Theoretical investigation of the pure and Zn-doped α and δ phases of Bi2O3 -: art. no. 205122

We have studied the atomic and electronic structure of pure and Zn-doped alpha and delta phases of Bi2O3 by first-principles calculations. For the pure alpha phase which is monoclinic, good agreement was obtained between the experimental and calculated structural parameters and, in addition, the calculated density of states in the valence band and the optical band gap correlated well with photoemission spectra. For the pure delta phase, which has a defective fluorite structure, the calculations suggest that of three possible oxygen vacancy structures, <100>-vacancy ordering is preferred. This phase, however, must be considered as a supercooled phase at T=0 K since we found that a single displaced vacancy (i.e., one that deviates from <100> ordering) can trigger a delta-alpha phase transition. Similarly, a Zn substitutional impurity in the delta phase can also trigger this phase transition. The formation energy of a Zn impurity in the alpha phase was found to be 1.34 eV, resulting in a maximum impurity concentration of 7.1x10(-6) at. % Zn at T=1000 K. The low solubility of Zn in the alpha phase of Bi2O3 is consistent with the observed phase separation between ZnO and Bi2O3.