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

18.1 Plutonium halide compounds 36518.1.4.1.bStandard entropy and heat capacityThere are no low-temperature measurements of the heat capacity of PuBr 3 (cr), andthe standard entropy is derived from two reasonably consistent estimates: Latimer’smethod and the spin-only contribution [2000RAN/FUG]. From these two estimates of205.3 and 196.4 J·K −1·mol−1 , the selected value isSm ◦ (PuBr −13, cr, 298.15 K) = (198 ± 6) J·K−1·mol18.1.4.1.c High temperature heat capacityLacking experimental data, the heat capacity of PuBr 3 (cr) is assumed to be similar toUBr 3 (cr). The suggested values for PuBr 3 (cr) are:Cp,m ◦ (PuBr 3, cr, T) = (104.5 + 15.0 × 10 −3 T − 6.38 × 10 5 T −2 −1) J·K−1·molfrom 298.1 to 935 K, and henceCp,m ◦ (PuBr 3, cr, 298.15 K) =−1(101.8 ± 6.0) J·K−1·mol18.1.4.1.d Fusion dataThe melting point is taken to be (935±10) K from the analysis of the vaporisation datafor the solid and liquid measured by Phipps et al. [50PHI/SEA], as discussed in Section18.1.4.2. As noted there, this and the derived enthalpy of fusion have been givenquite large uncertainties because of the unexplained differences in the vaporisation datafor PuF 3 ,PuCl 3 and PuBr 3 . The selected values are:T fus = (935 ± 10) K fus Hm ◦ (PuBr 3, cr, 935 K) = (47.1 ± 10.0) kJ·mol −1The heat capacity of the liquid has been taken to be (150±15) J·K −1·mol −1 , basedon the values for lanthanide trichlorides and tribromides.18.1.4.2 PuBr 3 (g)18.1.4.2.a Enthalpy of formation and standard entropyPhipps et al. (50PHI/SEA) have measured the vapour pressure of PuBr 3 crystal andliquid from 795 to 1084 K, using a tantalum effusion cell. As with their studies of PuCl 3and PuF 3 , these data are precise enough to warrant correction to the 1990 Temperaturescale and the latest values of the Fundamental Constants and half-lives of the plutoniumisotopes. The least-square fits to the corrected data are:log 10 (p/bar) = −15062.2/T + 10.256