chemical thermodynamics of neptunium and plutonium - U.S. ...

21.1 Plutonium carbon compounds and complexes 429Holley et al. [84HOL/RAN] discuss in detail the many measurements of the stabilityof Pu 2 C 3 (cr), including two measurements of the enthalpy of combustion. As notedearlier, the many studies of the Gibbs energy of formation at temperatures from 900 to1933 K consistently give more positive values than those calculated from the enthalpyof formation and the thermal functions. As for the monocarbide, we have acceptedthe values from the thorough analysis by Holley et al. [84HOL/RAN] (from theirTable A.6.12), which lead finally to the selected values. f Hm ◦ (Pu 2C 3 , cr, 298.15 K) = −(149.4 ± 16.7) kJ·mol −1 f G ◦ m (Pu 2C 3 , cr, 298.15 K) = −(156.5 ± 16.7) kJ·mol −121.1.1.3 Triplutonium dicarbideThis phase, whose structure is unknown, but is related to the fcc monocarbide, is foundin Pu-rich alloys below 848 K, where it decomposes into PuC 1−x and Pu(cr, bcc). Intheir study of plutonium carbides, Hall et al. [79HAL/HAI] measured the low temperatureheat capacity of two samples with compositions PuC 0.64 and PuC 0.68 ; the resultsare again shown on Figures 6.21 and 6.22 of Holley et al. [84HOL/RAN]. From thesedata, the entropy increment for the Pu 3 C 2 (cr) is interpolated to beSm ◦ (298.15 K) − S◦ −1m (0K) = (194.1 ± 4.0) J·K−1·molHolley et al. [84HOL/RAN] derive limits for the stability of this phase from the reliablephase diagram information from 300 to 850 K, which suggest fairly strongly that thisphase is completely disordered, giving the selected valuesSm ◦ (Pu −13C 2 , cr, 298.15 K) = (210.0 ± 5.0) J·K−1·mol f H ◦ m (Pu 3C 2 , cr, 298.15 K) = −(113 ± 30) kJ·mol −1 f G ◦ m (Pu 3C 2 , cr, 298.15 K) = −(123.5 ± 30.0) kJ·mol −1where the uncertainties in the enthalpy and Gibbs energies of formation include thoseof the mono- and sesquicarbides.For these calculations, the heat capacity was estimated to beCp,m ◦ (Pu 3C 2 , cr, T) = 120.67 + 4.686 10 −2 T + 1.9456 10 5 T −2 −1J·K−1·mol(298.15 to 850 K)extrapolated from the values from Hall et al. [79HAL/HAI]; the value at 298.15 K isCp,m ◦ (Pu −13C 2 , cr, 298.15 K) = (136.8 ± 2.5) J·K−1·mol21.1.1.4 Plutonium dicarbideThe invariancy involving Pu 2 C 3 (cr), PuC 2 (cr) and graphite at (1933 ± 5) K gives, withthe current data f G ◦ m (PuC 2, cr, 1933 K) = −(87.2 ± 10.0) kJ·mol −1Since there are no experimental heat capacity data for this phase, which is stable onlyabove 1933 K, no further analysis of its thermodynamic properties is warranted.