Polycrystalline boron nitride constructed from hexagonal boron nitride

Two classes of structural families of boron nitride allotropes (named as X-BNs and L-BNs) are proposed to shed light on the polycrystalline structure of high pressure h-BN. We find that the previously proposed pct-BN and Z-BN belong to our currently proposed family (L-BNs). X-BNs are much more stable at zero pressure and more energetically favorable than L-BN and w-BN. X-BNs can be viewed as a mixture of w-BN and c-BN. Surprisingly, the transition from h-BN to X-BNs can occur simply through sliding and buckling of BN sheets. The calculated bulk modulus and densities of X-BNs match well with the experimental data. Our calculations predict that X-BNs are transparent wide band-gap insulators.