Actinide sublimation enthalpies

Table V. Melting temperatures and sublimation enthalpies of lanthanides and actinides...

From these measured properties, thermodynamic state functions (enthalpies of sublimation, entropies, and free-energy functions) have been derived. For several actinides, these properties have been critically reviewed by Hultgren et al. [24] and more recently by Getting, Rand, and Ackermann [14]. The latter compilation includes properties for the metallic state and for ideal gas to 5(XX) K for the elements Th through Cm. Sublimation enthalpies (determined from measured vaporization behavior for Th through Es) and standard entropies (determined from measured heat capacities for Th through Am) of the actinide metals are compiled in Table 17.1. Sublimation enthalpies have been correlated with metal structures and electronic energy levels [27,28]. 

The influence of unoccupied 5/band states on the Fermi surface of thorium metal has been evaluated from precise calculations of the Fermi surface areas and effective masses.Comparison of the results with those obtained from experiments showed agreement when the 5/electrons were treated as itinerant. The nature of the bonding, electronic configurations and degree of localization of 5/ electrons in the actinide elements have been described in terms of a phenomenal model constructed from a knowledge of the crystal structure, the metallic radii, melting points and enthalpies of sublimation of the elements. The 5/electrons in the elements between protactinium... 

Fig. 1. Enthalpies of sublimation of rare earth and actinide metals and enthalpies of formation of their 3 + aquo-ions. Values of As bi/f for Md-Lr and A,// [Es " (aq)-Lr (aq)] are estimates.

Californium is the heaviest actinide for which data like the enthalpy of sublimation have been determined directly with bulk quantities of about 2 mg of pure metal. Due to the limited availability of the heaviest actinides down to the one-atom-at-a-time scale, the preparation of the metals becomes an integral part of an experiment for studying the metals. Unusual experimental approaches like the measurement of partial pressures of the actinide under study over an alloy, studies of diffusion of actinide atoms in metals, and adsorption studies of actinide atoms onto metal surfaces by thermochromatography have been reported. 

An important correlation between trivalent f-block ions and their Trivalent atoms (f" ds ) is the P(M) function proposed by Nugent et al. [29]. This function has been utilized for predicting enthalpies of sublimation of metals and enthalpies of formation of aqueous ions. David et al. [27] used heavy-actinide thermodynamic properties to establish a P(M) function relating all of the actinide metals and their 3 + aquo ions. Morss and Sonnenberger [ 103] used newer data to refine this P(M) and to develop similar P(M) plots relating f-block metals and their sesquioxides and trichlorides (Figs 17.5 and 17.6). 

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