. At intermediate values of from 2 to 10 compound [(UO2)8•O2(OH)12](H2O)n should be regarded as a crystalline phase of variable composition, related to the class of solid solutions with different content of molecular water. The dehydration product with formula [(UO2)8O2(OH)12](H2O)2 (UO3•H2O) isomerizes in to α-modification of uranyl hydroxide α-UO2(OH)2. The condensation of hydroxide groups in α-UO2(OH)2 and formation UO3 occurs at the third endotherm effect at 289–361°С. This oxide decomposes to U3O8 at 500°С.
Thus schoepite, meta-schoepite and uranyl hydroxide contain the similar structure units such as Н2О molecules, uranyl and hydroxide groups. Different combination of these structural units is a reason of existence of a variety of chemical forms of UO3•nH2O.
Primary author
Ms
Kseniya Chaplieva
(Aleksandrovna)
Co-authors
Ms
Elena Kostrova
(Leonidovna)
Prof.
Nicholay Chernorukov
(Georgievich)
Ms
Oxana Nipruk
(Valentinovna)
