Polymorphism of Bismuth Sesquioxide. I. Pure Bi₂O₃

Area of Science:

• Materials Science
• Solid State Chemistry
• Inorganic Chemistry

Background:

• Bismuth oxide (Bi₂O₃) exists in multiple polymorphic forms, each with distinct structural and thermal properties.
• Understanding the phase transformations and stability of Bi₂O₃ polymorphs is crucial for its applications.

Purpose of the Study:

• To elucidate the stability relationships among the four known polymorphs of bismuth oxide.
• To characterize the transition temperatures and thermal expansion coefficients of different Bi₂O₃ phases.

Main Methods:

• Differential Thermal Analysis (DTA) was employed to study phase transitions.
• High-temperature X-ray diffraction (XRD) was used to determine crystal structures at elevated temperatures.
• Controlled cooling experiments were performed to identify metastable phases.

Main Results:

• The stable monoclinic Bi₂O₃ transforms to the stable cubic form at 730 ±5 °C, melting at 825 ± 5 °C.
• Metastable tetragonal and body-centered cubic (b.c.c.) phases were observed around 645 °C upon controlled cooling.
• Tetragonal Bi₂O₃ can be synthesized by decomposing bismutite at 400 °C.
• Cubic Bi₂O₃ exhibits the highest coefficient of volume expansion, with significant expansion during the monoclinic to cubic transition.
• Exposure to air leads to carbonation, forming bismutite and an unidentified phase.

Conclusions:

• The study provides a comprehensive phase diagram for bismuth oxide polymorphs under specific conditions.
• The thermal expansion properties of cubic Bi₂O₃ are significant, impacting its material behavior.
• Environmental factors like air exposure can induce phase transformations and degradation in Bi₂O₃.