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Phase formation in the mixed sulfate-selenate aqueous system of uranyl nitrate and cesium nitrate has been investigated. Two types of crystalline compounds have been obtained and characterized using a number of experimental (single crystal XRD, FTIR, SEM) and theoretical (information-based complexity calculations, topological analysis) techniques. No miscibility gaps have been observed for Cs 2 [(UO 2 ) 2 ( T O 4 ) 3 ] ( T = S, Se), which crystallizes in tetragonal system, P -42 1 m , a =9.616(1)–9.856(2), c =8.105(1)–8.159(1) Å, V =749.6(2)–792.5(3) Å 3 . Nine phases with variable amount of S and Se have been structurally characterized. The structures of the Cs 2 [(UO 2 ) 2 ( T O 4 ) 3 ] ( T = S, Se) compounds are based upon the [(UO 2 ) 2 ( T O 4 ) 3 ] 2- layers of corner-sharing uranyl pentagonal bipyramids and T O 4 tetrahedra. The layers contain two types of tetrahedral sites: T 1 (3-connected, i.e. having three O atoms shared by adjacent uranyl polyhedra) and T 2 (4-connected). The Se-for-S substitution in tetrahedral sites is highly selective with smaller S 6+ cation showing a strong preference for the more tightly bonded T 2 site. Crystallization in the pure Se system starts with the formation of Cs 2 [(UO 2 )(SeO 4 ) 2 (H 2 O)](H 2 O) crystals, its subsequent dissolution and formation of Cs 2 [(UO 2 ) 2 (SeO 4 ) 3 ]. The information-based structural complexity calculations for these two phases support the rule that more topologically complex structures form at the latest stages of crystallization. [ABSTRACT FROM AUTHOR]

Cs 2 [(An O 2) 2 (T O 4) 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. The crystal chemistry review and properties of probably the most outstanding family of actinyl-bearing isotypic compounds Cs 2 [(An O 2) 2 (T O 4) 3 ] (where An ​= ​U, Np and T ​= ​S, Se, Cr, Mo), which is the only known to date example of structural type preservation with such a large diversity in chemical composition. Image 1 • Thermal behaviour of Cs uranyl sulfate and selenate was analysed. • Enthalpy of formation and luminescence properties of Cs uranyl sulfate and selenate were determined. • Crystal-chemical review of a family of Cs 2 [(An O 2) 2 (T O 4) 3 ] (An ​= ​U, Np; T ​= ​S, Se, Cr, Mo) compounds was prepared. [ABSTRACT FROM AUTHOR]