Its hardness is 46 GPa, slightly harder than commercial borides but softer than the cubic form of boron nitride. Since only small amounts of the mineral exist in nature, this has not yet been experimentally verified. Earlier optimistic reports predicted that the wurtzite form was very strong, and was estimated by a simulation as potentially having a strength 18% stronger than that of diamond. In the cubic form all rings are in the chair configuration, whereas in w-BN the rings between 'layers' are in boat configuration. In the wurtzite form, the boron and nitrogen atoms are grouped into 6-membered rings. As in the cubic form, the boron and nitrogen atoms are grouped into tetrahedra. The wurtzite form of boron nitride (w-BN point group = C 6v space group = P6 3mc) has the same structure as lonsdaleite, a rare hexagonal polymorph of carbon. The cubic form has the sphalerite crystal structure (space group = F 43m), the same as that of diamond (with ordered B and N atoms), and is also called β-BN or c-BN. Consistent with diamond being less stable than graphite, the cubic form is less stable than the hexagonal form, but the conversion rate between the two is negligible at room temperature, as it is for diamond. Cubic form (c-BN) Ĭubic boron nitride has a crystal structure analogous to that of diamond. BC 6N hybrids have been synthesized, where carbon substitutes for some B and N atoms. Therefore, h-BN and graphite are very close neighbors, and the material can accommodate carbon as a substituent element to form BNCs. ![]() Likewise, many metastable forms consisting of differently stacked polytypes exist. ![]() ![]() This registry reflects the local polarity of the B–N bonds, as well as interlayer N-donor/B-acceptor characteristics. The interlayer "registry" of these sheets differs, however, from the pattern seen for graphite, because the atoms are eclipsed, with boron atoms lying over and above nitrogen atoms. Within each layer, boron and nitrogen atoms are bound by strong covalent bonds, whereas the layers are held together by weak van der Waals forces. Hexagonal boron nitride (point group = D 6h space group = P6 3/mmc) has a layered structure similar to graphite. The most stable crystalline form is the hexagonal one, also called h-BN, α-BN, g-BN, and graphitic boron nitride. It is analogous to amorphous carbon.Īll other forms of boron nitride are crystalline. The amorphous form of boron nitride (a-BN) is non-crystalline, lacking any long-distance regularity in the arrangement of its atoms. Boron nitride has potential use in nanotechnology.īoron nitride exists in multiple forms that differ in the arrangement of the boron and nitrogen atoms, giving rise to varying bulk properties of the material. īecause of excellent thermal and chemical stability, boron nitride ceramics are used in high-temperature equipment and metal casting. The rare wurtzite BN modification is similar to lonsdaleite but slightly softer than the cubic form. ![]() The cubic ( zincblende aka sphalerite structure) variety analogous to diamond is called c-BN it is softer than diamond, but its thermal and chemical stability is superior. The hexagonal form corresponding to graphite is the most stable and soft among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN.
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