Effect of Al³⁺ and H⁺ on the elastic properties of stishovite

X-ray diffraction, Brillouin, and Raman scattering measurements were performed on Al³⁺ and H⁺-bearing stishovite at ambient conditions. Samples with different Al³⁺ and H⁺ contents were used to examine the effects of these minor constituents on the density, acoustic velocities, single-crystal and aggregate elastic moduli. The X-ray diffraction and compositional data suggest that the incorporation mechanism of Al³⁺ into stishovite involves the formation of oxygen vacancies, in addition to the incorporation H⁺ in the structure. Our data show an overall linear decrease of the acoustic velocities, single crystal (c_ij) and aggregate (K_o, G_o) elastic moduli as a function of Al³⁺ concentrations. For the sample of stishovite containing 6.07(5) wt% Al₂O₃ and 0.24(2) wt% H₂O we obtained: zero-pressure adiabatic bulk modulus, K_so = 290(3) GPa, and the shear modulus, G_so = 207(2) GPa, with a calculated density of ρ = 4.16(1) g/cm³ based on X-ray diffraction. Stishovite containing 4.37(12) wt% Al₂O₃ and 0.29(3) wt% H₂O possesses higher aggregate moduli: K_so = 296(3) GPa and G_so = 213(2) GPa, with a calculated density of ρ = 4.21(1) g/cm³ based on X-ray diffraction. We conclude that the formation of oxygen vacancies has a stronger effect on the density and thus elastic properties of stishovite than does the incorporation of hydrogen.