Raman spectrum and lattice parameters of ${\mathrm{MgB}}_2$ as a function of pressure

We report Raman spectra and synchrotron x-ray diffraction measurements of lattice parameters of polycrystalline ${\mathrm{MgB}}_2$ under hydrostatic pressure conditions up to 15 GPa. An anomalously broadened Raman band at $620{\mathrm{cm}}^{-1}$ is observed that exhibits a large linear pressure shift of its frequency. The large mode damping and Grüneisen parameter indicate the vibration is highly anharmonic, broadly consistent with theoretical predictions for the $E_{2g}$ in-plane boron stretching mode. The results obtained may provide additional constraints on the electron-phonon coupling in the system.