Low-temperature heat capacity of a-UC2-x was studied by adiabatic calorimetry between 5 and 350 K,8 yielding the already reported temperature coefficient of the electronic heat capacity, and values of the Debye temperature 0D of 302 for UC190 and 304 K for UC1.94.

Assessed thermodynamic functions for a-UC194 are reported in Table 6. The randomization entropy of UC1.90 was calculated by Storms2 taking into account the disordered and incomplete sublattice: S° = 2.7J K-1 mol-1.

Heat capacity measurements at higher tempera­tures could be affected by partial decomposition of a-UC2 into UC and C below 1790 K. Results were critically assessed by Holley et a/.4 (Table 6 and Figure 15), as well as the latent enthalpy and entropy of the UC2-x a! p transition.

Fink et a/.43 proposed a value for the enthalpy of solid p-UC2-x at the melting point, H°(2720) — H°(298) « 256 kJ mol-1.

The Gibbs energy of formation values given by Holley et a/, for UC2-x are higher than those of Chevalier and Fischer.101 The expression in Table 6 is recommended here.

A complete treatment of the vapor pressures over the UC 1+x-UC2-x phase field is reported by Holley. Norman and Winchell144 proposed the following values for the enthalpy and entropy of sublimation of p-UC2-x at the average T = 2500 K: AsubH° (2500) « 774 kJ mol-1 and AsubS° (2500) « 160 kJmol-1 K-1.

The a-UC2-x low-temperature values for the average linear thermal expansion coefficients yielded by neutron diffraction spectra at temperatures between 5 and 298 K127 were aTa = (9 ± 1)x 10-6K-1 and aTc = (6± 1)x 10-6K-1. These values for a-UC2-x

were observed to increase at higher temperature, aTa = (16.6± 0.2)x 10-6K-1 and aTc = (10.4± 0.2)x 10-6K-1 between 298 and 1208 K. Neutron scatter­ing studies between 1273 and 2573 K yielded the following average coefficient of thermal expansion for quasi-isotropic a-UC2: aTa = 18×10-6K-1

(up to 2050 K) and aTa = 25×10-6K-1 for p-UC2 at higher temperature. The coefficient of average cubic thermal expansion for p-UC2-x between 298 K and the melting point was suggested by Storms[3] [4] [5] as g = 4x 10-5 K-1.

The bulk modulus of a-UC2-x was measured to be B = 216 GPa.145