Résumé
In the reaction mechanism leading to the formation of the new mixed valence uranium uranyl phosphate, U(UO2)(PO4)(2), we found that an initial product UCIPO4 . 4H(2)O plays a decisive role in the synthesis. This uranium chloride phosphate tetrahydrate has never been mentioned in the literature, and so we investigated its chemical properties, X-ray powder diffraction patterns and its spectroscopic characteristics. We followed the evolution of these properties from room temperature to 1000-1200 degrees C. Its transformation during heat treatment in air corresponds to the oxidation of 50 mol percent of the total uranium amount, which gives the final compound U(UO2)(.)(PO4)(2). The redox reactions implied in the solid-state chemistry are described and discussed. Under the same heating conditions, but in an inert atmosphere, UCIPO4 . 4H(2)O gives a known oxouranium diphosphate (UO)(2)P2O7.