The crystal structure and sodium disorder of high-temperature polymorph β-Na3PS4

Abstract

Solid sodium conductors are of great interest as electrolytes for all-solid sodium batteries and also for sodium–sulphur batteries. Here we provide the dynamic structures of crystalline Na₃PS₄ by high-temperature powder X-ray diffraction (XRD) and molecular dynamics (MD) simulations. At room temperature, Na₃PS₄ adopts a tetragonal structure: α-Na₃PS₄. [Jansen and Henseler, J. Solid State Chem., 1992, 99(1), 110] α-Na₃PS₄ transforms to a cubic superionic phase β-Na₃PS₄ at ca. 530 K, but its detailed structure has not been solved so far. The overall structure of β-Na₃PS₄ is understood as Tl₃VS₄-type with characteristic spreading of Na distribution, reflecting the dynamic motion of Na. The α to β structural transition involves expansion of bottlenecks of Na migration especially along the tetragonal c axis, and enables three-dimensional ionic transport.