New hexagonal boron nitride polytypes with triple-layer periodicity

Regular hexagonal boron nitride (h-BN) samples present a few of intrinsic stacking faults, which result in a long-standing controversy about their electronic properties. To resolve this controversy, we designed eight possible BN polytypes with triple-layer periodicity. Under ambient pressure, the energies of all the proposed polytypes are between those of observed AA and Aa (h-BN) structures. Two proposed polytypes with direct bandgaps might be responsible for the direct bandgap observed in the h-BN samples. A model was proposed to show how the proposed structures might exist in the h-BN samples by analyzing the stacking characteristics and the previous experimental micrographs of h-BN samples.