Synthesis, structure determination, and ionic conductivity of sodium tetrathiophosphate

Single-phase colorless sodium thiophosphate powders and transparent colorless single crystals of the low-temperature phase of Na3PS4 have been synthesized by solid-state reaction from sodium metal, sulfur, and tetraphosphorusdecasulphide. At 261°C a phase transition from α- to β-Na3PS4 has been established via differential thermal analysis and temperature-dependent X-ray powder diffraction. X-ray structure analysis has been performed for the low-temperature phase at 25°C. The space group is P4¯21c, with a = 695.20(4)pm and c = 707.57(5)pm. The compound consists of sodium cations and isolated PS3−4 anions with two formula units per unit cell. AC-conductivity measurements show Na3PS4 to be a good ionic conductor with conductivities between σ = 4.17 × 10−6 Ω−1 cm−1 (at 50°C) and σ = 8.51 × 10−2 Ω−1 cm−1 (at 510°C). The activation energies for ion transport are 40.1 kJ mole−1 for α-Na3PS4 and 38.8 kJ mole−1 for β-Na3PS4. Above 490°C there is evidence for a second high-temperature phase existing with dynamically disordered anions, causing a steep increase in conductivity.