Dissociation Energy of Diatomic Cerium and Predicted Stability of Gaseous Intermetallic Cerium Compounds

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Abstract
The Knudsen‐cell mass spectrometric method has been used to study gaseous equilibria over the Au–Ce–CeS–BN–C system. The third‐law enthalpies, ΔH 0 ° , in kcal mole−1, of the reactions: (1) 0.5 Ce2(g)+0.5Au2(g)=CeAu(g) (2) Ce2(g)+Au(g)=CeAu(g)+Ce(g) and (3) Ce2(g)=2 Ce(g), were found to be 24.7 ± 0.14, 23.0 ± 0.16 and 58.0 ± 1.5, respectively. The corresponding second‐law value for Reaction (3) is 53.4 ± 10. The reactionenthalpies for Reactions (1) and (2) were combined with the literature value for D 0 °( Au 2 ) and the value of D 0 °( CeAu ) obtained in the present investigation to yield the dissociation energy of diatomic cerium. The selected value for the dissociation energy of Ce2 is D 0 ° = 57 ± 4 or D 298 ° = 58 ± 4 kcal mol −1 . Using the latter value and the literature value for the standard heat of sublimation of cerium, ΔH υ,298 ° = 101 ± 1 kcal mol −1 , the standard heat of formation, ΔH f,298 ° , of diatomic cerium has been calculated as 144 ± 6 kcal mol−1. The stability of selected gaseous intermetallicceriumcompounds has been predicted on the basis of the Pauling model of a polar bond and the experimental value for the dissociation energy of diatomic cerium. A reassessment of the dissociation energies, D 0 ° , of LaAu, CeAu, PrAu, and NdAu [J. Chem. Phys. 52, 2956, (1970)] resulted in the values of 79.5 ± 5, 77.0 ± 3.5, 72.0 ± 5 and 70.6 ± 5 kcal mol−1, respectively.