Understanding the Structure and Thermal Stability of Cs₃UCl₆(s) in the CsCl-UCl₃ System

The present effort details the first synthesis and structural characterization of Cs₃UCl₆(s), a compound for which no crystallographic information has previously been reported, despite its significance in the thermochemical evaluation of the CsCl-UCl₃ system. The compound was synthesized by a solid-state reaction and characterized by powder X-ray diffraction (XRD) with Rietveld refinement, with its purity analyzed by ICP-MS and oxygen quantified by inert gas fusion analysis. The phase crystallizes in a monoclinic unit cell (space group C2/c) with lattice parameters a = 27.303(11) Å, b = 8.204(3) Å, and c = 13.386(5) Å. The thermal stability of this compound was investigated by high-temperature XRD, which indicated a possible low-temperature polymorphic transformation. The structural information reported here resolves a long-standing deficiency in the crystallographic description of the CsCl-UCl₃ system and provides a robust reference for thermochemical assessments. Moreover, the accurate thermal characterization of Cs₃UCl₆(s) has broader implications for actinide science, particularly in high-temperature molten salt systems, where cesium and uranium chlorides coexist.