Abstract
AA h boron nitride (BN) crystal, assigned to an orthorhombic space group (No. 31, Pm21), is reported here. This new AA h BN crystal exhibits a 'linear' morphology for high-resolution transmission electron microscopy (HRTEM) and a (non-hexagonal) 'diagonal' 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 ∼15 meV higher than that of stable AB or AA' BN. It is shown that the typical scanning electron microscope 'fluffy cotton-like' 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.
Highlights
Synthetic boron nitride (BN) exhibits unique properties comparable in nature with those of graphite because of the similarity in crystal structure between these materials
The tube model for multi-walled carbon nanotubes (MWNTs) has been accepted as the structure of BN nanotubes (BNNTs) since their first report in 1995 (Chopra et al, 1995) because the high-resolution transmission electron microscopy (HRTEM)-determined morphologies and X-ray diffraction (XRD) and electron diffraction (ED) patterns of the latter are very similar to those of MWNTs
The analysis is proven by the unique fast Fourier transform (FFT) pattern for AAh (FFT-1) where the strong (020) spots are evident with h(100)
Summary
Synthetic boron nitride (BN) exhibits unique properties comparable in nature with those of graphite because of the similarity in crystal structure between these materials Both materials feature a 2D van der Waals (sp2) structure forming different crystalline structures, typically AA (or AA0) and AB (AB0 or A0B) (Constantinescu et al, 2013; Lee et al, 2021), according to the stacking sequence of the planes. Celik-Aktas et al (2005) reported single-crystalline (revealed by the spot ED pattern) BNNTs, suggesting a double-helix model with a zigzag structure, as determined by their unique TEM morphology, which showed regular dark spots (revealing a helical trace) on BNNTs. On the other hand, the existence of AA0 stacked graphite as helical (MWNT) or ribbon structures (Lee et al, 2013, 2016) suggests the possibility of the presence of another BN crystal, equivalent to AA0 graphite. XRD patterns of all the structures were generated by the FullProf suite for comparison with the experimental data (Rodrıguez-Carvajal, 2001)
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