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. Dibismuth trioxide is commonly used to produce the "Dragon's eggs" effect in fireworks, as a replacement of red lead. Structure The structures adopted by differ substantially from those of arsenic(III) oxide, , and antimony(III) oxide, . Bismuth oxide, has five crystallographic polymorphs. The room temperature phase, α- has a monoclinic crystal structure. There are three high temperature phases, a tetragonal β-phase, a body-centred cubic γ-phase, a cubic δ- phase and an ε-phase. The room temperature α-phase has a complex structure with layers of oxygen atoms with layers of bismuth atoms between them. The bismuth atoms are in two different environments which can be described as distorted 6 and 5 coordinate respectively. β- has a structure related to fluorite. γ- has a structure related to that of (sillenite), where a fraction of the Bi atoms occupy the position occupied by SiIV, and may be written as . δ- has a defective fluorite-type crystal structure in which two of the eight oxygen sites in the unit cell are vacant. ε- has a structure related to the α- and β- phases but as the structure is fully ordered it is an ionic insulator. It can be prepared by hydrothermal means and transforms to the α- phase at 400 °C. The monoclinic α-phase transforms to the cubic δ- when heated above 729 °C, which remains the structure until the melting point, 824 °C, is reached. The behaviour of 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- always forms on cooling the β-phase. Even though when formed by heat, it reverts to α- when the temperature drops back below 727 °C, δ- can be formed directly through electrodeposition and remain relatively stable at room temperature, in an electrolyte of bismuth compounds that is also rich in sodium or potassium hydroxide so as to have a pH near 14. Conductivity The α-phase exhibits p-type electronic conductivity (the charge is carried by positive holes)