Ytterbium atomization enthalpy

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,... 

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. 

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