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.

中文翻译：

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立方 10BP、natBP 和 11BP 晶体中温度相关的光学声子位移和分裂

立方磷化硼 (BP) 单晶的红外反射光谱显示出非常窄的 Reststrahlen 带，表明小的 TO-LO（横向光学-纵向光学）分裂。研究${\rm TO}(\Gamma)$和${\rm LO}(\Gamma)$的${^{10}}{\rm BP}$的声子热行为，${^ {\rm nat}}{\rm BP}$和${^{11}}{\rm BP}$块状单晶，85–500 K 中的温度相关红外反射光谱在这里应用。随着温度升高，Reststrahlen 带变宽。${\rm TO}(\Gamma)$和${\rm LO}(\Gamma)$的频率表现出非线性红移，并且 TO-LO 分裂逐渐增加。我们的研究发现，热膨胀在低温下起主导作用，而声子非谐性在高温下逐渐发生。