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High-yield production of high-quality boron-nitride nanotubes (BNNTs) has been reported recently in several publications. A boron-rich material is evaporated using a laser or plasma in a nitrogen-rich atmosphere to supply precursor gaseous species for nucleation and growth of BNNTs. Either hydrogen was added or pressure was increased in the system to achieve high yield and high purity of the synthesized nanotubes. According to the widely-accepted “root grow” mechanism, upon gas cooling, boron droplets form first, then they adsorb nitrogen from the surrounding gas species, and BNNTs grow on their surfaces. However, what are the precursor species that provide nitrogen for the growth is still an open question. To answer this question, we performed thermodynamic calculations for determining the B–N mixture composition considering a broad set of gas species. For the first time, condensation of boron was …