Targeted crystal growth of uranium gallophosphates via the systematic exploration of the UF4–GaPO4–ACl (A = Cs, Rb) phase space

Abstract

The molten flux synthesis of a uranium gallophosphate and a uranium gallate, Cs₄[UO₂Ga₂(PO₄)₄] (1) and Cs₂UO₂Ga₂O₅ (2), and four uranium phosphates, [Rb₂Rb_3.93Cl_0.93][(UO₂)₅(PO₄)₅] (3), Rb₁₁[(UO₂)₈(PO₄)₉] (4), Rb_7.6[(UO₂)₈O_8.6F_0.4(PO₄)₂] (5), and Rb₆[(UO₂)₅O₂(PO₄)₄] (6), is reported. A systematic exploration of the UF₄–GaPO₄–ACl (A = Cs, Rb) phase space resulted in the synthesis of targeted Cs₄[UO₂Ga₂(PO₄)₄] (1) and Cs₂UO₂Ga₂O₅ (2), which are gallium analogs of the previously reported aluminates, Cs₄[UO₂Al₂(PO₄)₄] and Cs₂UO₂Al₂O₅. The exploration of this phase space simultaneously led to the synthesis and characterization of four new uranium phosphate phases: [Rb₂Rb_3.93Cl_0.93][(UO₂)₅(PO₄)₅] (3), a salt inclusion material, and Rb₁₁[(UO₂)₈(PO₄)₉] (4), both of which have complex 3D, porous, framework structures, and Rb_7.6[(UO₂)₈O_8.6F_0.4(PO₄)₂] (5) and Rb₆[(UO₂)₅O₂(PO₄)₄] (6), both of which are layered structures related to the U₃O₈ topology. Fluorescence spectroscopy data are reported for all the compositions and are found to be typical for uranyl compounds.