Strain stiffening, high load-invariant hardness, and electronic anomalies of boron phosphide under pressure

New refractory hard materials with a favorable band gap are in high demand for the next-generation semiconductors capable of withstanding high temperature and other hostile environments. Boron phosphide (BP) is such an attractive candidate with exceptional properties; however, it has mainly been studied theoretically because of the difficulty in sample preparation. In this work, we report successful synthesis of large millimeter-sized single-crystal BP. The final product has a zinc-blende structure with a unique electronic structure and is optically transparent with a moderate band gap of similar to 2.1 eV. Our experiments, in conjunction with ab initio simulations, reveal that the compound exhibits extraordinary strain stiffening and unusually high load-invariant hardness of similar to 38 (3) GPa, which is close to the 40-GPa threshold for superhard materials, making BP the hardest among all known semiconductors. Based on the first-principles calculations, the fracture mechanisms in BP under tensile and shear deformations can be attributed to the formation of a metastable hexagonal phase. Further spectroscopic measurements indicate that an unusual electronic transition occurs at high pressures of similar to 13 GPa, resulting in an asymptotically enhanced covalent bonding state. The pressure dependence of multiphonon processes is also determined by Raman measurement. In addition, our studies suggest a phonon-assisted photoluminescence process and evidence for the photon-pumped etalon effect at 707 nm.