Structural properties of hexagonal boron nitride

The electronic and structural properties of hexagonal boron nitride (BN) were studied using density functional theory calculations. Three different approximations for the exchange - correlation energy ( the local density and two forms of the generalized gradient) - were used to calculate properties such as the bulk modulus, cohesive energy and lattice constants to determine their relative predictive abilities for this system. In general, calculations using the local density approximation produced properties slightly closer to experimental values than calculations with either generalized gradient approximations. Different stackings, or arrangements of one basal plane with respect to another, were examined to determine the equilibrium stacking(s) and it was found that the different stackings have similar cohesive energies and bulk moduli. Energy versus volume curves were calculated for each stacking using two different methods to determine their relative efficacy. Bulk moduli values obtained assuming no pressure dependence were closer to experimental values than those obtained from three common equations of state. Comparisons between the cohesive energies of hexagonal BN and cubic BN show that the cubic phase is more stable. The pressure/volume dependence of the band structure was studied for several different stackings and all showed similar behaviour, specifically a 3 - 4.5 eV band gap that was nearly independent of pressure in the - 500 to + 500 kb regime. These calculated results of the pressure/volume dependence of the band structure are the first reports for this system.