The thermodynamic functions of gaseous actinide elements. Effects of unobserved energy levels

The actinide gases have a large number of unobserved energy states - up to 3 x 106 for Pu(g) - which could contribute to the partition function and its derivatives, from which the thermal functions of these gases are calculated. Existing compilations have simply ignored these levels. By making reasonable assumptions as to the distribution of these energy states, their effect on the functions can be calculated. It is concluded that the existing compilations will be inadequate above approximately 2000 K. The effect is particularly marked on the heat capacity. For example, when unobserved levels for Pu(g) are included, the heat capacity of Pu(g) reaches a maximum value of more than 12R at 3200 K. Similar considerations will apply to the gaseous actinide ions. (author)