Bismuth(III) oxide
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![Existence domains of the four polymorphs of Bi2O3 as a function of temperature. (a) The α-phase transforms to the δ-phase when heated above 727 °C, which remains the structure until the melting point, 824 °C, is reached. When cooled, the δ-phase transforms into either the β-phase at 650 °C, shown in (b), or the γ-phase at 639 °C, shown in (c). The β-phase transforms to the α-phase at 303 °C. The γ-phase may persist to room temperature when the cooling rate is very slow, otherwise it transforms to the α-phase at 500 °C.[2]](/uploads/202412/24/Bi2O3_phases.svg4212.png)
![(a) Sillén model; vacancies ordered along<111>,[6] (b) Gattow model; vacancies completely disordered in oxygen sub-lattice, with each oxygen site having 75% occupancy,[7] (c) Willis model; oxygen atoms displaced from regular 8c sites (for example, the atom marked A in (b)) along<111> to 32f sites. The Bi3+ ions labelled 1–4 in (c) correspond to those labelled 1–4 in (b).[8]](/uploads/202412/24/Bi2O3_models4212.png)
Bismuth(III) oxide is perhaps the most industrially important compound of bismuth. It is also a common starting point for bismuth chemistry. It is found naturally as the mineral bismite (monoclinic) and sphaerobismoite (tetragonal, much more rare), but it is usually obtained as a by-product of the smelting of copper and lead ores. Bismuth trioxide is commonly used to produce the "Dragon's eggs" effect in fireworks, as a replacement of red lead.