Summary: Metropolis Simulation of Bi2O3
Course Project Computational Physics (Dr. Joan Adler)
This project centers upon the computer simulation of the oxide Bi2O3, a material of interest
for oxygen separation/generation and oxygen sensors due to its high conductivity for oxygen ions.
The phase diagram of this material is shown in Fig. 1.
Fig. 1 Bi2O3 Phase diagram
We see a line phase at 60 mol% O, corresponding to Bi2O3, with phase transition - at 1002°K
(729°C) and melting point at 1098°K (825°C). The d-phase is the one with high oxygen
conductivity. From x-ray and other studies, it has been determined that Bi2O3 is of the 'defect
fluorite' (fcc) structure. As shown in Fig. 2, the fluorite structure packs a molecule of stoichiometry
AB2. It is a defective structure since there are two vacant oxygen ion sites per unit cell thus in
Fig. 2, two of the oxygen sites (on average) will be unoccupied, giving Bi4O6 for each unit cell.
These sites are shown in black in the figure at opposite corners, although their exact placement was
the subject of some debate in the early investigation of this material. In fact these vacant sites are
being occupied and vacated once again at great frequency, which allows for the high oxygen ion
conductivity, and gives a picture of constantly shifting oxygen vacancy positions; since all the
oxygen positions are equivalent, any oxygen can hop into a neighboring vacancy, leaving behind a
new vacancy in its place.