Single-crystal elastic constants of magnesium difluoride (MgF2) to 7.4 GPa Original Research Article

The six independent elastic constants (C11, C12, C13, C33, C44, and C66) of single-crystal MgF2 in the rutile structure have been measured by Brillouin spectroscopy at room temperature from ambient conditions to 7.4 GPa. Measurements were performed on two monocrystals with perpendicular faces, (001) and (100). A quasi-linear fit from finite strain theory was applied to the experimental data revealing the pressure dependence of the six elastic constants of MgF2. The shear modulus CS=1/2(C11−C12), and the aggregate shear (Voigt–Reuss–Hill) modulus G show a softening with increasing pressure, indicating the approach of the rutile-to-CaCl2-type structural phase transition at P~9 GPa. The adiabatic bulk modulus (Reuss average) and its pressure derivative have been determined: K0S=105.1±0.3 GPa, (∂K0S/∂P)T=4.14±0.05. The pressure–volume equation of state of MgF2 was computed self-consistently from the Brillouin data. Our results are in good agreement with X-ray diffraction data. As the phase transition is approached, MgF2 becomes strongly anisotropic and develops partially auxetic behavior (a negative Poissonʹs ratio in certain directions).