Computational investigation into the properties of potentially ultra hard boride materials

Abstract

Boron exists in many different structures with an important similarity that they all contain connected boron icosahedron formed from twelve B atoms. Many have the potential to be important hard materials. The icosahedra are arranged in planes and are bonded to each other by three centered bonds within a plane and by two centered bonds to icosahedra in adjacent planes. It is likely that the two center bonds are stronger than any other bonds in the crystals and may contribute signi cantly to the strength of these materials. Various structures that hold potential for super hard material properties are examined in the present work using ab-initio computational techniques. Systematic trends are established. The charge density between B-B bonds in each structure are examined and it is suggested that hardness of the material, in part, relate to the average charge density contained on these bonds. Atoms connecting the B12 icosahedra can donate charge that enhance the strength of the B-B bonds. The structural and thermodynamics properties of boron icosahedral materials are also studied using molecular dynamics (MD)simulation with the use of bond order Terso¤ potentials and are compared with ab-initio computational results and exper-iments. Various physical quantities including the elastic constants of boron carbide (B4C), thermal expansion coe¢ cient, speci c heat are predicted at high temperatures. The linear thermal expansion coe¢ cient for the a and c axis are examined. Predicted speci c heats for B4C and boron suboxide (B6O) structures obey the classical Dulong- Petit result which is obtained at high temperatures for all solids. Moreover, thermo- dynamic properties obtained in this work are used to estimate Gr uneisen parameters for these potentially ultra-hard boride materials. Finally we examined the elastic constants of an ultra hard boride B6O and some defects in the crystal structure using the rst principle calculations. The single crys- tal elastic constants calculated were used to estimated polycrystalline properties and thermodynamic properties such as the melting and Debye temperaturwell as sound wave velocities and melting temperatures of B6O and defect structures. Single crystal elastic constants are found to be comparable with that estimated previously from theoretical calculations and polycrystalline elastic moduli were also calculated and analyzed systematically in comparison with available theoretical and experimen- tal data. We also estimated the formation energies of the various structures using chemical potentials. Analysis of the computed results shows that the formation en- ergies of substitutional defect, nitrogen to oxygen are smaller that those of carbon and vacancies and the low values of nitrogen substitutional defect suggest a possible great solubility of nitrogen in B6O.