Uranyl phosphate structure

Figure 51 Phosphate coordination geometries and cocrystallized template from the structure-directed preparation of a layered uranyl phosphate (Francis, Drewitt et al. Chem. Commun. 1998, 279-280).

The structure of a-[(U02)(0H)2] [54] contains sheets of uranyl hexagonal bipyramids (Fig. 17), and the sheet anion topology consists only of hexagons. This is the only uranyl oxide hydrate that contains hexagonal bipyramids, although this coordination type is common in uranyl carbonates, uranyl nitrates, and uranyl phosphates of the phosphuranylite group. 

The uranyl phosphate chain in the structure of Cu2[(U02)(P04)2] is topologically identical to the uranyl arsenate chain found in walpurgite [104], Copper occurs in sixfold (4+2) coordination as elongated tetragonal bipyramids to form face-sharing chains that extend along the [010] direction. The chains of Cu polyhedra share vertices with adjacent phosphate tetrahedra of the parallel uranyl phosphate chains (Fig. 25). 

In the structure of autunite, Ca[(U02)(P04)]2(H20)n, (Fig. 31), Ca is in sevenfold coordination by H2O groups and is further coordinated by two O atoms of the uranyl phosphate sheet at somewhat longer distances. There are two additional H2O groups located in the interlayer where they are held in position only by hydrogen bonding [125]. 

A wider range of structural variation is exhibited by the autunite-type compounds that contain Ba. The barium uranyl phosphate decahydiate mineral uranocircite, Ba[(U02)(P04)]2(H20)io, is likely isostructural with its arsenate analogue, heinrichite, Ba[(U02)(As04)]2(H20)io, (Tables 12 and 13) [128, 131],... 

The structure of CS2UO2P2O7 consists of uranyl square bipyramids that share equatorial vertices with pyrophosphate groups to form an open sheet (Fig. 47). The Cs atoms are centered below the cavities in the sheets and occupy the interlayer region [181]. The uranyl pyrophosphate sheet in CS2UO2P2O7 can be compared to the uranyl phosphate sheet found in autunite-type compounds (Fig. 28) the pyrophosphate group of the [(U02)(P207)] sheet occupies the same position as the single phosphate tetrahedron in the autunite-type sheet, [(U02)(P04)]. 

Preparation and Characterization of Lanthanide and Actinide Solids. Crystalline / element phosphates were prepared as standards for comparison to the solids produced in the conversion of metal phytates to phosphates. The europium standard prepared was identified by X-ray powder diffiaction as hexagonal EuP04 H20 (JCPDS card number 20-1044), which was dehydrated at 204-234 °C and converted to monoclinic EUPO4 (with the monazite structure) at 500-600 °C. The standard uranyl phosphate solid prepared was the acid phosphate, U02HP04 2H20 (JCPDS card number 13-61). All attempts to prepare a crystalline thorium phosphate failed, though thorium solubility was low. In the latter case the solids were identified as amorphous Th(OH)4 with some minor crystalline inclusions of Th02. 

The preparation of uranyl phosphates has also been a subject of lively discussion. The presence of the heavy uranium atom hinders the observation of some structural modifications and the fact that there are many different complexes in solution complicates the situation. Schreyer Baes , studying the solubility of phosphate in phosphoric add in the 0.001-15 M range, have identified three stable phases of uranyl phosphate... 

FIGURE 12.19 Molecular structures of uranyl phosphate esters (a) [U02(Bu0)2P02-Bu3P0(N03)]2 and (b) [U02 (Bu0)2P02 2] . Filled circles=P, large unfilled circles=U. 

Fig. 12 Uranyl phosphate chain and sheet structure as found in 3 2 uranyl phosphates (uranium 2-5 and 2-6 polyhedra ruled and PO4 tetrahedra stippled)... 

Fig. 13 Ideal uranyl phosphate sheet structure as found in autunite and meta-autunite families, (a) (top), structure of [(U02)(T04)] sheet (b) (centre), stacking of sheets in autunite family (c) (bottom), stacking of sheets in meta-autunite family. After Beintema °...

Zolensky M. E. The crystal structure and twinning of barium uranyl phosphate hexahydrate (meta-uranocircite), and comparison to the other members of meta-autunite group. Ph.D. thesis, Pennsylvania State University, 1983. 

Emission due to excited state energy transfer has been reported in layered low dimensional rare earth complexes such as uranyl phosphates [50],cryptates [51] and platinum tetracyanides [52, 53]. The luminescence properties of the low dimensional rare earth compounds of the type RE[M(CN)2]3 where RE = Eu, Dy, Gd " and M = Au, Ag are also interesting in this regard. We recently reported [16] that efficient excited state energy transfer from the Au(CN)2 and Ag(CN)j ions to Eu " enhances the luminescence observed from the rare earth ion. A similar result is obtained from other rare earth salts [17]. In some cases vibronic structure appears in the luminescence spectrum. 

The concepts developed through the pillaring of layered silicate clays and group-IV metal phosphates should be extended to other classes of layered host structures. Some work has been initiated in the area of related layered phosphates (e.g., vanadyl and uranyl phosphates), and this approach should be extended. Layered compounds with well-defined structures should receive first priority because they present the best possibilities for a molecular design approach. 

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