10th International Workshop on Infrared Microscopy and Spectroscopy with Accelerator Based Sources Proceedings

Long observed anomalies in the reststrahlen region of reflectivity spectra of cubic ionic crystals cause the failure of the harmonic scenario and find clear explications in the multi-phonon scattering processes[1,2]. Indeed, anharmonic effects are responsible for the finiteness of the phonon linewidths as well as the temperature and pressure dependence of the phonon frequencies. The need of an advanced description of the normal modes multiphonon interactions is not only of fundamental interest but also has an impact on many physical macroscopic properties such as electric, thermal conductivity and thermoelectricity[3,4]. For these reasons, in the last two decades, theoretical and numerical tools have been dramatically improved and now require a combined approach with the experimental side to accomplish an effective confirmation[5,6]. These macroscopic quantities under investigation depend on the entire phonon spectrum and are sensitive to the approximations used in the ab-initio simulations and to the quality of individual samples, jeopardizing the possibility to draw easy comparison. Combined experimental techniques are required to probe the entire phonon dispersion: Raman and infrared spectroscopy at zero wavevector and neutron or x-ray scattering at finite wavevector. Despite the simplicity of cubic ionic crystal systems, they will allow to frame the problem and present practical advantages from both the theoretical and experimental standpoints: they require modest computational efforts, they present enhanced anharmonic effects, they dispose phonons as unique heat carriers, they have direct interest for geological sciences and technological applications. To our best knowledge, most of the results of this research field are based on numerical studies, few on experiments and even fewer on the combination of the two approaches[7,8]. Here we present a comprehensive study of the optical phonon dispersion in the whole Brillouin zone of MgO single crystal as a function of temperature and pression, by combining ab-initio calculations, synchrotron based IR reflectivity and inelastic x-ray scattering experiments. For the first time, the outcomes of experiments and numerical results are systematically compared for the determination of the anharmonic contribution to the normal modes energy and lifetime.

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