Elasticity of stishovite and acoustic mode softening under high pressure by Brillouin scattering

Brillouin scattering measurements on single-crystal stishovite, a high-pressure polymorph of SiO2, were carried out to 22 GPa. Acoustic velocities in three 30-μm thick crystal platelets of synthetic stishovite were measured in a forward symmetric scattering geometry, and the full set of elastic constants were retrieved to 12 GPa. The measured velocity data were fit to Christoffel’s equation, yielding ambient-pressure elastic constants of C11 = 455(1) GPa, C33 = 762(2) GPa, C12 = 199(2) GPa, C13 = 192(2) GPa, C44 = 258(1) GPa, and C66 = 321(1) GPa. The elastic modulus (C11 − C12)/2 was observed to decrease with pressure, indicating acoustic mode softening, consistent with theoretical predications for the behavior of stishovite as it approaches the transition to the CaCl2-type phase. The bounds on the aggregate adiabatic bulk and shear moduli are KS0 = 315(1) GPa, G0 = 240(1) GPa for the Voigt bound, KS0 = 301(1) GPa, G0 = 216(1) GPa for the Reuss bound. Pressure derivatives of aggregate bulk and shear moduli were constrained to be (∂KS/∂P)T0 = 4.34(16) and (∂G/∂P)0 = 0.7(1) for the Reuss bound, and (∂KS/∂P)T0 = 4.0(1) and (∂G/∂P)0 = 1.1(1) for the Voigt–Reuss–Hill (VRH) average, respectively, by fitting the data to Eulerian finite strain equations. The volume compression curve obtained from our Brillouin measurement is in very good agreement with previous compression studies up to 50 GPa.