Temperature-dependent optical phonon shifts and splitting in cubic 10BP, natBP, and 11BP 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 T O ( Γ ) and the L O ( Γ ) of 10 B P , n a t B P , and 11 B P 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 T O ( Γ ) and the L O ( Γ ) 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.