A Transition Phase in the Transformation from α-, β- and ∊- to δ-Bismuth Oxide

As an important functional material, Bi2O3 is found in seven polymorphs. We systematically analyze the transformation from α-, β- and ∊- to δ-Bi2O3 by using crystallographic group theory and structural simulations. A transition phase is proved to exist in the transformation from the α-, β- and ∊-Bi2O3 to δ-Bi2O3. The transition phase is supposed to be tetragonal phase Bi2O2.7 (Z = 2) with the 139th space group.