Carbon also forms three oxides&colon; carbon monoxide, carbon suboxide (CO), and carbon dioxide. Boron suboxide (BO) has a hardness of about 35 GPa. By-products include red phosphorus suboxide. The suboxide of carbon adopts an unremarkable structure. Besides this pentoxide there is an additional niobium suboxide, niobium dioxide (NbO). Composites used may include BiN, GeN, and Pd-doped tellurium suboxide. Atomic carbon can be produced by carbon suboxide decarbonylation. An electric discharge in carbon suboxide produces about 11 ppm CO. Other less known oxides include carbon suboxide (CO) and mellitic anhydride (CO). Heating Lead 2,4-dinitroresorcinate also produces CO along with CO, CO and carbon suboxide. Disulfur monoxide or sulfur suboxide is an inorganic compound with formula SO. Carbon suboxide, or tricarbon dioxide, is an oxide of carbon with chemical formula or O=C=C=C=O. Carbon suboxide polymerizes spontaneously to a red, yellow, or black solid. Cumulenes containing heteroatoms are called heterocumulenes; an example is carbon suboxide.