Uranium structural parameters

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. 

Spectral parameters of the structured green luminescence (Fig. 4.4d) are absolutely similar to those of luminescence in fluorite after thermal treatment (Tarashchan 1978). Principally, during the calcination of the sedimentary phosphates new mineralogical phases, including fluorite, may be formed. Taking these data into accoimt, it is possible to conclude that after thermal treatment uranium is concentrated in the fluorite lattice in the form of... 

Between the two possible defects which may be responsible for hyperstoichiometry (i.e. uranium interstitials or oxygen vacancies) the latter is well evidenced by measurements of lattice parameter and densityand neutron diffraction Oxygen interstitials order in U4O9 to provide a crystal structure which can be derived from the fluorite structure of U02+x-... 

Uranium(IV) oxide, UOj, crystallizes with the fluorite structure as shown in Figure 6.5. On heating in oxygen, additional oxygen can be taken up by the lattice to form the non-stoichiometric material 002+. There is a gradual increase in the lattice parameters as additional oxygen is added to the structure. 

Energy-band structure studies have been reported on bee 7-uranium metal only the room-temperature phase has a low symmetry which adds to its complexity. The calculations have been performed for three possible configurations,/ d s, f d and f d s, and with two values of the exchange parameter, a = 1 and a = 2/3 (40). 

To the defect structure of UO2. Aitken et al. (19) applied statistical mechanical calculations based on the relation derived by Anderson (249) and evaluated the energy parameters. They suggested the possibility of an ordering of vacancy pairs in hypostoichiometric uranium oxide, in view of the relatively large interaction energy between vacancy pairs. Similar treatment for the hypostoichiometric U02 j phase also has been reported by Winslow (259). 

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