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In the course of a systematic investigation of some compounds of uranium (IV), several physico‐chemical methods have been applied to the salts of the series UX4 and M2UX6 (with X = Cl, Br, I and M = Li, Na, K, Rb, Cs). ‐ The results obtained in the present work refer to UCl4; they have been obtained by electrical conductivity measurements, by differential thermal analysis and by differential enthalpic analysis.

As with Na,UBr,, previously investigated, Li,UBr, undergoes several transitions before melting. These were studied using electrical-conductivity measurements, differential thermal analysis and differential enthalpic analysis performed in the temperature range 500-800 K.For Li,UBr, the thermal effects before fusion are insignificant and are much smaller than those found for Na,UBr, (1.7 f 0.4 kJ mol-l at 505 & 2 K and 0.35 f 0.15 kJ mol-1 at 740 2 K), while the enthalpy of fusion is larger (48 f 2 kJ mol-' at 781 k 2 K). For both compounds, comparable values were found by summing the entropies of transformation (61.9 J mol-l K-' for Na,UBr, and 65.2 J mol-1 K-l for Li,UBr,).The variation of electrical conductivity with temperature is very different for these compounds. While continuous evolution is observed for Li,UBr,, each transition of Na,UBr, corresponds to a jump in conductivity.The results obtained for Li,UBr, are discussed by comparison with those previously obtained for Na,UBr,.

Calorinietr); / I . Crystal StructureUsing several experimental techniques (electrical conductivity measurement, differential thermal analysis and differential enthalpic analysis), the LizUC16, NazUC16 and Cs2UC16 compounds have been investigated. -The joint examination of thermograms and phase diagrams suggests that the fusion of Li2UCI6 and Na2UCI6 compounds is preceeded by small thermal effects which are not always reproducible. with AH&,, = 40.9 f 0.3kJ -mol-'These new results were compared with those previously found for similar compounds. The calorimetric and electical experimental values obtained for Li2UBr, and Li,UCI, allowed a parallelity to be established: the very close values of entropy of fusion, 61 and 57 J'rnol-l K -I, respectively, suggest that the melting mechanism is similar for both compounds. -O n the other hand, in spite of similar entropies of fusion, Li,UCI6 and NazUC16 exhibit a strongly different behaviour of the electrical conductivity. The electrical properties of Li2UCI6, NazUCl, and Cs2UC16 were also tentatively compared with those of Li2UBr6 and Na2UBr6 but only a qualitative interpretation can be put forward. -However the similar premelting eNects and entropies of fusion (13.8 and 15.8 J -m o l -l . K -' respectively) found for NalUBr6 and cs2uc16 suggest a similar melting mechanism for both compounds. Therefore high temperature structural investigations would be essential and the clue to the electrical and thermodynamic behaviours of Li2UC16, Na2UCI6 and Cs2UCl6.

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