Polytypism and oxo-tungstate polyhedra polymerization in novel complex uranyl tungstates

Abstract

The chemistry of actinides is an important and currently active field of inorganic chemistry. Actinides and their compounds play a prime role in nuclear waste management and safety assessment. [1][2][3] In particular, investigations of complexation of uranyl ions 4-6 with oxygen-based complexes consisting of highvalent elements such as S, Cr, Se, Mo or W have high relevance for nuclear waste disposal, owing to their use in nuclear engineering (e.g., Mo 7 and W 8 are used in refractory alloys) or due to their formation in burn-up processes of nuclear fuel (Mo, Se). 9 Actinide sulfates, chromates and selenates are known to form diverse and complex structures with such interesting structural features as cation-cation interactions, microporous frameworks and nanotubules. [10][11][12][13][14][15][16] Actinide molybdates and tungstates may adopt structure types more resembling dense complex oxides than oxosalts. 4 In comparison to molybdates, uranyl tungstates are less studied with relatively few reports available in the literature. 17 The mixture was heated to 820°C for 4 h and cooled down to room temperature with the rate of 7°C h −1 . The obtained product consisted of yellowish crystals of 1 and 2 immersed within an amorphous yellow solid. Powder samples consisting of both 1 and 2 phases were prepared by heating a mixture of CsNO 3 (0.273 g, 1.40 mmol), UO 3 (0.400 g, 1.40 mmol) and WO 3 (0.324 g, 1.40 mmol) in a 1 : 1 : 1 molar ratio at 750°C for 72 h in a porcelain crucible. X-ray powder diffraction indicated that the sample is a mixture of α-and β-Cs 2 [(UO 2 ) 2 (W 2 O 9 )] in a ratio 1 : 4 (Fig. S1 †). Our attempts to obtain pure powders of either α-or β-Cs 2 [(UO 2 ) 2 (W 2 O 9 )] were not successful. Single crystals of 3 were obtained by high-temperature solid-state reaction of an initial mixture of RbNO 3 (0.103 g, 0.70 mmol), UO 3 (0.200 g, 0.70 mmol) and WO 3 (0.162 g, 0.70 mmol) with a Rb : U : W ratio equal to 1 : 1 : 1. The mixture was slowly heated to 870°C in a porcelain crucible and then cooled down to 400°C at a rate of 4°C h −1 . The product consisted of yellowish crystals of 3 in a glassy amorphous mass. Single crystals of the obtained phases were investigated by means of single-crystal X-ray diffraction analysis. It is noteworthy that the crystals of 1 were observed to be of considerably lower quality in comparison to the crystals of 2, which may indicate its lower stability.The structures of all three compounds are based upon 2D uranyl tungstate layers (Fig. 1). The layers in these structures are based on UO 2 2+ linear groups linked with oxo-tungstates polyhedra. In all three structures, U 6+ cations form linear uranyl groups, [OvUvO] 2+ , equatorially coordinated by five O atoms. In the structures of 1 and 2, UO 7 pentagonal bipyramids share edges to † Electronic supplementary information (ESI) available: Includes powder diffractograms and IR spectra. CCDC 881326-881328. For ESI and crystallographic data in CIF or other electronic format see

Hydrothermal syntheses and characterization of uranyl tungstates with electro-neutral structural units

“…Although more than twenty synthetic uranyl tungstate compounds are described in the literature [2][3][4][5][6][7][8][9][10][11][12][13][14], there are no reports of oxy-fluoride uranyl tungstates. The fluoride anion from hydrofluoric acid is commonly used in hydrothermal synthesis reactions as a mineralizer [25,26].…”

“…The structural complexity of uranyl compounds with group (VI) metals offers insights into the relationships between cation coordination, structure type, and electronic configuration [2]. The crystal chemistry of uranyl tungstates is particularly rich, with several reported structures having novel connectivities [2][3][4][5][6][7][8][9][10][11][12][13][14]. Most reported uranyl tungstates were obtained by high-temperature solid-state reactions using UO 3 , WO 3 , and alkali metal salts as starting materials [2-8, 10-12, 14].…”

A calorimetric and thermodynamic investigation of potassium uranyl tungstate K2[(UO2)(W2O8)]

“…In contrast to uranyl molybdates, uranyl tungstates have been less studied with relatively few reports in the literature [17][18][19][20][21][22][23][24][25]. Within the family of known uranyl tungstates the group with the general formula A 2 [(UO 2 )(W 2 O 8 )], where A = Li, Na, K and Ag, is the largest one.…”

Studies on thermophysical properties of ThW2O8 and UWO6

“…Structural studies of stable actinide compounds and other inorganic compounds containing hexavalent cation such as Mo 6? and W 6? are relevant to the safe disposal of nuclear waste [8]. These compounds are likely to be formed during burn-up of nuclear fuel or during alteration of spent nuclear fuel and thus have high relevance for nuclear waste management and safety assessment [9]. Tungsten has very high neutron displacement energy and melting temperature (3665 ± 14 K).…”