Structural complexities and sodium-ion diffusion in the intercalates Na x TiS 2 : move it, change it, re-diffract it.

After momentary attention as potential battery materials during the 1980s, sodium titanium disulphides, like the whole Na-Ti-S system, have only been investigated in a slapdash fashion. While they pop up in current reviews on the very subject time and again, little is known about their actual crystal-structural features and sodium-ion diffusion within them. Herein, we present a short summary of literature on the Na-Ti-S system, a new synthesis route to Na 0.5 TiS 2 -3 R 1 , and results of high-temperature X-ray and neutron diffractometry on this polytype, which is stable for medium sodium content. Based thereon, we propose a revision of the crystal structure reported in earlier literature (missed inversion symmetry). Analyses of framework topology, probability-density functions, and maps of the scattering-length density reconstructed using maximum-entropy methods (all derived from neutron diffraction) reveal a honeycomb-like conduction pattern with linear pathways between adjacent sodium positions; one-particle potentials indicate associated activation barriers of ca. 0.1 eV or less. These findings are complemented by elemental analyses and comments on the high-temperature polytype Na 0.9 TiS 2 -2 H . Our study helps to get a grip on structural complexity in the intercalates Na x TiS 2 , caused by the interplay of layer stacking and Na-Ti-vacancy ordering, and provides first experimental results on pathways and barriers of sodium-ion migration.