Growth of rhombohedral boron nitride crystals using an iron flux

We report the growth of high-quality rhombohedral boron nitride (rBN) crystals by the iron flux method at atmospheric pressure. In contrast to the lamellar structure of standard hexagonal boron nitride (hBN) covering the metal ingot, the current synthesized BN shell is composed of many triangular bulk crystallites, of submillimeter size, predominantly in the rhombohedral phase, with only weak traces of hBN detected by high-resolution x-ray diffraction. The low-temperature photoluminescence emission confirms the excellent quality of the rhombohedral stacking with unprecedented width for all phonon replicas at the band edge. The Raman spectra are characterized by the absence of any low-energy modes at $\ensuremath{\sim}50 {\mathrm{cm}}^{\ensuremath{-}1}$, the presence of an extended band around $800\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, and an asymmetric high-energy mode at $1366\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. These observations are in very good agreement with the phonon dispersion calculated for the rhombohedral primitive cell with ${C}_{3v}$ symmetry.