A New Family of Fast Sodium Ion Conductors: Na2M2TeO6 (M = Ni, Co, Zn, Mg)

The four compounds Na2M2TeO6 (M2+ = Ni, Co, Zn, Mg) have been prepared by solid-state reactions in air at 600−820 °C and characterized by powder X-ray diffraction, redox titration, impedance, and polarization measurements on ceramic samples. All of them are superstructures of the well-known hexagonal layered P2-type with ordering of M and Te in octahedral brucite-like layers. They have similar parameters of the hexagonal cells: a = 5.20−5.28 Å, c = 11.14−11.31 Å, but different stacking sequences along c. With M = Co, Zn, Mg (P6322), there are columns Te M Te M and M M M M, but with M = Ni (P63/mcm), there are columns Te Te Te Te and Ni Ni Ni Ni. As little as 5% Li substitution for Ni induces transformation to the P6322 structure. Sodium ions in the interlayer gaps are disordered over a number of trigonal prisms sharing faces and exhibit high conductivity: 4−11 S/m at 300 °C, despite relatively low densities of the ceramics. The materials are purely ionic conductors; the largest electronic contribution (0.1% at 300 °C) has been found for the Co compound, presumably due to a minor admixture of Co(3+).