Polymorphism of Bismuth Sesquioxide. I. Pure Bi(2)O(3)

Stability relationships of the four polymorphs of bismuth oxide have been determined by means of DTA and high-temperature x-ray studies. The stable low-temperature monoclinic form transforms to the stable cubic form at 730 ±5 °C, which then melts at 825 ± 5 °C. By controlled cooling, the metastable tetragonal phase and/or the metastable body-centered cubic (b.c.c.) phase appear at about 645 °C. Whereas b.c.c. can be preserved to room temperature, tetragonal will transform to monoclinic between 550 and 500 °C. Tetragonal Bi(2)O(3), however, is easily prepared by decomposing bismutite (Bi(2)O(3)·CO(2)) at 400 °C for several hours. The greatest transition expansion occurs at the monoclinic to cubic inversion, and cubic Bi(2)O(3) shows the greatest coefficient of volume expansion. With exposure to air, Bi(2)O(3) carbonates and partially transforms to bismutite and an unknown phase.