Abstract
Hexagonal boron nitride (h-BN) nanosheets exhibit remarkable mechanical resilience, chemical inertness, and thermal stability, making them quintessential two-dimensional dielectric materials. In this study, we investigate the structural stability and pressure-induced phase transformations of high-purity, few-layer h-BN nanosheets under quasi-hydrostatic compression up to 35.2 GPa using a symmetric diamond anvil cell (DAC) coupled with in situ synchrotron angle-dispersive X-ray diffraction (XRD) and high-resolution micro-Raman spectroscopy. At ambient conditions, the nanosheets exhibit the characteristic intra-layer E2g optical phonon mode at 1366.2 cm⁻¹. Under hydrostatic compression, this mode undergoes a continuous blueshift with a pressure coefficient of 4.12 cm⁻¹/GPa up to approximately 12.8 GPa. Beyond this critical pressure, a pronounced peak broadening, intensity attenuation, and the emergence of a broad shoulder near 1280 cm⁻¹ signify the onset of an sp²-to-sp³ hybridization transition. In situ synchrotron XRD patterns corroborate this transition, showing the rapid collapse of the interlayer c-axis lattice parameter and the appearance of diffraction peaks indexable to a wurtzite-like boron nitride (w-BN) phase above 13.4 GPa. Fitting the pressure-volume data to a third-order Birch-Murnaghan equation of state yields an ambient bulk modulus of B₀ = 34.6 ± 1.8 GPa with B₀' = 6.4 for the low-pressure h-BN phase. Upon decompression, a marked structural hysteresis is observed, retaining metastable sp³-bonded domains at ambient conditions. These findings elucidate the role of dimensional confinement in lowering the kinetic barriers of phase transitions in two-dimensional group-III nitrides.
