Crystal Structure and Ionic Conductivity of Na₂MnP₂S₆

Abstract

Green needlelike single crystals of Na 2 MnP 2 S 6 were grown by crystallization after melting a mixture of Na 2 S, Mn, and P 2 S 5 . 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. Na 2 MnP 2 S 6 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) Å 3 , and Z = 4. The Na and Mn atoms are octahedrally coordinated to the S atoms. They form [Na 2 MnS 6 ] 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 P3̅ m1-Li 4 P 2 S 6 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 (P3̅ m1 to Pbcn). The ionic conductivity of Na 2 MnP 2 S 6 is 6.2 × 10 −7 S cm −1 (Ea = 0.41 eV) at 31 °C.