Sodium Ion Conduction in Plastic Phases: Dynamic Coupling of Cations and Anions in the Picosecond Range
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Sodium Ion Conduction in Plastic Phases: Dynamic Coupling of Cations and Anions in the Picosecond Range D. Wilmer, H. Feldmann R.E. Lechner1 , and J. Combet2 M¨unster University, Institute of Physical Chemistry and Sonderforschungsbereich 458, Schlossplatz 4/7, 48149 M¨unster, Germany 1 Hahn-Meitner-Institut, Glienicker Str. 100, 14109 Berlin, Germany 2 Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France
ABSTRACT Results of simple computer simulations and model calculations for ion conducting rotor phases are compared to quasi-elastic neutron scattering data from solid solutions of sodium orthophosphate and sodium sulphate, x Na2 SO4 · (1 − x) Na3 PO4 . These materials are not only sodium fast-ion conductors in their high-temperature cubic phases but also show considerable dynamic reorientation disorder of their tetrahedral anions. At an elastic energy resolution of about 100 µeV, neutron spectrometry monitored oxygen scattering due to anion reorientation which occurs on the picosecond time scale. This thermally activated process exhibits activation energies between 0.184 eV (x = 0.0) and 0.052 eV (x = 0.5). Analysis of the quasielastic intensities as a function of scattering vector Q gives clear evidence of the involvement of cations in the anion reorientation. Increasing the elastic resolution to about 1 µeV FWHM (thereby shifting the dynamic window to the nanosecond scale) allowed to examine sodium diffusion in x Na2 SO4 · (1 − x) Na3 PO4 . This process consists predominantly of thermally-activated jumps between tetrahedrally coordinated sites, the activation energies ranging from 0.64 eV for x = 0.0 to 0.30 eV for x = 0.5. INTRODUCTION The characterization of ion dynamics in fast ion conductors is both a fascinating and complicated task since we are dealing with a large number of interacting charged particles in a disordered environment. The situation is even more complex if we consider fast cation conducting rotor phases which combine high cation mobility with dynamic rotational disorder of their polyatomic anions. The conduction mechanism in the plastic phases of compounds like, e.g., Li2 SO4 , LiNaSO4 , LiAgSO4 , and Na3 PO4 has thus been discussed for a long time, and an intense debate focused on the relevance of a possible dynamic coupling between the rotational and translational motion of anions and cations [1–10]. Since most of the fast cation conducting rotor phases exist at elevated temperatures, the elementary steps of cation transport and anion reorientation typically occur on the nanosecond to picosecond time scale. These processes are, therefore, ideally suited for investigation by quasielastic neutron scattering. Moreover, this technique offers the additional advantage of providing spatial information, e.g., jump distances for the cation hopping process. In our contribution we present results from simple computer simulations and model calculations for ion conducting rotor phases and compare them to our experimental findings in several quasielastic neutron scatterin
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