Gaseous lanthanide halides

As electron diffraction provides direct measurement of interatomie distanees, it is an ideal method for the determination of the molecular stmcmres of gases. Moreover, moleeules in the gas phase are free from the intermolecular interactions and the influence of fields that ean distort a structure (particularly the conformation) in the crystalline state, or even change it completely. We will see some examples of this in the case histories presented in Chapter 12. But there are, of course, limits to the usefiilness of electron diffraction, of which the most obvious is that gaseous samples are needed. The essential requirement is that the compound to be smdied should have a vapor pressure of about 1 mbar at a temperature at which it is stable. Lower vapor pressures can be used, but the experiments are more difficult to perform. As long as no decomposition occurs, the temperature does not really matter, and such involatile species as alkali metal halides or lanthanide halides and some metal oxides have been studied at temperatures of up to 2000 K or more. However, we should remember that raising the sample temperamre increases amplitudes of vibration, and can change the relative populations of isomers or conformers. 

The thermochemistry of rare-earth trifluorides was summarized in Gmelin Hand-buch (1976) and the thermochemistry of rare-earth tribromides and triiodides was summarized in Gmelin Handbuch (1978). The thermochemistry of trivalent rare-earth trichlorides was critically assessed by Morss (1976). Enthalpies of formation of most of the lanthanide tribromides were determined by Hurtgen et al. (1980). Thermodynamic properties for europium halides were assessed by Rard (1985). Only enthalpies of formation of Sc, Y, Dy and Tm triiodides have been redetermined since the classical work of Hohmann and Bommer (Morss and Spence 1992). A recent set of literature values of enthalpies of formation of rare-earth solid and gaseous trihalides has been published, accompanied by Born-Haber cycle estimated values for all trihalides (Struck and Baglio 1992). 

---