Cation Disorder and Large Tetragonal Supercell Ordering in the Li-Rich Argyrodite Li<sub>7</sub>Zn<sub>0.5</sub>SiS<sub>6</sub>
Bernhard T. Leube, Christopher M. Collins, Luke M. Daniels, Benjamin B. Duff, Yun Dang, Ruiyong Chen, Michael W. Gaultois, Troy D. Manning, Frédéric Blanc, Matthew S. Dyer, John B. Claridge, Matthew J. Rosseinsky
Abstract
High Resolution Image Download MS PowerPoint Slide A tetragonal argyrodite with >7 mobile cations, Li 7 Zn 0.5 SiS 6, is experimentally realized for the first time through solid state synthesis and exploration of the Li–Zn–Si–S phase diagram. The crystal structure of Li 7 Zn 0.5 SiS 6 was solved ab initio from high-resolution X-ray and neutron powder diffraction data and supported by solid-state NMR. Li 7 Zn 0.5 SiS 6 adopts a tetragonal I 4 structure at room temperature with ordered Li and Zn positions and undergoes a transition above 411.1 K to a higher symmetry disordered F 4 3 m structure more typical of Li-containing argyrodites. Simultaneous occupation of four types of Li site (T5, T5a, T2, T4) at high temperature and five types of Li site (T5, T2, T4, T1, and a new trigonal planar T2a position) at room temperature is observed. This combination of sites forms interconnected Li pathways driven by the incorporation of Zn 2+ into the Li sublattice and enables a range of possible jump processes. Zn 2+ occupies the 48 h T5 site in the high-temperature F 4 3 m structure, and a unique ordering pattern emerges in which only a subset of these T5 sites are occupied at room temperature in I 4 Li 7 Zn 0.5 SiS 6 . The ionic conductivity, examined via AC impedance spectroscopy and VT-NMR, is 1.0(2) × 10 –7 S cm –1 at room temperature and 4.3(4) × 10 –4 S cm –1 at 503 K. The transition between the ordered I 4 and disordered F 4 3 m structures is associated with a dramatic decrease in activation energy to 0.34(1) eV above 411 K. The incorporation of a small amount of Zn 2+ exercises dramatic control of Li order in Li 7 Zn 0.5 SiS 6 yielding a previously unseen distribution of Li sites, expanding our understanding of structure–property relationships in argyrodite materials.