What is Bismuth oxide?

Bismuth oxide has seen interest as a material for solid oxide fuel cells or SOFCs, since it is an ionic conductor, i.e. oxygen atoms readily move through it. Pure bismuth oxide, Bi2O3 has four crystallographic polymorphs. It has a monoclinic crystal structure, designated α- Bi2O3, at room temperature. This transforms to the cubic fluorite-type crystal structure, δ-Bi2O3, when heated above 727 °C, which remains the structure until the melting point, 824 °C, is reached. The behaviour of Bi2O3 on cooling from the δ-phase is more complex, with the possible formation of two intermediate metastable phases; the tetragonal β-phase or the body-centred cubic γ-phase. The γ-phase can exist at room temperature with very slow cooling rates, but α- Bi2O3 always forms on cooling the β-phase.
Bismuth is a brittle metal with a white, silver-pink hue, often occurring in its native form with an iridescent oxide tarnish showing many colors from yellow to blue. The spiral, stair-stepped structure of a bismuth crystal is the result of a higher growth rate around the outside edges than on the inside edges. The variations in the thickness of the oxide layer that forms on the surface of the crystal causes different wavelengths of light to interfere upon reflection, thus displaying a rainbow of colors. When combusted with oxygen, bismuth burns with a blue flame and its oxide forms yellow fumes.
Bismuth compounds (accounting for about half the production of bismuth) are used in cosmetics, pigments, and a few pharmaceuticals. Bismuth has unusually low toxicity for a heavy metal. As the toxicity of lead has become more apparent in recent years, alloy uses for bismuth metal (presently about a third of bismuth production), as a replacement for lead, have become an increasing part of bismuth's commercial importance.
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