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

• We have measured the single-crystal elastic constants of MgF2 to 7.4 GPa.
• The pressure dependence of all the individual and aggregate elastic moduli are constrained above 1 GPa for the first time.
• We observe softening of the shear modulus with increasing pressure indicating the approach of the rutile-CaCl2 type phase transition.
• The pressure–volume equation of state of MgF2 was determined self-consistently from the Brillouin data.
• MgF2 becomes highly elastically anisotropic and partially auxetic at high pressures.

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 Poisson's ratio in certain directions).