Crystal Structure and Ionic Conductivity of Na₂MnP₂S₆

Green needlelike single crystals of Na2MnP2S6 were grown by crystallization after melting a mixture of Na2S, Mn, and P2S5. The crystal structure of the new compound was determined from single-crystal X-ray diffraction data. The chemical composition was confirmed by energy-dispersive X-ray spectroscopy, and the ionic conductivity was measured using electrochemical impedance spectroscopy. Na2MnP2S6 crystallizes in the orthorhombic system, space group Pbcn (no. 60) with a = 7.3850(6) Å, b = 21.6682(13) Å, c = 6.2495(4) Å, V = 1000.04(12) Å3, and Z = 4. The Na and Mn atoms are octahedrally coordinated to the S atoms. They form [Na2MnS6]8– honeycomb-like structure within which P–P dimer units are located. Two of the three sodium atomic positions are partially occupied. The group-subgroup transformations emphasized a strong relationship with the P3̅m1-Li4P2S6 structure. Indeed, 4Li atoms were replaced by 2Na, Mn, and □ (□: vacancy), and half of the P–P dimer units have the same distribution within the tunnels. The new cationic ordering is at the origin of the trigonal to orthorhombic symmetry transformation (P3̅m1 to Pbcn). The ionic conductivity of Na2MnP2S6 is 6.2 × 10–7 S cm–1 (Ea = 0.41 eV) at 31 °C.