Boron suboxide (chemical formula B6O) is a solid compound with a structure built of eight icosahedra at the apexes of the rhombohedral unit cell.

Due to its short interatomic bond lengths and strongly covalent character, B6O displays a range of outstanding physical and chemical properties such as great hardness (close to that of rhenium diboride and boron nitride), low mass density, high thermal conductivity, high chemical inertness, and excellent wear resistance.

[1] These boron suboxide materials formed at or near ambient pressure are generally oxygen deficient and non-stoichiometric (B6Ox, x<0.9) and have poor crystallinity and very small grain size (less than 5 μm).

[2][5][6] B6O of the α-rhombohedral boron type has been investigated because of its ceramic nature (hardness, high melting point, chemical stability, and low density) as a new structural material.

This seems to be due to the expense of synthesizing high-quality B6O powder via the reaction of B2O3 with B and further difficulties in densifying B6O parts via standard industrial sintering and hot-pressing techniques.