THE ELASTIC PROPERTIES OF FINE-GRAINED POLYCRYSTALLINE SAMPLES BY BRILLOUIN SCATTERING

The crust and mantle of the Earth are polycrystalline in nature, whereas in the laboratory we often measure properties on near-perfect single crystals when available. The relationship between the properties of single-crystal and polycrystalline samples is therefore of fundamental importance in applying lab measurements to the Earth. In this study we have investigated the sound velocities and elastic properties of fine-grained polycrystalline materials using Brillouin scattering. Our initial experiments were performed on polycrystalline MgO with a grain size of approximately a few microns or less. Samples were prepared using spark plasma sintering, resulting in dense fine-grained material that is sufficiently transparent to transmit the visible light used in Brillouin scattering. Samples were ground and polished into plate-like shapes for symmetric geometry Brillouin scattering at room conditions, and also at high pressure using a diamond anvil cell. Brillouin spectra from these samples show broad peaks that are roughly symmetric in shape. Our results thus far indicate that the velocities of the fine-grained material are consistent with the velocities obtained from single-crystal measurements using Voigt-Reuss-Hill averaging.