High-Pressure High-Temperature Synthesis of Hexagonal Boron Nitride Single Crystals from a Sr₃B₂N₄ Solvent

We report on high-pressure high-temperature (HPHT) synthesis of high-quality hexagonal boron nitride (hBN) single crystals (SCs) grown out of the novel strontium-based solvent Sr3B2N4. Colorless and transparent hBN SCs with lateral sizes up to ∼1.2 mm were recovered after HPHT synthesis at 4.5 GPa and 1500 °C. The quality of the obtained hBN SCs was characterized using X-ray diffraction (XRD), Raman spectroscopy, cathodoluminescence (CL), and X-ray photoelectron spectroscopy (XPS). The narrow peaks observed in both the XRD and Raman spectra indicate excellent crystalline quality for the hBN SCs. Intense CL peaks centered around 5.76 eV were detected in the hBN SCs grown from high-purity precursors. Additionally, the low residual oxygen level in the hBN precursor powder was identified as a critical factor for achieving a high yield of hBN SCs. We also verified the viability of this route in both a Kawai-type two-state multianvil apparatus with a small-sized chamber and a cubic anvil apparatus with a large-sized chamber. Our work demonstrates that the strontium-based Sr3B2N4 solvent provides a low-cost alternative for growing high-quality hBN SCs under HPHT conditions.