Crystal uranium oxides

The crystal structures of a number of diphosphine disulphides (121) and (122) show a remarkable constancy in the bond lengths. Two types of molecule are observed in the crystal of the tetramethyl compound (121, X = Y = Me). The crystal structure of triphenylphosphine oxide (P—C 176 pm, P—O 164 pm) varies little from that observed in the uranium oxide complexes, and does not confirm P—O bond lengthening in complexes, as indicated by vp=.o (see Section 3C). 

Uranium oxides are of importance in the nuclear industry, and for this reason considerable effort has been put into understanding their nonstoichiometric behavior. The dioxide, U02 crystallizes with the fluorite structure with an ideal composition MX2 (Fig. 4.7a) but is readily prepared in an oxygen-rich form. In this state it is... 

TJew references to uranium borates appear in the literature. Larson (12) reported that yellow crystals, whose composition was assumed to be 3UO3 B2O3 (uranyl orthoborate), were obtained among other products from a melt of uranium niobate in boric oxide. Bruhat and Dubois (2) stated that perborate solutions react with uranium oxide to give an anhydrous stable yellow salt of the composition UBO4. No further information has appeared on either of these compounds. 

Reaction of Aldehydes Over Titanium and Uranium Oxide Single-Crystal Surfaces... 

Senanayake SD, Chong SV, Idriss H (2003) The reactions of formaldehyde over the surfaces of uranium oxides. A comparative study between polycrystaUine and single crystal materials. Catal Today 85 311... 

More detailed experiments were conducted by Bowden and Singh [38, 39]. Crystals of lead and cadmium azides and nitrogen iodide (coated with uranium oxide) were irradiated with slow neutrons only nitrogen iodide exploded. Lead... 

Uranium oxides are considered to be difficult to produce in single crystals by chemical transport reaction, because of their large heat of formation. Naito, Kamegashira, and Nomura 405 have, however, succeeded in preparing single crystals of uranium oxides, such as U02+,(, and U30g j... 

Accordingly, the O/U composition of the crystal produced varies with chlorine pressure (403). It was shown that the nonstoichiometry of the uranium oxide single crystal could be successfully controlled. 

Besides uranium oxides, single crystals of other compounds of uranium and thorium have been prepared by chemical transport reaction jJ-USj by Smith and Cathey (407)-, USej, USej, UTcj, UTcj and USj by Slovyanskih et al. 

I submitted to a fractionated crystallization two kilograms of purified radium-bearing barium chloride that had been extracted from half a [metric] ton of residues of uranium oxide ore. ... 

Kovba L. M. Crystal structure of potassium uranium oxide K2U7O22. Zh. strukt. khim., 13, 1972, 256-9. 

In the Phanerozoic uranium oxide mineralization in granites tends to occur as a lower-temperature phase, such as low thorium pitchblende in veins, which is less resistant to weathering than euhedral thorian uraninite crystals and, hence. 

Sohd uranium—phosphate complexes have been reported for the IV and VI oxidation states, as well as for compounds containing mixed oxidation states of U(IV) and U(VI). Only a few sohd state stmctures of U(IV) phosphates have been reported, including the metaphosphate U(P03)4, the pyrophosphate U(P202), and the orthophosphate, CaU(PO4)2. The crystal stmcture of orthorhombic CaU(POis similar to anhydrite (194). Compounds of the general formula MU2(PO4)3 have been reported for M = Li, Na, and K, but could not be obtained with the larger Rb and Cs ions (195). In the sohd state, uranium(IV) forms the triclinic metaphosphate, U(P03)4. Each uranium atom is eight-coordinate with square antiprismatic UOg units bridged by... 

Uranium, too, is widely distributed and, since it probably crystallized late in the formation of igneous rocks, tends to be scattered in the faults of older rocks. Some concentration by leaching and subsequent re-precipitation has produced a large number of oxide minerals of which the most important are pitchblende or uraninite, U3O8, and camotite, K2(U02)2(V04)2.3H20. However, even these are usually dispersed so that typical ores contain only about 0.1% U, and many of the more readily exploited deposits are nearing exhaustion. The principal sources are Canada, Africa and countries of the former USSR. 

Spinels. There are limited experimental data on uranium and thorium partitioning between magnetite and melt (Nielsen et al. 1994 Blundy and Brooker 2003). Both studies find U and Th to be moderately incompatible. Blundy and Brooker s results for a hydrous dacitic melt at 1 GPa and 1025°C give Du and D h. of approximately 0.004. The accuracy of these values is compromised by the very low concentrations in the crystals and the lack of suitable SIMS secondary standards for these elements in oxide minerals. Nonetheless, these values are within the range of Djh of magnetites at atmospheric pressure 0.003-0.025 (Nielsen et al. 1994). It is difficult to place these values within the context of the lattice strain model, firstly because there are so few systematic experimental studies of trace element partitioning into oxides and secondly because of the compositional diversity of the spinels and their complex intersite cation ordering. 

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