First-principle investigation of electronic structure and mechanical properties of AlMgB 14
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First-principle investigation of electronic structure and mechanical properties of AlMgB14 L. F. Wan and S. P. Beckman Department of Material Science and Engineering, Iowa State University Ames, IA 50010, U.S.A. ABSTRACT The structural and electronic properties of AlMgB14 are investigated using ab initio methods. The impact of vacancies and electron doping on the crystal’s atomic and electronic structure is investigated. It is found that removing metal atoms does not influence the density of states, except for changes to the Fermi energy. The density of states of the off-stoichiometric Al0.75Mg0.75B14 crystal and the AlMgB14 crystal with five electrons removed are nearly identical. The removal of six electrons results in an 11% contraction in the crystal’s volume. This is associate with the removal of electrons from the B atoms’ 2p-states. INTRODUCTION There is an immense interest in hard materials for technological applications, such as machine tools[1], grinding medium[2], and wear resistant coatings[3]. The manufacturing industry is a multi-billion dollar per year industry, which means that there is a continued push to increase efficiency, because even small improvements lead to substantial cost savings.[4, 5] Superhard materials, which offer to reduce manufacturing time and increase the lifetime of tools, are naturally an attractive technology.[3, 4, 6] The boron-rich borides are promising materials for possible use as superhard crystals because they are typically very hard and boron is known to be chemically resistant in situations where diamond is susceptible to chemical attack.[7] The basic unit is the boron icosahedra B12, which is strongly bonded other icosahedra and arranged to create a regular lattice of B12 units. By counting the bonds within the icosahedra and the number of valence electrons, it is found that an additional two electrons are needed to satisfy the bonds in the B12 structure. These electrons typically come from additional atoms that are located between the icosahedra. A promising class of superhard boron-rich borides is based on the AlMgB14 crystal that has been synthesized at the Department of Energy, Ames Laboratory.[8, 9] This crystal class is very different from other boron-rich borides because the Al and Mg metal atoms do not share strong covalent bonds with the boron network.[10, 11] The atomic structure is shown in FIG. 1. The unit cell contains 64 atoms and is composed of four B12 icosahedra and eight intericosahedra B atoms that are each bonded to three B12. Between the layers of B are four Al and four Mg atoms. The Ames Lab specimen is found to have a baseline hardness of 32-35 GPa, and with the addition of a small amount of Ti, the hardness is measured to be as large as 46 GPa. Surprisingly, aside from the Ames Lab group there have been few experimental studies of this system that focus on the mechanical properties. The primary experimental thrust has emphasized the synthesis of other crystals within the same family, including MgB12[12], MgB12C2[13], Mg2B24C[14], M
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