Thermochemical properties of the gaseous scandium, yttrium, and lanthanum fluorides

Abstract

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 LaF3 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.