Europium atomization enthalpy

The stabilities of the Eu2+, Yb2+, and Sm2+ ions correlate with the third ionization enthalpies of the atoms and the sublimation enthalpies of the metals. The Eu2+(aq) ion is readily obtained by reducing Eu3+(aq) with Zn or Mg, while preparation of the others requires use of Na/Hg or electrolysis. The aqueous Eu2+ solutions are easily handled, but those of Sm2+ and Yb2+ are rapidly oxidized by air and by water itself. The Ln2+ ions show many resemblences to Ba2+, giving insoluble sulfates, for example, but soluble hydroxides. Europium can be easily separated from other lanthanides by Zn reduction followed by precipitation of the other Ln3+ hydroxides. 

Gschneidner [28,29] showed that the enthalpies of formation of several classes of lanthanide compounds can be correlated systematically as a function of atomic number. He pointed out [30] that the correlations for europium and ytterbium are anomalous because they are divalent in their metallic state but trivalent in the compounds. As shown in Figure 1, the enthalpies of formation of the lanthanide sesquioxides (or of any other class of compounds of R ) do not change in a smooth fashion as a function of Z or of the ionic radius of R. These enthalpies of formation correspond to the reactions that appear to be similar throughout the rare earths,... 

A crucial issue is to determine the valence (the number of electrons participating in metallic bonding) of each metal (i.e., the pure, solid metal at 298 K and 1 atm pressure). For example, most of the solid rare earths are trivalent metals (with 6s 5d bonding) but europium metal is divalent (6s 5d°). Many of the gaseous atoms (table 2, second column, e.g., Pr, Nd, Pm,...) are also divalent. (One electron of a filled subshell is readily promoted.) This situation was described and explained by Brewer (1971,1983a, b, 1987) and has been further elaborated by Johansson and co-workers (Johansson 1978, Brooks et al, 1984) and by Gschneidner (1993). Haire and co-workers (Kleinschmidt et al. 1984, Haire and Gibson 1989) established that both Es and Fm metals are divalent and they predicted enthalpies of vaporization of Md and No to be the same (ca. 143 kJ mol ) based on their expected divalency. 

The reliability of measurements of the partial pressures of R2CI6 can in principle be verified by a standard procedure based on changes in the thermod)mamic characteristics of these molecules along the lanthanide series. However, the enthalpy of atomization AatH°(298) is not the most convenient parameter for such a check since it does not vary monotoni-cally with the number of the lanthanides in the series. The plot of this dependence is a broken line with maxima at lanthanum, gadolinium, and lutetium compounds and minima at europium and ytterbium compounds. In addition, the enthalpy of atomization usually increases in going from dysprosium to erbium dimers. 

The calculation scheme for the enthalpies of formation of lanthanide dihalides proposed by Kim and Oishi (1979) is based on the assumption that the formation of these compounds, except for europium and ytterbium, is accompanied by an electronic transition 4f 5d 6s 4f + 5d°6s in the lanthanide atom. This results in the observation of an irregularity in the variation of the thermodynamic parameters, including the enthalpies of formation, as a function of the lanthanide atomic nmnber. 

The reference enthalpies of formation for barimn, europium, and ytterbium plotted as a fimction of the atomic nmnber (Ba, La-Lu = 1-16) and for the dichlorides of these elements (see Table 52) were fitted to smooth curves by means of a second-order polynomial. 

Usually, trends in AatH°(298) variations depending on the atomic numbers of lanthanides are considered. This dependence, however, has the form of a broken line with maxima at lanthanum, gadolinium, and lutetium and minima at europium and ytterbium. In addition, an increase in AatH°(298) is usually observed when going from dysprosium to erbium compoimds. A smoother dependence on the atomic number of lanthanides was obtained for the enthalpies of sublimation of lanthanide trifluorides and trichlorides. We believe that the use of this feature allows Asubhf°(298) values to be predicted more accurately for separate lanthanide dichlorides. Accordingly, the reliability of all the thermodynamic data can then be estimated. 

We believe that the calculation of the enthalpies of atomization of samarium and europium fluorides from the data reported by Kleinschmidt et al. (1981) is more correct and that the reactions involving SmF and EuF studied by Zmbov and Margrave (1967a) should be considered when discussing AatH°(HoF, 0). In addition, we tried to calculate both AatH°(RF, 0) and AatH°(RF2, 0) from the enthalpies of reactions with low stoichiometric coefficients. 

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