PHASE-EQUILIBRIA IN THE BISMUTH-OXYGEN SYSTEM

The bismuth-oxygen system was studied in the crystalline and gaseous s tate by means of high-temperature vaporization studies. Vaporization e xperiments were performed using Knudsen effusion in a high-vacuum envi ronment and using thermogravimetric analysis with a flowing pure nitro gen environment. Oxygen was lost from samples of Bi2O3(delta). Composi tions of residues from the vaporization experiments were determined by using thermogravimetric analysis with pure oxygen to oxidize the resi dues 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 syst em. 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 inte nsities and the congruently effusing composition. Mass spectrometric d ata combined with measurements by Sidorov et al. [High Temp. Sci. 1980 , 12, 175] enabled the calculation of the dissociation energy at 0 K f or BiO(g) of 333.1 +/- 3.0 kJ/mol. Pressures of Bi and O-2 were calcul ated using vacancy and interstitial models for the solid solution fiel ds for the composition range 1.23-1.5 O/Bi. In addition, differential thermal analyses and quenched sample phase determinations provided dat a enabling construction of the Bi-O phase diagram.