High-purity cubic boron nitride (cBN) and hexagonal boron nitride (hBN) single crystals were synthesised at 4.5 GPa and 1500 degrees C using barium boron nitride as a solvent. Secondary ion mass spectrometry was used to analyse impurities in the crystals. Fine cBN and hBN crystals, whose oxygen and carbon concentrations were less than 10(18) atoms/cm(3), were obtained, and their band-edge optical properties were measured by cathodoluminescence spectroscopy. High-purity hBN single crystals exhibited intense ultraviolet emission, demonstrating their promise for use as deep ultraviolet-light emitters. (C) 2007 Elsevier B.V. All rights reserved.