X-ray thermal diffuse scattering at high pressure

Abstract

In single-crystal diffraction images, thermal diffuse scattering (TDS) produces a highly structured background, which contains a wealth of information about the lattice dynamics of the system. Synchrotron facilities with high x-ray brilliance and modern area detectors allow high-quality TDS data to be collected in less than one hour, even from samples enclosed in high-pressure diamond anvil cells. Compared to inelastic x-ray scattering, these much shorter data collection times open the way to more detailed studies of crystalline systems as a function of pressure, temperature, and/or other parameters, albeit at the cost of a much more demanding data analysis. A specific challenge in high-pressure TDS experiments is to account for the Compton-scattering and TDS background from the diamond anvils. TDS data analysis software has been developed to simulate TDS patterns and extract the full phonon dispersion relations at high pressure from experimental TDS images.

The TDS method was tested with the element Ge. TDS analysis performed on data collected under ambient and high-pressure conditions yielded phonon dispersion relations with an excellent agreement with existing frequency measurements. Despite the additional challenges associated with the high-pressure data analysis, the TDS method revealed pressure-induced softening effects, which were also observed in previous studies.

The full phonon dispersion relations for caesium in the bcc-Cs, fcc-Cs and Cs-IV phases were obtained from the TDS method. The lattice dynamics from this method were consistent with observations of a pressure-induced frequency plateau from a previous study that measured a single mode at high pressure. The full lattice dynamics obtained from the TDS method were used to determine the mode-Gr¨uneisen parameters, speed velocities and the phonon density of states for many pressures across the three Cs phases.

Finally, lead telluride was examined at a range of temperatures and pressures. The data analysis for this system proved to be more challenging than previously encountered for the other systems in this work. The phonon dispersion relations obtained from the TDS data analysis agreed well with previous experimental studies at lower-frequency phonon branches but deviated from these studies at higher-frequency branches. A mix of temperature- and pressure-induced softening and hardening is observed for several different modes. The mode-Gruneisen parameters of several modes were determined to study the anharmonicity of the system.