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HRTEM study has been carried out on BN nanotubes produced by laser heating of hBN in a diamond anvil cell under high nitrogen pressure of 8.3 GPa. Special attention has been paid to their tip-end capping features and the epitaxial relationship with the mother phase during their growth. The observed tip-end cone angles ranged from 22° to 45 °. The tip-end structural models have been proposed that are relied on the existence of 4 and 8 member BN rings in the nanotube caps instead of 5 and 7 member rings which are typical for carbon nanotubes. The base material in the bottom of the nanotubes mostly revealed cubic BN structure, though, occasionally, hexagonal BN was found as the nanotube mother phase. In the former case, cBN in the bottom of the nanotube and the nanotube //BN sheets exhibit orientation relationship as follows: (110)cubic//(0002)hex and [ 1¯11 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003063056/c87f8d57-bd6e-40f8-a651-90760b6f3961/content/ieq0345.tif"/>11]cubic//[10 101¯0 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003063056/c87f8d57-bd6e-40f8-a651-90760b6f3961/content/ieq0346.tif"/>0]hex·
