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.