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Rediscovering the Crystal Chemistry of Superhard Dodecaborides and Other Higher Borides


Almost all items used in everyday life are a result of the processing industry, as seen in the machining of modern metals to form appliances and car parts, precision machining parts for phones and computers, and drill tools for extracting oil to be used to power cars and provide reagents for plastics and chemical synthesis. Many tools used today are made using diamond. Diamond is the hardest mineral due to its structure, where the high density of carbon atoms and the large number of short covalent bonds produce both ultra-incompressibility and a large shear modulus. Due to both of these factors, diamond is superhard (Vickers hardness above 40 GPa), and is of utmost importance for such multi-billion dollar industrial processes as cutting (machining) and drilling (oil industry). However, diamond is expensive, both natural and man-made, due to supply limitations and synthetic costs of production from the high-pressure high-temperature conditions needed. Moreover, diamond readily forms carbides with iron and as such cannot be used to cut steel or other ferrous metals. As such, the majority of tools for cutting and machining are made using less expensive tungsten carbide (WC). Unlike diamond, WC is a metal, therefore it can be processed (shaped and cut) using common electric discharge machining. Moreover, tungsten carbide is not superhard, having a hardness of only 25 GPa; therefore, the development of superhard substitutions is an important area of exploration.

The primary focus for the development of superhard metals to date has been on new compositions and crystal structures, as well as intrinsic and extrinsic hardening effects using solid-solution formation and morphological control of the surface and grain structure. The search for new superhard metals stands on the shoulders of boride crystallography. Metal borides come in a large variety of possible structures, primarily defined by the way the boron atoms are arranged: isolated boron atoms (e.g. Cr4B), single and double chains (e.g. CrB and Ta3B4), networks of boron atoms (e.g. AlB2, ReB2, WB4, ZrB12), and structures based on boron icosahedra, B12 (e.g. ZrB50 and GdB66). The first reported superhard metal boride was our work on rhenium diboride (ReB2) however, due to the limited supply and cost of rhenium, there has been a great incentive to investigate the possibility of using less expensive metals. This resulted in the cheaper alternative superhard borides: tungsten tetraboride (WB4) and dodecaborides (ZrB12 and YB12). This in turn resulted in the metal boride field expanding to the use of alloys and solid-solutions via doping with metals analogous to tungsten (i.e. group 5, e.g. Ta and Nb), thus enhancing the bulk modulus and using structures with a 3D backbone of boron atoms (“cage”-structures) increasing the material’s shear resistant due to it becoming more isotropic. However, metal borides also possess a large variety of ternary and higher order metal borides with unique structures and properties and this constitutes a significant part of the current work reported here. Such ternary metal borides have essentially been “forgotten” as apart from papers on the crystal structure of such phases, over the past 50 years there have been only 2 papers on properties, both of which only reported electronic properties. Therefore, these phases represent a treasure trove of research opportunities.

The goal of this project has been to investigate higher borides (i.e. dodecaborides) for their properties. Furthermore, the project has attempted to expand the field of superhard material compositions by delving into the unstable/metastable phase space stabilizing high-pressure, high-temperature phases through solid solutions formation at ambient pressure.

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