Thermodynamic properties and decomposition of lithium hexafluoroarsenate, LiAsF6

The heat capacity of lithium hexafluoroarsenate is determined in the temperature range 50-750 K by adiabatic and differential scanning calorimetry techniques. The thermodynamic properties of LiAsF6 under standard conditions are evaluated: C-p(0) (298.15 K) = 162.5 +/- 0.3 J/(K mol), S-0(298.15 K) = 173.4 +/- 0.4 J/(K mol), Phi(0)(298.15 K) = 81.69 +/- 0.20 J/(K mol), and H-0(298.15 K) - H-0(0) = 27 340 +/- 60 J/mol. The C-p(T) curve is found to contain a lambda-type anomaly with a peak at 535.0 +/- 0.5 K, which is due to the structural transformation from the low-temperature, rhombohedral phase to the high-temperature, cubic phase. The enthalpy and entropy of this transformation are 5.29 +/- 0.27 kJ/mol and 10.30 +/- 0.53 J/(K mol), respectively. The thermal decomposition of LiAsF6 is studied. It is found that LiAsF6 decomposes in the range 715-820 K. The heat of decomposition, determined in the range 765-820 K using a sealed crucible and equal to the internal energy change DeltaU(r)(T), is 31.64 +/- 0.08 kJ/mol.