Unusual Fast Cation Conduction in the High-Temperature Phase of Lithium Sodium Sulfate
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Unusual Fast Cation Conduction in the High-Temperature Phase of Lithium Sodium Sulfate H. Feldmann, R.E. Lechner1 , and D. Wilmer 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
ABSTRACT Lithium sodium sulfate (LiNaSO4 ) belongs to a group of simple inorganic salts exhibiting fastcation conducting high-temperature phases with rotationally disordered anions. The analysis of a combination of quasielastic neutron scattering and high-frequency (10 MHz to 60 GHz) conductivity measurements in the high-temperature phase of LiNaSO4 reveals an unusual cation conduction mechanism: the Haven ratio, HR = D∗ /Dσ , turns out to be considerably larger than one. This behavior, to our knowledge detected for the first time in a typical fast ion conductor, can be traced back to a charge correlation factor clearly smaller than unity, indicating that charge transport is less effective than tracer transport in this material. INTRODUCTION A number of crystalline materials have high-temperature phases which exhibit both high cation conductivity and rotational anion disorder. These are known as fast ion conducting plastic phases. The mechanisms of ion conduction in the plastic phases of compounds like Li2 SO4 , LiNaSO4 , LiAgSO4 , or Na3 PO4 have been discussed for a long time. It has been suggested that the transport of cations is greatly enhanced by an intimate coupling to the reorientational motion of the complex anions. There is an ongoing debate between the proponents of this so-called “paddle-wheel” mechanism [1] and those who favor an explanation in terms of a percolation-type mechanism [2]. In order to determine the relevance of the dynamic coupling in plastic-phase fast-ion conductors, it is highly desirable to examine the dynamics of cations and anions in entirely dynamic experiments. In this paper, we give a short report on experiments on LiNaSO4 using quasielastic neutron scattering and high frequency conductivity measurements. The room temperature β-phase of LiNaSO4 is stable up to Ttr = 788 K. Its space group symmetry is P 31c with Z = 6. The structure is built by SO4 and LiO4 tetrahedra. Upon transition into the α phase, the cation conductivity rises by more than two orders of magnitude to about 0.07 Ω−1 cm−1 . In α-LiNaSO4 , the rotationally disordered anions form a BCC arrangement with ˚ (829 K). Here, the sodium ions are located in octahedral sites (0, 1 , 1 ), while the smaller a = 5.75 A 2 2 lithium ions prefer a tetrahedral environment ( 14 , 0, 12 ). EXPERIMENTAL Sample preparation Powder samples of LiNaSO4 were prepared by fusing Li2 SO4 and Na2 SO4 at 973 K in a platinum crucible. Isotope-enriched 7 Li2 SO4 was used for the neutron scattering samples in order to reduce neutron absorption by 6 Li. After cooling to room temperature the crystalline samples were ground to a powder and kept in an evacuated drying chamber at 473 K for 72 hours. Sample identification
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