IS RADIAL X-RAY DIFFRACTION A RELIABLE TOOL TO DETERMINE ELASTIC PROPERTIES AT HIGH PRESSURE?

Radial X-ray diffraction of materials under nonhydrostatic pressure and lattice strains theory [1] allow us to investigate mechanical properties at pressures relevant to the deep Earth's interior. Recently, in the case of e-Fe, the disagreement between the results from radial X-ray diffraction and those of high-pressure inelastic X-ray scattering, Raman scattering, and theoretical computations [2] has raised questions about the reliability of radial X-ray diffraction as a means to study the high-pressure elasticity of minerals. We tested the reliability of the results of radial X-ray diffraction performing simultaneous single-crystal Brillouin scattering to 25 GPa and powder radial X-ray diffraction to 65 GPa of CaO (lime). The elastic constants determined at ambient conditions are consistent with existing low-pressure ultrasonic data and within 10% of theoretical values [3]. From radial X-ray diffraction, we determined the shear strength, which increases from 0.3 at 1 bar to 1.8 GPa at 65 GPa, and we constrained the 300 K isotherm in excellent agreement with previous results [4]. The elastic anisotropy of CaO increases 85 % in magnitude from 1 bar to 65 GPa. At pressure above 10 GPa we observed softening of C₄₄, which drives, at 58 GPa, a B1 - B2 structural transition. The agreement between radial diffraction results and the extrapolation of Brillouin data is better than 5% up to 35 GPa. At pressures above 35 GPa C₁₁ is as much as 12 % softer and C₁₂ as much as 40% stiffer than the extrapolation of Brillouin results. The results can be reconciled by 18 % decrease of the degree of stress continuity across grain boundaries in the sample. This study shows that radial X-ray diffraction can be a reliable means to obtain the elastic tensor at high-pressure for materials with cubic symmetry. References [1] A.K. Singh et al. J. Appl. Phys. 83, 7567 (1998). [2] A.K. Singh et al. Phys. Rev. Lett. 80, 2157 (1998); H.-K. Mao et al. Nature. 396, 741 (1998); (1999); Steinle-Neumann et al. Phys. Rev. B. 60, 791 (1999); Merkel et al. Science. 288, 1626(2000); Fiquet et al. Science. 291, 468 (2001). [3] B.B. Karki, and J. Crain. J. Geophys. Res. 103, 12405 (1998); H. Oda et al. Phys. Chem. Minerals. 19, 96 (1992). [4] P. Richet et al. J. Geophys. Res. 93, 15279 (1988).