Identification of TO and LO phonons in cubic natBP, 10BP and 11BP crystals

Cubic boron phosphide (BP) has the physical advantages of ultra-high hardness, high melting point, and high thermal conductivity, which make it greatly potential in device applications under extreme conditions. However, there are still problems about phonon identification of BP with a curious phenomenon found in the Raman spectra of isotopic lattice (the small peak in the Raman spectra of natBP almost or completely disappears in the Raman spectra of isotopic lattice). Faced with this problem, here we report the extremely narrow reststrahlen bands of natBP, 10BP, and 11BP single crystals via infrared reflectance spectrum different from that of cubic BN, α-GaN, and 4H–SiC, and identify their transverse optical phonon (TO) and longitudinal optical phonon (LO), which breaks traditional understanding in this aspect. Through the classical phonon polariton model fitting, the TO-LO splitting in three BP crystals is calculated less than 3 cm−1, which is attributed to their weak ionicity. This research helps to identify the Raman spectra of BP, where the strongest Raman peak is found as a mixed mode of TO(Γ) and LO(Γ).