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

138 9. Neptunium group 17 (halogen) compounds and complexesWith these values the pressures of NpF 6 (g) in the above decomposition reaction are ca.3 × 10 −5 bar at 400 K and 5 × 10 −3 bar at 500 K, in accord with the observation ofMalm et al.[93MAL/WIL].9.1.2.7 NpF 6 (cr)Osborne et al.[70OSB/WEI] have given an almost complete, consistent description ofthe relative properties of NpF 6 (cr, l, g) based on their measurements of the low temperatureheat capacity (7 to 350 K) and the vapour pressure measurements by Weinstock etal.[59WEI/WEA] from 273.15 to 350 K. Since there has been no other thermodynamicwork on this compound, we have accepted the values of Osborne et al. [70OSB/WEI]with only essentially trivial updating, but have also ventured to estimate the enthalpiesof formation of the crystal and gas.9.1.2.7.aEnthalpy of formationThe enthalpy of sublimation of NpF 6 (cr) is well defined below, but the individualenthalpies of formation of the solid and gas are not known accurately.Approximate values have been estimated by interpolating the difference f Hm ◦ (MF 6, cr) − f Hm ◦ (MO2+ 2, aq) for M = U, Np, Pu, using the ionic radii ofthe M(VI) ions (for co-ordination number VI) [76SHA] as the interpolator. Sincethese values are 0.73 × 10 −10 m, 0.72 × 10 −10 mand0.71 × 10 −10 m for U, Npand Pu, the value of the enthalpy difference, f Hm ◦ (NpF 6, cr) − f Hm ◦ (NpO2+ 2 , aq),for Np becomes the mean of the corresponding values for U and Pu, namely−(1109.0 ± 15.0) kJ·mol −1 , where the uncertainty is estimated here. With f Hm ◦ (NpO2+ 2 , aq, 298.15 K) =−(860.7 ± 4.7) kJ·mol−1 selected by this review,this gives finally f Hm ◦ (NpF 6, cr, 298.15 K) = −(1969.7 ± 15.7) kJ·mol −1 . Theselected value is the rounded value, with a somewhat increased uncertainty f H ◦ m (NpF 6, cr, 298.15 K) = −(1970 ± 20) kJ·mol −19.1.2.7.bStandard entropy and heat capacityThe low temperature heat capacity of NpF 6 (cr) has been measured by Osborne et al.[70OSB/WEI] from 7 to 350 K. No anomalies were observed in the heat capacity curveover this temperature range. Because NpF 6 (cr) has one non-bonding electron, therewill be a twofold degeneracy in the ground-state, and it is anticipated that at sometemperature below the current range of measurements, there will be a co-operativephenomenon to remove this degeneracy. Thus a term of R ln 2 has been added to theentropy obtained by extrapolation of the observed heat capacity curve from 7 to 0 K.The validity of this addition is confirmed by the excellent agreement for the entropy ofthe gas as calculated from the calorimetric data and from statistical mechanics, as describedin the following section (discussion on NpF 6 (g)). The derived standard entropy