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

422 20. Plutonium group 15 compounds and complexes20.5 The plutonium-antimony systemTwo compounds are reported for this system, Pu monoantimonide (PuSb, cr) and Pudiantimonide (PuSb 2 ,cr)20.5.1 PuSb(cr)20.5.1.a Crystal structurePuSb has a face-centred cubic rock-salt structure (space group Fm3m) with reportedlattice parameters, a 0 = 6.2405±0.0005×10 −10 m[67KRU/MOS], and a 0 = 6.2397±0.0001 × 10 −10 m[95ARA/OHM]. The face-centred cubic structure transforms to thecubic CsCl structure at pressures above 17 GPa and at 42 GPa to a tetragonal structurewith space group P4/mmm [90DAB/BEN].20.5.1.b Heat capacityThe heat capacity of PuSb has been measured from below 20 K using a transient techniqueand from 13 to 300 K using an adiabatic method [90HAL/MOR]. The samplewas annealed at 1273 K for 2 h in argon in order to remove the radiation damage.Transitions were found at 75 K and 87 K and the values of the entropy associated withthese transitions are 1.51 kJ·mol −1 and, 0.76 kJ·mol −1 respectively.A decrease in transition temperatures with irradiation damage has been reported[88BLA/HAL], [86HAL/JEF].The electronic component of the heat capacity (γ )is5± 5J·K −2·mol−1 .The standard entropy was calculated from the measurement of heat capacity[90HAL/MOR]as:C ◦ p,m(PuSb, cr, 298.15 K) = (52.8 ± 3.5) J·K−1·mol−1S ◦ m(PuSb, cr, 298.15 K) = (106.9 ± 7.5) J·K−1·mol−120.5.1.c Enthalpy of formationThere are no measurements of the enthalpy of formation of PuSb(cr), but an estimatecan be made from the data for USb(cr ) and other systematic trends for the actinidepnictides. The estimated value is, f Hm ◦ (PuSb, cr, 298.15 K) =−(150 ± 20) kJ·mol−1Another estimated value [76MIE] lies within the above uncertainty band.20.5.2 Plutonium diantimonide, PuSb 2 (cr)There is no structural information for this compound and the thermodynamic informationis sparse.