Actinide melting temperatures

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

Schick s work includes the study of borides, carbides, nitrides, and oxides of some elements in Groups IIA, IIIB, IVA, IVB, VB, VIIB, and VIII as well as selected rare earths and actinides. As far as possible, the tables have been made compatible with the JANAF tables. Among the subjects treated are phase diagrams, heat capacities, enthalpies, entropies, enthalpies of phase transformation, formation, and reaction, melting temperatures, triple points, free energies of formation, vapour pressures, compositions of vapour species, ionization and appearance potentials, e.m.f. of cells, and enthalpies of solution and dilution. Volume 1 summarizes the techniques used to analyse data and cites the data analysed, and Volume 2 gives tables of values produced by this study. 

With Ag, the compound which possesses the highest melting temperature has the formula R Agsi for the early elements of the lanthanide series (La — Nd), it has the formula RAg for the other lanthanide elements (except Eu and Yb) and also for Y and Sc. The stoichiometry of this compound is Ag3Th and Aga.ePu for the actinides. 

With tin the compound which possesses the highest melting temperature has either the stoichiometry (R, An)5Sn3 or (R, An)5Sn4. Considering the structures of the intermetallic compounds of tin with rare earths and actinides a clear similarity appears for the composition (R, An)Sn3 all the compounds have the AuCu3 structure type. 

Efficient refining of the more volatile actinide metals (Pu, Am, Cm, Bk, and Cf) is achieved by selective vaporization for those (Pu, Am, Cm) available in macro quantities. The metal is sublimed at the lowest possible temperature to avoid co-evaporation of the less volatile impurities and then deposited at the highest possible temperature to allow vaporization of the more volatile impurities. Deposition occurs below the melting point of the metal to avoid potential corrosion of the condenser by the liquid metal. Very good decontamination factors can be obtained for most metallic impurities. However, Ag, Ca, Be, Sn, Dy, and Ho are not separated from Am metal nor are Co, Fe, Cr, Ni, Si, Ge, Gd, Pr, Nd, Sc, Tb, and Lu from Cm and Pu metals. 

Table XI gives the room-temperature, atmospheric pressure crystal structures, densities, and atomic volumes, along with the melting points and standard enthalpies of vaporization (cohesive energies), for the actinide metals. These particular physical properties have been chosen as those of concern to the preparative chemist who wishes to prepare an actinide metal and then characterize it via X-ray powder diffraction. The numerical values have been selected from the literature by the authors.

Sinha, S.R, Spectrochemical and electrochemical properties of some lanthanides and actinides in room temperature melt, In Mamantov, G., Mamantov, C.B., and Brausnstein, J. (Eds), Advances in Molten Salt Chemistry, 6, Elsevier, Proc. Electrochem. Sci. (Proc. Int. Symp. Molten Salts) 87-7, 458-468,1987. 

The monoclinic AM2(P04)3 actinide phosphates were obtained by the following methods 1) chemical interactions in boron oxide melt at 1200 °C (48 h), 2) precipitation from the A-M-chloride melts by addition of soluble A-phosphates at 700-800°C (approximately 1 h), 3) interactions between the MO2 (M = Np, Pu) oxides and aqueous solutions of alkaline element salts with H3PO4 followed by several stages of heating, resuspension in water, and finally the product was calcined at temperatures up to 1100 C. 

Additional requirements to high level and actinide waste forms are due to transportation to repository and long-term storage. From this point of view such properties as density, porosity, thermal and temperature conductivity, tensile, flexural and compressive strength. Young s and bulk modules, and microhardness are measured [24]. For the waste forms whose production via melting is suggested some properties of their melts (viscosity, electric resistivity, surface tension) are also important. 

The polymorphism of the lanthanide and actinide metals reflects transitions between configurations of the outer electrons, and, hence, is determined also by a possible participation of f electrons in the chemical bonding. The bonding forces are evidenced also in the temperatures and heats of melting and of evaporation. 

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