Uranium trihalides

Only a few X-ray diffraction and XAFS studies at high temperatures can be found in the literature mainly due to technical difficulties in experimental work. E.g. Okamoto et al. (1998, 1999) measured the X-ray diffraction of some molten rare earth and uranium trihalides. The obtained X-ray diffraction data were analyzed using molecular dynamics technique. This procedure is almost standard in the structural analysis of molten salt systems. 

The only reported complexes are those with uranium and plutonium trihalides. The products of composition UCl3-(6.8-6.9)NH3 and UCl3-(7.0-7.4)NH3 lose ammonia at 20 or 45 °C to... 

The compounds known are summarized in Table 10.1. The only compound of an early actinide in the -1-2 state is Thl2, a metallic conductor which is probably Th + (e )2 (D)2-Certain heavier actinides form MX2 (Am, Cf, Es), which usually have the structure of the corresponding EuX2 and are thus genuine M + compounds. All four trihalides exist for all the actinides as far as Es, except for thorium and protactinium. Tetrafluorides exist for Th-Cm and the other tetrahalides as far as NpX4 (and in the gas phase in the case of PuCE). Pentahalides are only known for Pa, U, and Np whilst there are a few MFe (M = U-Pu), uranium is the only actinide to form a hexachloride. The known actinide halides are generally stable compounds most are soluble in (and hydrolysed by) water. 

UF3 is the most difficult of the trihalides to obtain in the pure state. It can be obtained by the reduction of uranium(IV) fluoride by aluminum (137), hydrogen (138-141), or uranium metal (139, 141-144). Reduction by aluminum can be carried out at 1173 K, when the aluminum fluoride can be sublimed off as the monofluoride. But the product is not very pure. 

With the exception of thorium, the actinides form trihalides. For uranium and neptunium, reduction of the MX4 compounds with hydrogen is necessary, but for the elements from plutonium onwards the action of the carbon tetrahalide or aluminium halide on the dioxide is usually employed. The trifluorides are insoluble but the rest dissolve to give solutions containing ions. 

Isomorphism among compounds of the actinides is common and only a few examples need be given. The dioxides, MO2, of thorium, uranium, neptunium, plutonium and americium all have a fluorite lattice. The trihalides of the transuranic elements are isomorphous not only with the corresponding trihalides of actinium and uranium but also with those of the lanthanides. Isomorphism is also exhibited in many complex halides thus thorium, ura-... 

Halides of uranium. Our present knowledge of the structures of the majority of the 5f halides is summarized (with references) under Trihalides, Tetrahalides, etc. in Chapter 9. The halides of U include ... 

Trichlorides, tribromides, and tri-iodides of all elements from uranium through einsteinium are known. Solution-calorimetry enthalpies of formation are known for these trihalides of uranium through plutonium, although that of NpCl3 requires an estimate of its heat of solution [20]. The enthalpy of formation of AmCl3 is known from solution calorimetry that of other americium trihalides as well as heavier trihalides must be estimated. 

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