Abstract
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 ${\rm TO}(\Gamma)$ and the ${\rm LO}(\Gamma)$ of ${^{10}}{\rm BP}$, ${^{\rm nat}}{\rm BP}$, and ${^{11}}{\rm BP}$ 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 ${\rm TO}(\Gamma)$ and the ${\rm LO}(\Gamma)$ 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.
© 2021 Optical Society of America
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