The crystal chemistry of uranium

The topic of this chapter is crystal chemistry of uranium vanadates. It should be noted that the cation often has tetrahedral coordination and forms compounds that are isotypic to phosphate and arsenate analogs. However, in its association with the (U02) uranyl ion, vanadium adopts different coordination environments resulting in interesting compounds that do not have equivalents among phosphates and arsenates. 

This book is a series of reviews of recent results on the state-of-art of structural and crystal chemistry of uranium and transuranium element compounds. 

The hexavalent-uranium phosphate minerals (Table 14) are important and widespread uranyl-oxysalt minerals. Their structures and behavior are dominated by the crystal chemistry of the (U 02) uranyl group they have been described in detail by Burns (1999) and will not be considered any further here. 

Mikheev (1988,1989,1992) has obtained extensive evidence through cocrystallization that almost all the tripositive lanthanide (except for Sm, Eu, Tm and Yb) and actinide ions (U, Np, Pu, Cm, Bk) can be reduced and have (M /M ) in the neighborhood of — 2.5 to — 2.9 V. The lanthanide potentials are not consistent with the experimentally confirmed generalized f electron energetics scheme developed by Nugent (1975). The potentials are not consistent with potentials inferred from pulse-radiolysis studies (Sullivan et al. 1976,1983,1988). If the potentials (M /M ) proposed by Mikheev for uranium, —2.54 V, and plutonium, —2.59 V, at macroscopic concentrations (Mikheev etal. 1991) were correct, phase diagram studies and electrochemistry ih molten salts should have revealed the ions and Pu ", but no evidence other than cocrystallization has been presented. In fact, the crystal chemistry of the reduced uranium halide NaUjCl (Schleid and Meyer 1989) is consistent with ions and metallic electrons. The cocrystallization model (Mikheev and Merts 1990) may not be transferable to aqueous solution and thus Mikheev s (M /M ) potentials are not cited in table 5. 

Chemically, the uranyl silicates form three groups depending on the uranium/silicon ratio. The most populated group, the 1 1 group, is one of the best studied. Stohl and Smith and Sidorenko and co-workers reviewed the crystal chemistry of these minerals. They showed that all 1 1 minerals have essentially the same basic structural unit [(U02)Si04]J" , an infinite chain of edge-shared uranyl pentagonal dipyramids and silicate... 

The uranyl vanadates form mineral groups distinct from the phosphates and arsenates because of the markedly different chemistry of the vanadium ion. Like uranium, vanadium shows several valence states in nature, and its detailed mineralogy is very complex. The crystal chemistry of vanadium was reviewed by Evans.In its lower valence states it forms distinct vanadium minerals, but in its higher valence state 5-1- it com-... 

Bums PC, Ewing RC, Hawthorne FC (1997) The crystal chemistry of hexavalent uranium polyhedron geometries, bond-valence parameters and polymerization of polyhedra. Can Mineral 35 1551-1570... 

Coordination compounds of diphosphazane dioxides with uranyl or thorium ions were synthesized [475], The crystal structure of [U02(N03)2L1] [L, = Ph2P(0) N(Ph)P(0)Ph2] reveal the bidentate chelating mode of binding of the diphosphazane dioxide to these metals. The chemistry of other uranium organophosphorus and related compounds is described [476-479]. Some of the actinide complexes are presented in Table 5.16. 

Uranium hexachloride is a black solid melting at 177.5°. Since it is hygroscopic and reacts vigorously with water, it should be handled only in dry-boxes. The crystal structure has been determined hexagonal symmetry, space group D a-C7> (m, n = 3), with an almost perfect octahedron of chlorine atoms around each uranium atom. Uranium hexachloride can be sublimed at 75-100° at low pressures, but normally some thermal decomposition results. The ultraviolet-visible spectrum of gaseous uranium hexachloride has been determined. No fine structure was observed in the spectrum. Because previously available preparative methods were inadequate, there has been very little study of the chemistry of uranium hexachloride. It reacts with hydrogen... 

Lead is a member of Group IVB of the periodic table with two oxidation states (Pb and Pb ), but the chemistry of the element is dominated by Pb ion. Lead has four isotopes with three of them being the tdtimate decay products of uranium and and, therefore, widely used in geological dating. The crystal structure of lead in solid form is face-centered cubic (FCC) with a lattice parameter of 4.95 A at 20°C. Lead is a blue-gray metal with density (11.3 g/cm ) 50 % more than that of steel and four times that of aluminum. However, lead is malleable, soft, and melts at only 327°C, and therefore, readily cut and shaped into pipes and sheets since ancient times. 

Crystal chemistry of UOs. Uranium trioxide in an aqueous slurry can exist as one of three hydrates, depending on the temperature at which it... 

Many substituted uranocenes have been made and there is a substantial body of organometallic chemistry of uranocene derivatives now known 16, 17). Some of this chemistry will be mentioned in passing but wiU not be covered in a systematic way since other reviews of the organic chemistry are available 18). The only other actinide cyclooctatetraene complex structurally characterized to date is bis[(l,3,5,7-tetramethylcyclooctatetraenyl]uranium(IV) 19), which was of interest because the presence of methyl groups allowed the planarity and relative orientation of the dianion rings to be determined. Crystal and molecular parameters for these three actinide compounds are summarized in Table 1. 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

This is a convenient, facile, and high-yield preparative route for quantitative preparation of the complexes above no special equipment is required. UI3(THF)4 crystallizes in a P2x/c space group. It is mononuclear with pentagonal-pyramidal coordination geometry around the central uranium atom. This compound is stable until 75°C, then THF molecules are removed by steps, forming UI3 at 162°C. Lewis base adducts of uranium tri-iodide, such as UI3(THF)4 above, are synthetically useful precursors for trivalent uranium chemistry (see Sec. 5.3.2) [360]. 

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