Crystal structure and phase transitions of the lithium ionic conductor Li3PS4

The crystal structures of and phase transitions in the lithium ionic conductor, Li3PS4, which is part of the thio-LISICON (LIthium Super Ionic CONductor) family, were studied using high-temperature synchrotron X-ray diffraction measurements and thermal analysis. There are three transitions among the α, β and γ-phases of this material as a function of temperature. The low-temperature γ-phase transformed to the medium-temperature β-phase at 573 K, which then transformed to the high-temperature α-phase at 746 K. The γ-phase is iso-structural with β-Li3PO4 (SG: Pmn21) with an orthorhombic cell with a = 7.70829(11), b = 6.53521(10), and c = 6.1365(7) Å at 297 K. The β-phase has an orthorhombic cell with a = 12.8190(5), b = 8.2195(4), and c = 6.1236(2) Å at 637 K. The apexes of the PS4 tetrahedron are ordered in the same direction in the γ-phase, while in the β-phase, they are ordered in a zig-zag arrangement. The γ-phase showed an ionic conductivity of 3.0 × 10− 7 S cm− 1 with an activation energy of 21.3 kJ mol− 1 at room temperature. The β-phase showed an ionic conductivity of 3.0 × 10− 2 S cm− 1 at 500 K with an activation energy of 15.5 kJ mol− 1.

Research Highlights
► Structures of Li3PS4 were determined by high-temperature synchrotron XRD data.
► The beta-type structure indicated high lithium ionic conductivity.
► The arrangement of the PS4 tetrahedron is related to the lithium ionic conduction.