editorial More than a simple The expanding realm of and related superhard materials requires understanding their structural complexity and the correlation with synthesis conditions for potential properties .

ommonly known as the hardest allotropes have been observed or predicted natural material on , diamond in the literature. However, a fine control Ctakes billions of years to form under among these structures is difficult to achieve; the extreme and especially as many are metastable at ambient conditions deep beneath the surface. With pressure and temperature and substantially the name derived from the Greek ‘adamas’ impacted by synthesis routes which often meaning ‘untameable’, diamond is reminiscent involve extreme conditions. Moreover, of hard, pure and clear popularly the intermediate structures or hybrids used in jewellery. Yet the majority of mined that contain stacking disorders and mixed , or those artificially synthesized, bonding types are not individual phases, do not reach gem quality due to crystalline Credit: Björn Wylezich/Alamy Stock Photo and how to obtain them in usable quantities imperfections, but all have uses as an is yet to be explored. Understanding how important engineering material in industry. these different structures are related, Possessing outstanding hardness, thermal sp2-bonded domains, accompanied with their transformation mechanisms and the conductivity and chemical inertness, they intergrowth and mixture of all these transition energy barriers that determine are idea for cutting, polishing or grinding, defect-induced nanostructures, substantially the specific requirements of formation whereas diamonds with favourable optical, increases the structural complexity. In this conditions will be essential to master the electrically insulating and thermally issue, a Comment by Péter Németh and engineering of diamond-like materials, as conducting properties are found in optics colleagues classifies the nanostructures commented by Németh and colleagues. and optoelectronics applications. Also worth reported in diamond materials, and maps Computational methods including machine noting is the diamond anvil cell, a device them with other common crystalline learning can serve as efficient tools to search widely used in high-pressure research. materials in an energy–volume space for superhard structures and assess their Diamonds with different degrees that provides at a glance an overview of the stability and synthesizability, as well as to of crystallinity can be synthesized in relationship between diamond structures predict structural evolution and synthesis different ways to meet the quality and and other carbon polymorphs. The pathways, although this is challenging for cost requirements of various applications. mechanical, thermal and optoelectronic metastable materials that have to consider Large-size single crystals, for instance, properties associated with the presence kinetic barriers and a vast configuration have been grown under high-pressure and of these nanostructures are outlined, space. high-temperature (HPHT) conditions providing prospects for future development Beyond carbon, other types of using catalyst solvents. Mono-, poly- or of diamond-related materials based on elements can be brought into play to nanocrystalline diamond films can be nanostructure engineering, which they note form diamond-like compounds showing deposited on different substrates by is currently still in its infancy. superhardness and interesting electric/ chemical vapour-phase deposition in the Beyond crystalline diamonds, various electronic properties, including small presence of atomic . Carbon amorphous carbon materials containing elements like , and oxygen precursors can also be directly converted mixed sp2–sp3 or purely sp3 bonding1 also forming short, strong covalent bonds, as well into diamond polycrystals under static show excellent diamond-like mechanical as transition metal elements like osmium, HPHT mimicking the natural formation properties. The fraction of sp3 bonding and iridium or introducing a high conditions in the Earth, or under impact atomic configuration would naturally affect density of valence electrons3. In diamond events including detonation or shock the density and hardness of the resulting and related materials, the variety of bonding compression resembling the formation materials, while it is not an easy task to and versatility of atomic arrangements lead process of meteoritic diamonds. characterize different amorphous structures, to high levels of structural complexity. But The manufacturing process and the that is, the local atomic order they possess on the positive side, they also offer a high carbon sources used determine the type despite lacking long-range order. Similarly to degree of design freedom for reaching and amount of crystal defects present in the metallic glasses whose properties are strongly desired properties, which must be controlled diamonds, which ultimately influence their affected by short-range and medium-range on the basis of crucial fundamental mechanical and physical properties — for order2, the identification of amorphous understanding on the transformation instance, hardness, toughness, as well as structures and the understanding of their mechanisms and capability of structural electronic, thermal and optical properties. correlation with macroscopic behaviour control in practice. ❐ The structure of a perfect diamond crystal may help to design amorphous with consists of carbon atoms covalently bonded stronger or unique performance. Published online: 20 October 2020 3 in a tetrahedral network in sp hybridization, From a theoretical viewpoint, the https://doi.org/10.1038/s41563-020-00848-6 arranged in long-range order — a structure properties of diamond-like materials and called the diamond cubic lattice. However, more generally carbon materials can be References the occurrence of grain boundaries, tuned by regulating the bonding character 1. Lin, Y. et al. Phys. Rev. Lett. 107, 175504 (2011). 2. Hirata, A. et al. Nat. Mater. 10, 28–33 (2011). reflection or rotation twins, stacking faults and their structures from the atomic to 3. Kaner, R., Gilman, J. J. & Tolbert, S. H. Science 308, introducing hexagonality, or graphite-like micro scale. Diverse nanostructures and 1268–1269 (2005).

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