A Novel Family of Np(VI) Oxysalts: Crystal Structures, Calorimetry, Thermal Behavior, and Comparison with U(VI) Compounds

Abstract

Phase formation in the mixed sulfate−selenate Np(VI)-bearing aqueous system has been investigated. The obtained crystalline compounds, Cs 2 [(NpO 2 ) 2 (TO 4 ) 3 ] (T = S, Se), crystallize in the tetragonal system, space group P4̅ 2 1 m, with a = 9.5737(3)−9.817(3) Å, c = 8.0824(4)−8.111(3) Å, and V = 740.79(6)−781.8(6) Å 3 , and have been chemically characterized. Single-crystal X-ray diffraction experiments at various temperatures were used to define the thermal behavior of the crystal structures relative to the S/Se ratio. The thermal behavior of mixed sulfate−selenate and pure selenate compounds is anisotropic, with the highest thermal expansion in the direction perpendicular to the sheets of Np-bearing polyhedra, while the structure of the pure sulfate compound experiences almost isotropic thermal expansion with slightly higher expansion within the layer, which is explained by the influence of stronger Cs−O bonding. High-temperature drop solution calorimetry was used to derive the enthalpies of formation of the Cs + -bearing neptunyl oxysalts. Within this family of isotypic structures, cesium neptunyl sulfate has a more negative enthalpy of formation in comparison to the selenate compound.

Thermal behavior of uranyl selenite minerals derriksite and demesmaekerite

“…The results of current study allow to expand the common crystal chemical borders of the U 6+ -bearing minerals and to estimate rather rarely studied parameters such as the strength characteristics of crystal structures. By the current study, it has been documented that uranyl complexes are one of the most stable and rigid blocks in the structural architecture of natural and synthetic uranyl compounds, and they govern the thermal behavior of the whole structure, which is in agreement with the results of recent studies (Gurzhiy et al 2018(Gurzhiy et al , 2020Kornyakov et al 2019). These conclusions are valid, regardless of: 1) the dimensionality of U 6+ -bearing substructural units, 2) both the arrangement and the chemical composition of an interlayer structure.…”

Evolution of Chernobyl Corium in Water: Formation of Secondary Uranyl Phases

“…Such a distribution most likely comes from the fact that the P1 site is more tightly bonded than the P2 site, which prevents a larger Si cation from occupying it. Similar crystal chemical restrictions for the larger Se 6+ cations incorporation in tighter S 6+ sites were observed in a course of phase formation studies in the mixed actinyl sulfate–selenate aqueous systems [ 45 , 46 , 47 , 48 , 49 , 50 ].…”

Chemically Induced Polytypic Phase Transitions in the Mg[(UO2)(TO4)2(H2O)](H2O)4 (T = S, Se) System

“…It was shown for the structures forming in the K- and Cs-bearing mixed uranyl sulfate-selenate systems ,, that there is a certain correlation between the rates of Se incorporation and topology of different T sites. The refinement of the T 6+ sites occupancies in the structures of 1 – 11 verifies the selectivity of the Se-for-S substitution.…”

“…In this paper, we have reported 10 new Mg uranyl oxysalts, and this system appeared to be another example of mixed actinyl sulfate-selenate system (for K + , for Cs + , ), that demonstrate highly complicated crystallization behavior depending upon the degree of the Se-for-S selective substitution. Although the topology of the U-bearing chains and their arrangement into pseudo two-dimensional (2D) complexes remains the same, three structure types have been described within the system that differ in the relative arrangements of the layers that can be conditionally described as belonging to 1 M , 2 O , and 2 M polytypes (in the strict sense, polytypes must have the same chemical composition).…”

“…The phenomenon of polytypism is well-known and has been studied in detail for clay minerals, 47,48 micas, 49,50 layered double hydroxides, 51−53 silicon carbides, 54 Occupancy of the T 6+ Sites. It was shown for the structures forming in the K-and Cs-bearing mixed uranyl sulfate-selenate systems 20,21,55 that there is a certain correlation between the rates of Se incorporation and topology of different T sites. The refinement of the T 6+ sites occupancies in the structures of 1−11 verifies the selectivity of the Se-for-S substitution.…”