In crystallography, the rhombohedral (or trigonal) crystal system is one of the 7 lattice point groups. A crystal system is described by three basis vectors. In the rhombohedral system, the crystal is described by vectors of equal length, all three of which are not mutually orthogonal. The rhombohedral system can be thought of as the cubic system stretched along a body diagonal.
In some classification systems, the rhombohedral system is grouped into a larger hexagonal system.
The point groups that fall under this crystal system are listed below, followed by their representations in international notation and Schoenflies notation, and example crystals.
Rhombohedral system boron nitride is produced by heating a boron material selected from the group consisting of boron oxide, boric acid and an oxygen-containing boron compound capable of forming boron oxide when heated, at a temperature of from 1200.degree.
This rhombohedral system boron nitride was observed by an electron microscope, and it was found to be a coalescence of beard-like crystals elongated in the direction of the c-axis with hexagonal system notation and having good crystallinity and a diameter of about one micron.
Each rhombohedral system boron nitride obtained in each Example was mixed with copper powder, and exposed to a pressure region for the formation of cubic system boron nitride by a impact-pressure method.
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