Phonon Dispersions in Metals at High Pressure Investigated by Inelastic X-Ray Scattering

Abstract: Traditionally, experimental determination of lattice dynamics is the domain of inelastic neutron scattering, but the restrictions on sample size imposed by the technique relegated the achievable information to low or at most moderate pressures (~10 GPa). Characterizing the effect of pressure on the propagation of elastic wave is instead singularly important for understanding elasticity, mechanical stability of solids, material strength, inter-atomic interactions, phase transition mechanism and to constrain models of the Earth and planetary interior. The elastic properties and the sound wave anisotropy of hcp metals at high pressure have been experimentally investigated by Inelastic X-ray Scattering (IXS). This technique allows the collection of the phonon dispersion curve and is particularly well suited for extreme conditions. The case of hcp iron, the main constituent of the Earth's inner core, will be addressed, followed by the case of hcp cobalt, chosen as analogue to iron, due to the similar mechanical and thermal properties, but with the advantage to be available as a single crystal so that the complete elastic tensor has been determined and a comparative study of the single-crystalline and aggregate elastic properties has been performed. The case of molybdenum will be also discussed, as an example of a combined experimental and theoretical study of lattice dynamics at high pressure. In this regard compression provides a very convenient way to probe the inter-atomic potential, allowing the study of the nature of anharmonic contributions, as well as electron-phonon interactions.