New pentaborates for pyrolytic boron nitride

Direct pyrolysis of specific crystal precursors has long been a favored approach for synthesizing high-quality hexagonal boron nitride (h-BN). In this study, two types of single-crystal borates were identified as effective precursors for direct pyrolysis to h-BN. Single-crystal X-ray diffraction analysis revealed their structures as new pentaborates: [C₄H₁₁N₁₀][B₅O₆(OH)₄] (crystal-I) and [C₂H₅N₄][C₂N₄H₄B₅O₆(OH)₄] (crystal-II). The pyrolysis process and resultant products were thoroughly examined using TG-DTA, FT-IR, SEM,TEM and XRD, providing insights into the mechanism of h-BN formation. Notably, these pentaborates enable the preparation of spherical h-BN with a size ranging from 1μm to tens of micrometers. Crystal-II emerged as a more promising precursor due to its pre-existing dative B←N bonds, which facilitated a higher conversion rate to h-BN.