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

424 20. Plutonium group 15 compounds and complexes20.6.1.cEnthalpy of formationThere are no experimental determinations of the enthalpy of formation of PuBi(cr),however there is an estimated value [76MIE], f Hm ◦ (PuBi, cr, 298.15 K) =−(117 ± 20) kJ·mol−120.6.2 Pu dibismuthide (PuBi 2 , cr)20.6.2.aCrystal StructureThere is no information on the crystal structure of this compound.20.6.2.bEntropyThe entropy was estimated in an analogous manner to that of PuBi(cr). The estimatedvalue is,Sm ◦ (PuBi −12, cr, 298.15 K) = (163 ± 14) J·K−1·mol20.6.2.cEnthalpy of formationThere are no direct determinations of the enthalpy of formation of PuBi 2 (cr). There aremeasurements of the partial Gibbs energy of Pu in the two phase field, PuBi 2 (cr) + Bi(cror l) using an EMF cell [69LEB/BAB] from which the Gibbs energy of formation in thetemperature range 700-900 K can be calculated. The implied value of the enthalpy offormation at these temperatures would imply that the compound has a higher stabilitythan would be expected from the trends within the actinide pnictides [81CHI/AKH].There is also an estimate of the enthalpy of formation [76MIE]: f H ◦ m (PuBi 2, cr) =−(126 ± 22) kJ·mol −120.6.2.dThe Gibbs energy of formationThe Gibbs energy of formation of PuBi 2 (cr) has been evaluated [81CHI/AKH] frommeasurements [69LEB/BAB] using a molten salt EMF cell to measure the thermodynamicactivities in Pu unsaturated and saturated solution. With these data and thesolubility of Pu in liquid Bi the estimated value of this quantity for the temperaturerange 700-900 K is f G ◦ m (PuBi 2, cr) =−(216438 − 64.48T) J·mol −1All the experimental data described above for this system require confirmation.