Chemically-induced structural variations of a family of Cs₂[(AnO₂)₂(TO₄)₃] (An = U, Np; T = S, Se, Cr, Mo) compounds: Thermal behavior, calorimetry studies and spectroscopy characterization of Cs uranyl sulfate and selenate

Cs₂[(AnO₂)₂(TO₄)₃] (where An ​= ​U, Np; T ​= ​S, Se, Cr, Mo) is a remarkable family of actinide-bearing compounds due to the surprising persistence of its structure type despite the substitutions in actinide and oxyanion sites. Cesium uranyl sulfate and selenate compounds have been analysed using high-temperature powder X-ray diffraction, Raman and luminescence spectroscopy techniques; in addition, high-temperature oxide-melt calorimetry studies had been carried out to derive the enthalpies of formation of both compounds. The crystal structures of all six known isotypic compounds have been compared to reveal specific geometric parameters, which results in differences of thermal behavior and spectra. Chemical alteration in the An and T sites results in changes of the Cs⁺ coordination environment, which in turn leads to an increase of its influence on the layered structural units and the stability of the structure itself. These substitution mechanisms may be regarded as an example of the chemical composition selectivity for the preparation of compounds with the desired properties.