Lanthanides sublimation enthalpies

Figure 9.63 Properties of REEs (from above dissociation energies of lanthanide oxides ( ), sublimation enthalpies of REEs (U) and transition energies for the transition of the 5d electron from 4fn 5d6s2...

Fig. 39. The sublimation enthalpies at 298.15 K of the lanthanide trichlorides. - experimental results for SmCl3 and EUCI3 that are not considered (see text).

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. 

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

Early predictions of sublimation enthalpies made by analogy to trends in the lanthanides (25) have not always been confirmed. 

P(M) was defined by Nugent etal. (1973b) to explain the apparent irregularities in lanthanide-metal enthalpies of sublimation and lanthanide aquo-ion enthalpies of formation. P(M) is shown in fig. 2 on an energy-level scale values are tabulated in... 

The above-mentioned procedure is also useful with respect to the description of new relationships for series of similar compoimds. Some special features of changes in the sublimation enthalpy over series of such compoimds are well seen in the dependence of Asub °(298) on the position of the 4f metal in the lanthanide series. A decrease in the coordination number (CN) of a 4f metal in the crystal lattice of the corresponding compound leads to a lower Asubff°29s value. This is observed in going from P-YF3- to LaF3-t) e structures of the trifluorides. A similar phenomenon is characteristic for the lanthanide trichlorides and dichlorides. Moreover, the absolute value of the sublimation enthalpy reflects the stability of the compound and its resistance to thermal actions. As expected, such correlations are t) ical for other series of similar 4f compounds. 

TABLE 18 Recommended enthalpies of sublimation, enthalpies of formation and enthalpies of atomization for lanthanide trifluorides (kJ/mol)... 

TABLE 51 Enthalpies of sublimation of dimeric lanthanide trichlorides, enthalpies of formation, and enthalpies of atomization of the dimers under standard conditions, in Id/mol"... 

Kudin, L.S., Vorob ev, D.E., 2003. Mass spectrometric determination of sublimation enthalpies of lanthanide trichlorides in the form of monomer molecules. In Proceedings of the II International Symposium on High Temperature Mass Spectrometry (Plyos, 2003), Ivanovo, pp. 129-134. 

We assume, in this case, that the conduction band has become normal (that is, it has no longer any 5 f character). Thus, physical properties may be usefully compared with those of the lanthanides. In Table 5 we report known basic properties (metallic radii, crystal structures, melting temperatures and enthalpies of sublimation) of the transplutonium metals. 

The vapour pressures of M(dpm)3, where M = La-Lu, have been determined273 over a range of temperature within 410-530 °C and the heats of sublimation, and heats of vapourization ot the ligands, were calculated. The heats of sublimation of the lighter lanthanides (e.g. Pr, 165.4 kJ mol-1) are considerably greater than those of the heavier (e.g. Yb, 133.3 kJ mol-1) and doubtless this increase reflects the enthalpy of cleavage of the dimeric [M2(dpm)6].274... 

In the first approximation we have neglected the dimers in the vapour pressure analysis. On the basis of the mass spectrometric results this seems to be justified for the light lanthanide trichlorides, and good agreement is found for the enthalpies of sublimation derived from the... 

A distinct difference can be noted when the variation in the enthalpy of sublimation along the LnCh series is compared to the results of the critical evaluation by Myers and Graves (1977b). The trend for the lanthanide trichlorides is almost linear in our analysis which is not the case in the analysis of Myers and Graves (1977b), who reported a pronounced difference in enthalpy at the mid of the series separating the hexagonal and the monoclinic lanthanide trichlorides. This observation can clearly be related to the distinct different entropies for the solid phase that were selected here, based on new experimental results. 

The recommended enthalpies of formation of the gaseous trichlorides derived from the selected enthalpies of sublimation are shown in table 22. The mean bond energies derived from these values is shown in fig. 40, which reveals the same pattern as for the trifluorides, which should be explained by a f°-f7-f14 linear base variation and an excess contribution which is related to the changes in the electronic nature of the lanthanide(III) ions. 

Figure 19-1 Enthalpies of sublimation of the lanthanide metals (data from E. D. Cater, J. Chem. Educ., 1978, 55, 697).

Enthalpies of sublimation based on Knudsen effusion measurements have been determined for LaBe by Gordienko, et al. (57). The thermodynamics of formation of lanthanide hexaborides from oxides have been deduced by Portnoi, et al. (162) vaporization and stabilities have been studied by Smith (168). 

Enthalpies of sublimation of lanthanides have been shown to decrease regularly from La to Eu, then again from Gd to Yb (Gschneidner 1961). This monotonic variation in property from La to Eu and again from Gd to Yb is often referred to... 

The standard enthalpies of sublimation of rare-earth metals have been measured by a number of workers. Hultgren et al. (1973) have discussed the sources of data and error estimates in their tabulation. Later Morss (1976) has also briefly discussed these data in his comprehensive discussion on thermochemical properties of the lanthanides. Recently Bratsch and Lagowski (1985) have listed a set of values of the sublimitation enthalpies which are also listed in table 1. The values of AH°f for rare-earth metals recommended in table 1 have been used in the recalculation of D values. 

FIGURE 12 Enthalpies of sublimation of lanthanide trifluorides A, data from Kovacs and Konings (2003) , data from Myers and Graves (1977b) and O, data determined in this work. 

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