The prediction of phase diagrams of actinides is difficult because, in addition to the use of s, p and d electrons, the use of f electrons in binding by the metals from Pa to Cf must be considered. Also, the actinides have an unusual number of electronic configurations of comparable stability considering promotion energies and bonding energies. The availability of different configurations with different sizes also makes it possible to achieve higher densities than one would expect for close-packed structures. Calculations of the thermodynamic stabilities of various structures for all pure actinides will be published shortly. We are calculating the thermodynamics and phase diagrams of the binary systems. In the space available here, I will discuss only systems with no intermediate phases using a modification of the regular-solution model which is normally expressed in terms of energies of vaporization from the condensed phase to the gaseous atom in the ground electronic state, which is appropriate for organic compounds for which the energy of vaporization is a measure of the van der Waals bonding. For most metals, the energy of vaporization is not a proper measurement of bonding energy as the electronic configuration in the condensed phase is different from that of the ground state of the gaseous atom. The correct bonding energy is the difference between the energy of the condensed phase and the energy of the gaseous atom with the same electronic configuration as the condensed phase. As an example, the Delta H-298/R value of 33.4 kK for the sublimation of b.c.c. Am to the gaseous ground state (f(7)s(2)) must be increased by 29.5 kK, the promotion energy to f(6)d(2)s, to obtain the bonding energy of 62.9 kK. Of this total bonding energy, 23.8 kK is due to the s electron, 35 kK is due to the two d electrons and 4.2 kK is the contribution due to f electron bonding. Examples will be given of the use of the modified regular-solution model for calculation of phase diagrams.
