Temperature-dependent high-energy diffraction and thermal diffuse scattering from UO2 and tellurium single crystals

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We propose to exploit the new capabilities of the recently inaugurated high-energy diffraction beam line P21.1@PETRA III to determine from single crystal diffraction data the structure of UO2 below the Neel transition at TN = 30.8 K, to test a prediction concerning the intensities of Bijvoet pairs in tellurium and to obtain thermoelastic stiffness coefficients from thermal diffuse scattering from both compounds.Low temperature high-energy (100 keV) single crystal diffraction studies of UO2 will allowus to provide accurate structural data and test hypotheses on the oxygen position in the low temperature phase. The structural model for the low temperature phase is more than 40 year old and was derived from the dependence of the intensities of magnetic reflections on the scattering angle. This model has never been refined against high resolution diffraction data. With P21.1 we will be able to investigate the structural changes occurring at the first order transition at 30.8 K with unprecedented accuracy.With the same experimental set-up, we propose to test a prediction on relative intensities of Bijvoet pairs in tellurium. This would provide an alternative approach to determine the absolute structure of monatomic acentric crystals.For both compounds, thermoelastic coefficients will be obtained from thermal diffuse scattering, allowing us to understand the softening of elastic stiffness coefficients during cooling in UO2 and changes in the inter- and intrachain interactions in tellurium up to the melting point.