Temperature-dependent optical phonon shifts and splitting in cubic BP, BP, and BP crystals

The infrared reflectance spectrum of cubic boron phosphide (BP) single crystals shows a very narrow Reststrahlen band, indicating a small TO-LO (transverse optical-longitudinal optical) splitting. To study the phonon thermal behavior of the TO(0) and the LO(0) of BP, BP, and BP bulk single crystals, temperature-dependent infrared reflectance spectroscopy in 85–500 K is applied here. As the temperature increases, the Reststrahlen band broadens. The frequencies of the TO(0) and the LO(0) exhibit nonlinear red shifts, and the TO-LO splitting gradually increases. Our research found that thermal expansion plays a leading role at low temperatures while phonon anharmonicity gradually takes place at high temperatures.