Phase Equilibria in the Bismuth−Oxygen System

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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 Bi 2 O 3 (δ). Compositions of residues from the vaporization experiments were determined by using thermogravimetric analysis with pure oxygen to oxidize the residues to Bi 2 O 3 . 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 be explained without having a new phase present in the system. The new solid phase is in accord with Bi 14 O 16, a phase previously observed only in thin films. Pressures of the dominant gaseous species Bi 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.