THERMOCHEMICAL PROPERTIES OF THE GASEOUS TANTALUM FLUORIDES

The gaseous tantalum fluoride species TaFn, with n=1 to 5, were generated under equilibrium conditions by admitting SF6(g) to a tantalum effusion cell at temperatures in the range 1000-2500 K. Mass spectrometry was utilized to establish the species identities and then to study several reaction equilibria. Reaction enthalpies were derived primarily by second law analysis, from which the standard enthalpies of formation at 298 K of TaF5 (-424.6 kcal/mol), TaF4(-305.2 kcal/mol), TaF 3(-194.0 kcal/mol), TaF2(-68.7 kcal/mol) and TaF (69.2 kcal/mol) were derived, all ±3 kcal/mol. Estimated thermodynamic functions of the Ta-F species, based on data for the neighboring tungsten fluorides, were found to be quite compatible with the equilibrium data. Equilibrium gas phase compositions in the Ta-F system, calculated over a range of temperatures for several pressures using the data reported here, correlate closely with kinetic data on the reaction of Ta with F atoms. The sharp decline in reaction rate above 2000 K can be accounted for on purely thermodynamic grounds. © 1979 American Institute of Physics.