The electronic and mechanical properties of the XYB14 complex borides studied by first-principles methods
Intensive scientific efforts have been invested in searching and designing new superhard materials for applications as abrasives, polishing and cutting tools, wear-resistant and protective coatings. The best-known superhard material to date is diamond. However, its industrial applications are greatly limited by its high cost and susceptibility to chemical corrosion. Alternatively, boron compounds are promising candidates because of boron's hardness and excellent chemical and thermal stability. Conventional superhard materials, such as diamond and cubic boron nitride, are usually comprised of a strong, covalently bonded network of atoms. This bonding usually results in a dense, highly symmetric crystal structure that is stoichiometric. Therefore, it is particularly challenging to chemically modify the structure and their mechanical properties are thus, "as made", rather than, "by design". Recently, a class of complex borides, based upon the AlMgB14 crystal structure, has been proposed as a potentially superhard material. Unlike conventional superhard materials, the crystal structure of AlMgB14 is only loosely packed. It is also known that a variety of metal species and vacancies can occupy the metal atom sites. The measured Vickers hardness of the base compound exceeds 32 GPa, and it is experimentally observed that the addition of impurity species and second phases has a significant beneficial impact on the mechanical properties. At this time, there has been no systematic study aimed to explain the origin of the intrinsic hardness of the XYB14-type compound or to understand how to control its physical properties. The goal of this project is to provide a thorough understanding of the electronic structure of the XYB14-type metal borides, and in particular how the electronic structure is related to its chemical composition. Based on first-principles methods, a series of calculations are performed to examine the relationship between the chemical bonding and the mechanical properties of XYB14. The impact of substituting different atomic species into both metal and boron sites is examined. The success of this project will provide insight to the origin of its unexpected hardness and predict practical methods to control the mechanical properties of XYB14 crystals.
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