Pressure-induced depolarization and resonance in Raman scattering of single-crystalline boron carbide

We report polarized and resonant Raman scattering of single-crystal boron carbide $({\text{B}}_{4}\text{C})$ at high pressures. Significant intensity enhancements of 270 and $1086\text{ }{\text{cm}}^{\ensuremath{-}1}$ Raman bands of ${\text{B}}_{4}\text{C}$ have been observed at quasihydrostatic pressures higher than $\ensuremath{\sim}20\text{ }\text{GPa}$. The pressure-induced intensity change of the $1086\text{ }{\text{cm}}^{\ensuremath{-}1}$ band is mainly due to the resonance between excitation energy and electronic transition, whereas the intensity change of $270\text{ }{\text{cm}}^{\ensuremath{-}1}$ band is caused by the depolarization effect. Importantly, the first-order phase transition has not been found at high quasihydrostatic pressures and all the Raman intensity changes along with the corresponding high-pressure lattice distortion can be recovered during unloading.