From Order to Disorder and Back Again: In Situ Hydrothermal Redox Reactions of Uranium Phosphites and Phosphates

Five new uranium phosphites, phosphates, and mixed phosphate–phosphite
compounds were hydrothermally synthesized using H3PO3 as an initial reagent. These compounds are Cs4­[(UO2)8­(HPO4)5­(HPO3)5]·4H2O (1), Cs­[UIV­(PO4)­(H1.5­PO4)]2 (2), Cs4­[UIV6­(PO4)8­(HPO4)­(HPO3)] (3), Cs10­[UIV10­(PO4)4­(HPO4)14­(HPO3)5]·H2O (4), and Cs3­[UIV4­(PO4)3­(HPO4)5] (5). The
first contains uranium­(VI) and the latter four uranium­(IV). Of the
UIV structures, two have extensive disordering among the
cesium cation positions, one of which also contains disordering at
some of the phosphate–phosphite positions. These intermediate
compounds are bookended by nondisordered phases. The isolation of
these transitional phases occurred at the higher of the pH conditions
attempted here. Both the starting pH and the duration of the reactions
have a strong influence on the products formed. Herein, we explore
the second series of in situ hydrothermal redox reactions of uranyl
nitrate with phosphorous acid and cesium carbonate. The isolation
of these disordered crystalline products helps to illuminate the complex
reaction pathways that can occur in hydrothermal syntheses.