Amorphous carbon is free, reactive carbon that does not have any crystalline structure (also called diamond-like carbon). Amorphous carbon materials may be stabilized by terminating dangling-π bonds with hydrogen. These materials are then called hydrogenated amorphous carbon. As with all amorphous solids, some short-range order can be observed. Amorphous carbon is often abbreviated to aC for general amorphous carbon, aC:H or HAC for hydrogenated amorphous carbon, or to ta-C for tetrahedral amorphous carbon.
In mineralogy 
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In mineralogy, amorphous carbon is the name used for coal, soot, carbide-derived carbon, and other impure forms of carbon that are neither graphite nor diamond. In a crystallographic sense, however, these materials are not truly amorphous, but are polycrystalline materials of graphite or diamond within an amorphous carbon matrix. Commercial carbon also usually contains significant quantities of other elements, which may also form crystalline impurities.
In modern science 
With the development of modern thin film deposition and growth techniques in the latter half of the 20th century, such as chemical vapour deposition, sputter deposition, and cathodic arc deposition, it became possible to fabricate truly amorphous carbon materials.
True amorphous carbon has localized π electrons (as opposed to the aromatic π bonds in graphite), and its bonds form with lengths and distances that are inconsistent with any other allotrope of carbon. It also contains a high concentration of dangling bonds; these cause deviations in interatomic spacing (as measured using diffraction) of more than 5% as well as noticeable variation in bond angle.
The properties of amorphous carbon films vary depending on the parameters used during deposition. The primary method for characterizing amorphous carbon is through the ratio of sp2 to sp3 hybridized bonds present in the material. Graphite consists purely of sp2 hybridized bonds, whereas diamond consists purely of sp3 hybridized bonds. Materials that are high in sp3 hybridized bonds are referred to as tetrahedral amorphous carbon (owing to the tetrahedral shape formed by sp3 hybridized bonds) or as diamond-like carbon (owing to the similarity of many physical properties to those of diamond).
Experimentally, sp2 to sp3 ratios can be determined by comparing the relative intensities of various spectroscopic peaks (including EELS, XPS, and Raman Spectroscopy) to those expected for graphite or diamond. In theoretical works, the sp2 to sp3 ratios are often obtained by counting the number of carbon atoms with three bonded neighbors versus those with four bonded neighbors. (Note that this technique requires deciding on a somewhat arbitrary metric for determining whether neighboring atoms are considered bonded or not, and is therefore merely used as an indication of the relative sp2-sp3 ratio.)
Although the characterization of amorphous carbon materials by the sp2-sp3 ratio may seem to indicate a one-dimensional range of properties between graphite and diamond, this is most definitely not the case. Research is currently ongoing into ways to characterize and expand on the range of properties offered by amorphous carbon materials.
All practical forms of hydrogenated carbon (e.g. smoke, chimney soot, mined coal such as bitumen and anthracite) contain large amounts of polycyclic aromatic hydrocarbon tars, and are therefore almost certainly carcinogenic.
See also 
- "diamond-like carbon films" (pdf). (2nd ed.). Retrieved 2006-06-28.
- "amorphous carbon" (pdf). IUPAC Compendium of Chemical Terminology (2nd ed.). International Union of Pure and Applied Chemistry. 1997. Retrieved 2006-06-28.