AA(h) BN crystal, basic structure of boron nitride nanotubes

Abstract

AA(h) boron nitride (BN) crystal, assigned to an orthorhombic space group (No. 31, Pm2(1)), is reported here. This new AA(h) BN crystal exhibits a &apos;linear&apos; morphology for high-resolution transmission electron microscopy (HRTEM) and a (non-hexagonal) &apos;diagonal&apos; electron-diffraction pattern, which have been experimentally demonstrated in this article. It is also demonstrated that this new crystal is the basic structure of multi-walled BN nanotubes (BNNTs) existing in the form of a helix. The helical AA(h) BNNTs exist in a metastable phase owing to < 200 > texture growth of the orthorhombic crystal, where the energy is similar to 15 meV higher than that of stable AB or AA&apos; BN. It is shown that the typical scanning electron microscope &apos;fluffy cotton-like&apos; morphology of BNNTs is due to secondary growth of diverse BN sheets (including mono-layers) on incoherently scrolled wall strands of BNNTs, providing further evidence for the helical structure with HRTEM evidence for a left-handed helix.