Boron Trioxide

Boron trioxide (or diboron trioxide) is one of the oxides of boron. It is a white, glassy solid with the formula B₂O₃. It is almost always found as the vitreous (amorphic) form; however, it can be crystallized after extensive annealing. It is one of the most difficult compounds known to crystallize.

Glassy boron oxide (g-B₂O₃) is thought to be composed of boroxol rings which are six-membered rings composed of alternating 3-coordinate boron and 2-coordinate oxygen. This view is controversial, however, because no model has ever been made of glassy boron oxide of the correct density containing a large number of six-membered rings. The rings are thought to make a few BO₃ triangles, but mostly link (polymerize) into ribbons and sheets. The crystalline form (α-B₂O₃) (see structure in the infobox) is exclusively composed of BO₃ triangles. This trigonal, quartz-like network undergoes a coesite-like transformation to monoclinic β-B₂O₃ at several gigapascals (9.5 GPa).