Phase equilibria in the bismuth-oxygen system

The bismuth-oxygen system was studied in the crystalline and gaseous state by means of high-temperature vaporization studies. Vaporization experiments were performed using Knudsen effusion in a high-vacuum environment and using thermogravimetric analysis with a flowing pure nitrogen environment. Oxygen was lost from samples of Bi2O3(delta). Compositions of residues from the vaporization experiments were determined by using thermogravimetric analysis with pure oxygen to oxidize the residues to Bi2O3. The congruently effusing composition was determined to be at a composition of 1.23 O/Bi atomic ratio. The vaporization data, solid-state composition as a function of fraction of sample vaporized, could not he explained without having a new phase present in the system. The new solid phase is in accord with Bi14O16, a phase previously observed only in thin films. Pressures of the dominant gaseous species Pi and O-2 were determined by using two data: mass spectrometric intensities and the congruently effusing composition. Mass spectrometric data combined with measurements by Sidorov et al. [High Temp. Sci. 1980, 12, 175] enabled the calculation of the dissociation energy at 0 K for BiO(g) of 333.1 +/- 3.0 kJ/mol. Pressures of Bi and O-2 were calculated using vacancy and interstitial models for the solid solution fields for the composition range 1.23-1.5 O/Bi. In addition, differential thermal analyses and quenched sample phase determinations provided data enabling construction of the Bi-O phase diagram.