Electrical conductivity and enthalpies of transformation in Li2UBr6 between 500 and 800 K. Comparison with Na2UBr6

Abstract

As with Na₂UBr₆, previously investigated, Li₂UBr₆ 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 Li₂UBr₆ the thermal effects before fusion are insignificant and are much smaller than those found for Na₂UBr₆(1.7±0.4 kJ mol^–1 at 505±2 K and 0.35±0.15 kJ mol^–1 at 740±2 K), while the enthalpy of fusion is larger (48±2 kJ mol^–1 at 781±2 K). For both compounds, comparable values were found by summing the entropies of transformation (61.9 J mol^–1 K^–1 for Na₂UBr₆ and 65.2 J mol^–1 K^–1 for Li₂UBr₆).

The variation of electrical conductivity with temperature is very different for these compounds. While continuous evolution is observed for Li₂UBr₆, each transition of Na₂UBr₆ corresponds to a jump in conductivity.

The results obtained for Li₂UBr₆ are discussed by comparison with those previously obtained for Na₂UBr₆.