UF4 has been studied extensively. The heat of forma­tion of UF4 has been measured using fluorine bomb calorimetry by Hayman40 at 315 K (without tak­ing into account the impurities in the samples), by Wijbenga,41 and by Johnson.42 The value obtained by Hayman was recalculated by Wijbenga considering the impurities and the value of the heat of formation of UF6(s) measured by Johnson.26

Mal’tsev et a/43 determined the heat of forma­tion of the highest crystal hydrate of uranium tetrafluoride, UF4-2.5H2O, as the sum of the heats of nine reactions. Using the heat of hydration of UF4-2.5H2O given by Popov et a/.,44 Mal’tesev et a/. deduced the heat of formation of the anhydrous tet — rafluoride. Several works reported the enthalpy of formation by solution calorimetry:

• At first, Khanaev et a/45 used various (HCl + H3BO3 + FeCl3) aqueous solutions at 323 K.

• A solution of hydrofluoric acid, hydrochloric acid, and aqueous aluminum chlorate AlCl3-6H2O was used by Hu et a/.46

• Finally, Cordfunke et a/47 have chosen a mixed aqueous solvent containing sulfuric acid, boric acid, and ceric sulfate.

The results obtained are widely scattered from (—1884.9 ± 2.9) to (—1921.3 ± 4.2) kJ mol-1.

The selected value came from the IAEA review,37 which is a weighted average of the values published

Table 2 Thermodynamic properties of the crystalline uranium tetrafluoride

AfH° (UF4, cr, 298.15 K) (kJ mol-1) S0 (UF4, cr, 298.15 K) (J K-1 mol-1) Cp (UF4, cr, 298.15) (J K-1 mol-1) Cp (UF4, cr, T) (J K-1 mol-1)

by Johnson42 using fluorine combustion calorimetry (with the selected enthalpy of formation of UF6(s)) and Cordfunke et a/.47 using solution calorimetry (with the selected enthalpies of formation of HF (aq), U3O8(s), and g-UO3)24 The values differ by 10.7 kJ mol-1, therefore the NEA-TDB24 recom­mended further measurements to resolve the discre­pancies in the experimental values.

2.06.3.2.2.1 Heat capacity

The low-temperature heat capacity of crystalline uranium tetrafluoride was measured by:

• adiabatic calorimetry by Brickwedde et a/25 from 20 to 350 K;

• adiabatic calorimetry by Osborne et a/.48 between 5 and 300 K;

• and isothermal calorimetry by Burns et a/.49 in the 1.3-20 K temperature range.

The values are very close except for the values of Brickwedde eta/.25 extrapolated at T< 15 K (Figure 8).