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 alpha, beta and gamma-phases of this material as a function of temperature. The low-temperature gamma-phase transformed to the medium-temperature beta-phase at 573 K, which then transformed to the high-temperature alpha-phase at 746 K. The gamma-phase is iso-structural with beta-Li3PO4 (SG: Pmn2(1)) with an orthorhombic cell with a=7.70829(11), b=6.53521(10), and c=6.1365(7) angstrom at 297 K. The beta-phase has an orthorhombic cell with a=12.8190(5), b=8.2195(4), and c=6.1236(2) angstrom at 637 K. The apexes of the PS4 tetrahedron are ordered in the same direction in the gamma-phase, while in the beta-phase, they are ordered in a zig-zag arrangement. The gamma-phase showed an ionic conductivity of 3.0 x 10(-7) Scm(-1) with an activation energy of 21.3 kJ mol(-1) at room temperature. The beta-phase showed an ionic conductivity of 3.0 x 10(-2) S cm(-1) at 500 K with an activation energy of 15.5 kJ mol(-1). (C) 2010 Elsevier B.V. All rights reserved.