Rare-earth fluoride vapors

Extensive studies have been performed for the determination of the vapor pressures and the vaporization thermodynamics of rare-earth fluorides. The majority of the data are based on mass spectrometric work and originate from the laboratories of Suvorov, Margrave and Searcy. Table 1 summarizes the reported vapor pressures and thermodynamic functions of sublimation, vaporization and (in two cases) dimerization of rare-earth fluorides. The data are not in full agreement in those cases that the same compound has been studied by different research groups. The reader should also refer to the articles of Myers and Graves (1977a,b) for selected third-law values for the enthalpy of sublimation, A//5298-... 

Vapor pressures and vaporization thermodynamics of rare-earth fluorides... 

Gr. aktis, aktinos, beam or ray). Discovered by Andre Debierne in 1899 and independently by F. Giesel in 1902. Occurs naturally in association with uranium minerals. Actinium-227, a decay product of uranium-235, is a beta emitter with a 21.6-year half-life. Its principal decay products are thorium-227 (18.5-day half-life), radium-223 (11.4-day half-life), and a number of short-lived products including radon, bismuth, polonium, and lead isotopes. In equilibrium with its decay products, it is a powerful source of alpha rays. Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300-degrees G. The chemical behavior of actinium is similar to that of the rare earths, particularly lanthanum. Purified actinium comes into equilibrium with its decay products at the end of 185 days, and then decays according to its 21.6-year half-life. It is about 150 times as active as radium, making it of value in the production of neutrons. 

From a solution containing iron and some rare earth metals, Debierne precipitated a mixture of hydroxides. It was radioactive, an activity that could not have its origin in uranium, radium or polonium. A new element could be isolated by fractional crystallization of magnesium lanthanum nitrate. The element was named actinium after the Greek word aktinos, meaning ray . Actinium metal has been prepared by the reduction of actinium fluoride with lithium vapor at about 1100 to 1300°C. 

A large number of mixed metal or complex fluorides have been characterized for the rare earths. In part, these systems have been extensively studied because of their interesting optical, electrical and magnetic properties and because of their potential as host materials for ions of spectroscopic interest. The M(I)X-RfllDXj, the M(I)X-R(IV)X4 and the M(II)X2-R(III)X, (M(I)-aIkali metal, M(II) alkali earth or divalent rare earth) systems have been most extensively investigated however, a limited number of M(III)X3-R(III)X3 and M(IV)X4-R(III)X3 systems have been described. The mixed metal halides have interesting complex vapor species and several of the solid phases have structures which are closely related to those of binary halides. Phase equilibria of mixed metal systems have been determined primarily by thermal and X-ray diffraction analyses. The preparative procedures and many properties of the mixed halides have been reviewed by Brown (1968) and the mixed fluorides have been reviewed by tsanova (1971). 

Conversion of a fluoride to the metal can leave areas of the metal with fluorine content much higher than the average composition. It is not uncommon in these cases for the determination of fluorine to be in error by two to three orders of magnitude. Rare earths prepared or purified in a vaporization step produce a condensed metal which, if sampled directly for analysis by SSMS, can yield erroneous data due to heterogeneous impurity content since only a few mg of sample are consumed in the analysis by SSMS (Svec and Conzemius, 1968). 

Among the rare-earth halides, fluorides represent the most extensively studied systems with respect to the thermochemical properties of their vapors. Most of the studies took place in the period from the late 1960s to mid-1970s. 

Among the rare-earth halides, bromides constitute the group of systems which are studied to a much lesser extend compared to fluorides, chlorides and iodides. It is noteworthy that after Makhmadmurodov et al. (1975a,b) published their results on vaporization thermodynamics of some rare-earth bromides it was very recently that an extensive well-documented Knudsen effusion mass spectrometric investigation of the DyBr3 vaporization appeared in the literature (Hilpert et al. 1995). The successful characterization of the thermochemical properties of the dimer homocomplex Dy2Br6(g) by Hilpert et al. is taken as an indication that further vaporization studies are required for most rare-earth bromide systems with a view to establish the probable existence of vapor dimer homocomplexes and determine their thermochemical properties. Table 8 summarizes the vapor pressures and vaporization thermodynamics of rare-earth bromides. Most likely the vapor pressures reported so far could be in considerable error since the formation of dimers has not been taken into account. 

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