Flux Growth of Uranyl Titanates: Rare Examples of TiO₄ Tetrahedra and TiO₅ Square Bipyramids

Abstract

Single crystals of four new uranyl titanates have been grown via the flux growth method using mixed alkali halide fluxes. Na 2 (UO 2 )(TiO)O 3 and KNa(UO 2 )(TiO)O 3 have analogous layered structures containing titanyl (TiO 2+ ) units coordinated into TiO 5 square pyramids. Cs 2 (UO 2 )TiO 4 crystallizes in the Cs 2 USiO 6 structure type and is a rare example of a structure containing TiO 4 tetrahedra. Cs 2 (UO 2 )Ti 2 O 6 crystallizes in a new tunnel structure and contains the also rare TiO 5 trigonal bipyramids. DFT studies were performed to understand the bonding in the observed titanate polyhedra. Furthermore, the luminescence properties of the compounds are reported, and leaching studies are reported for Cs 2 (UO 2 )Ti 2 O 6 .

Uranyl Titanate Silicates: Syntheses, Structures, and Family Relations

“…A similar arrangement has been observed in other silicates including Cs 6 Si 10 O 23 and K 3 (NpO 2 ) 3 (Si 2 O 7 ) . While the average M–O bond distances for SiO 4 and TiO 4 tetrahedra differ by ∼0.2 Å, , no split oxygens are observed in ( 3 ) and ( 4 ) with instead the observed oxygens having intermediate M–O bond distances between those expected for Si and Ti tetrahedra. Shown in Figure a, the ADPS of the mixed Si/Ti atoms and three of the four coordinated oxygens in ( 3 ) are larger than typical, suggesting structural distortions to accommodate the different preferred M–O bond distances of Si and Ti.…”

“…This is the result of the lack of tilting of the uranyl columns. This can be seen by comparison with CsUSiO 6 and CsUTiO 6 , 12,13 both of which contain columns of corner-sharing uranyl square bipyramids connected by M 4 O 12 square units. Illustrated in Figure 9, in these structures the M 4 O 12 units are larger than the distance between U atoms in the uranyl column, forcing the uranyl polyhedra to tilt away from the M 4 O 12 units.…”

“…We have previous extensively studied the uranyl silicates, − uranyl titanates, and titanate silicates. , All three classes of materials were found to readily form using mixed alkali fluoride/chloride fluxes. Within uranyl silicates and titanates, uranyl cations predominately adopt square bipyramidal geometries, ,,, silicon universally forms SiO 4 tetrahedra, and titanium typically prefers five-coordinate or octahedral coordination environments, , although in rare cases it is known to form TiO 4 tetrahedra. ,− The suitability of mixed halide fluxes to uranyl, titanate, and silicate species, and the different coordination preferences for each species provides the alkali uranyl titanate silicates the potential to form diverse and complex structures. This prompted us to explore the uranyl titanate silicates via flux growth.…”

“…This can be observed in the uranyl phosphates, where the extended phosphuranylite structure is generated from the phosphuranylite structure by insertion of uranyl square bipyramids into the uranyl chains. 6 We have previous extensively studied the uranyl silicates, 7−12 uranyl titanates, 13 and titanate silicates. 14,15 All three classes of materials were found to readily form using mixed alkali fluoride/chloride fluxes.…”

“…The shared U−O−U bonds are stronger than typical equatorial U−O bonds, resulting in weaker uranyl bonds. 13,37 Indeed, the emission spectra of Cs (2), which contain isolated UO 6 square bipyramids.…”