Standard entropy, heat capacity and enthalpy of formation

1 Standard entropy, heat capacity and enthalpy of formation 

Thorium dicarbide has three polymorphs. Because of their range of homogeneity, unique transformation temperatures cannot be given those quoted below are for the phase in equilibrium with carbon. The low temperature form, stable below (1690 + 40) K, is monoclinic, space group C2/c (CuO type) with a = (6.692 + 0.003), b= (4.223 +0.003), c = (6.744 + 0.003) A, / = (103.12 + 0.11)° [1968BOW/KRI] at room temperature, as determined by neutron diffraction. These results confirmed the lattice parameters and space group established earlier by Hunt and Rundle 

Takahashi et al. [1965TAKAVES2] and Westmm et al. [1965WES/TAK2] have measured the low-temperature heat capacities of two samples of the dicarbide, with compositions of ThCi 9s(cr) and ThCi 93(01) respectively. The former sample probably contained some free carbon, but the consistent values of ((298.15 K) - S (() K)) of (68.6 + 0.3) J K mol from the two studies has been taken to refer to the ThCi.94(cr) composition. The assumption that this phase contains randomly mixed C and C2 groups then gives a randomisation entropy of (ThQ 94, cr, 0 K) = (1.9 + 0.3) J-K -mol, to give  

Holley et al. [1984HOL/RAN] give another (five-parameter) estimate by comparison specifically with uranium dicarbide (which has a pronounced upward curvature in the heat capacity at high temperatures). For convenience, their equation has been refitted to two extended Kelley equations to conform to NEA usage 

These equations have been used in the analysis of the Gibbs energy data, but are not selected. 

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