THERMOCHEMICAL PROPERTIES OF THE GASEOUS SCANDIUM, YTTRIUM, AND LANTHANUM FLUORIDES

Equilibria involving the gaseous mono- and difluorides of Sc, Y, and La were studied by effusion-beam mass spectrometry, and reaction enthalpies were derived by second and third law methods. In addition, the sublimation pressures of the crystalline trifluorides ScF3, YF3, and LaF 3 and related thermodynamics of the gaseous trifluorides were determined by the torsion-effusion method. Dissociation energies at 298 K in kcal mol-1 for the bonds M-F, FM-F, and F2M-F, respectively, were derived as follows: For Sc, 143.2±3.2, 148.8±4.1, and 155.7±6.4; for Y, 163.8±3.2, 145.6±3.2, and 151.6±4.7; and for La, 157.5±4.1, 153.2±3.2, and 149.2±5.4. These values do not fall into any obvious pattern. The dissociation energies of ScF and YF are in good agreement with the results of high-level theoretical calculations. Spectroscopic and molecular constants of the species are summarized, and the thermochemical results are compared with values in the literature. © 1995 American Institute of Physics.