Cation Disorder and Large Tetragonal Supercell Ordering in the Li-Rich Argyrodite Li₇Zn_0.5SiS₆

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.