Sodium ion conduction in plastic phases: Dynamic coupling of cations and anions in the picosecond range
- PDF / 165,010 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 50 Downloads / 160 Views
Sodium ion conduction in plastic phases: Dynamic coupling of cations and anions in the picosecond range D. Wilmera) and H. Feldmann Mu¨nster University, Institute of Physical Chemistry and Sonderforschungsbereich 458, 48149 Mu¨nster, Germany
R.E. Lechner Hahn-Meitner-Institut, 14109 Berlin, Germany
J. Combet Institut Laue-Langevin, 38042 Grenoble Cedex 9, France (Received 16 March 2005; accepted 19 May 2005)
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, xNa2SO4⭈(1 − x)Na3PO4. 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 eV, 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 eV full width at half-measure (FWHM) (thereby shifting the dynamic window to the nanosecond scale) allowed examination of sodium diffusion in xNa2SO4⭈(1 − x)Na3PO4. 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.
I. 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., Li2SO4, LiNaSO4, LiAgSO4, and Na3PO4 has thus been discussed for a long time, and an
a)
Address all correspondence to this author. e-mail: [email protected] This paper was selected as the Outstanding Meeting Paper for the 2004 MRS Fall Meeting Symposium K Proceedings, Vol. 835. DOI: 10.1557/JMR.2005.0277 J. Mater. Res., Vol. 20, No. 8, Aug 2005
http://journals.cambridge.org
Downloaded: 30 Jan 2015
intense debate focused on the relevance of a possible dynamic coupling between the rotational and translational motion of anions and cations.1–10 Part of the controversy was due to the fact that the term “paddle-wheel mechanism” was used in different meanings.11 From both experimental11 and theoretical12 studies it appears more probable that both effects enhance the ionic conductivity in rotor phases. Since most of the fast cation conducting rotor phases exist at elevated temperatures, the elementary st
Data Loading...
