Experimental and Computational Insights into the Anomalous Thermal Expansion of (Nh4)Reo4,

Abstract

The temperature dependence of the structure and the ground state properties of scheelite type NH 4 ReO 4 have been studied using neutron powder diffraction (NPD) and Density Functional Theory (DFT), respectively. Despite the large incoherent background in the experimental NPD, associated with the presence of hydrogen, accurate and precise structural parameters were obtained.  Comparison of the results of the NPD and DFT studies shows that the observed anomalous thermal contraction in NH 4 ReO 4 is a consequence of thermally induced rotational disorder of the NH 4 groups. Comparing the experimentally determined and optimized structures reveals deformation of the NH 4 tetrahedra that is responsible for the unusual tetragonal distortion of this material. The Raman Spectra of NH 4 ReO 4 is presented and the modes are assigned based on the DFT calculations.

Keywords: Scheelite, Neutron diffraction, Thermal Expansion, Density functional theory, Raman spectroscopy