High-Temperature Vaporization and Thermodynamics of the Titanium Oxides. VII. Mass Spectrometry and Dissociation Energies of TiO(g) and TiO2(g)

Abstract

Two experiments of the successive vaporization type, five mass spectrometric vaporization experiments, and two isomolecular exchange reaction experiments were undertaken to resolve discrepancies in the thermodynamics of the titanium—oxygen system. The first two types of experiments confirmed that Ti₃O₅(s) is the congruently vaporizing phase. The second type of experiment showed that the principal reaction accounting for the congruent vaporization is $${\mathrm{Ti}}_3{\mathrm{O}}_5(\mathrm{S}\mathrm{)=}\mathrm{TiO(g)}\text{+2}{\mathrm{TiO}}_2(\mathrm{g}\mathrm{).}$$ In the third type of experiment, measurements on the isomolecular exchange reaction $$\mathrm{TiO(g)}+\mathrm{Sc(g)}=\mathrm{Ti(g)}+\mathrm{ScO(g)}$$ showed that the dissociation energy of TiO(g) is smaller than that for ScO(g) and on the basis that ${\mathrm{D}}_0^{\mathrm{o}}(\mathrm{ScO})$ is $\text{160.4±2.0}\mathrm{kcal}/\mathrm{mole}$ , ${\mathrm{D}}_0^{\mathrm{o}}(\mathrm{TiO})$ becomes $\text{156.9±2.2}\mathrm{kcal}/\mathrm{mole}$ , or $\text{6.80±0.10}\mathrm{eV}$ . This result may be combined with earlier vapor pressure measurements on Ti₃O₅(s) and the congruency condition to yield ${\mathrm{D}}_0^{\mathrm{o}}({\mathrm{TiO}}_2\text{)=311.7±1.8}\mathrm{kcal}/\mathrm{mole}$ , $\text{13.52±0.08}\mathrm{eV}$ , or very nearly twice ${\mathrm{D}}_0^{\mathrm{o}}(\mathrm{TiO})$ . One of the two previously discussed discrepancies in the thermodynamics of the titanium oxides appears to have been resolved, and the likely resolution of the other is discussed.