Heterovalent substitutions in Na2M2TeO6 family: Crystal structure, fast sodium ion conduction and phase transition of Na2LiFeTeO6

A new solid electrolyte Na2LiFeTeO6 has been prepared by solid-state reactions in air at 700 °C and characterized by powder X-ray diffraction, redox titration and impedance measurements on ceramic samples. It is a new superstructure type of the well-known hexagonal layered P2 type with complete ordering of Li, Fe and Te in octahedral brucite-like layers: orthorhombic, P212121, a = 5.2109(1), b = 8.9791(2), c = 11.2389(2) Å, Z = 4. At room temperature, sodium ions in the interlayer gap are concentrated in two of the six non-equivalent trigonal prisms; at elevated temperatures, they exhibit high ionic conductivity, e.g., 4 S/m at 300 °C. Orthorhombic distortion decreases on heating, presumably due to sodium movement, and vanishes at ca. 400 °C giving rise to the hexagonal Na2Co2TeO6-type structure. Analogous Na2LiRTeO6 (R = Al or Ga), Na2LiFeWO6, Na2Mn2TeO6, Na2M2WO6 (M = Ni or Cu), Na2NiMSbO6 (M = Al or Fe) and Na2NiAlTO6 (T = Nb or Ta) could not be prepared.

► Na2LiFeTeO6 is a new orthorhombic superlattice of the known hexagonal layered P2 type.
► The distortion seems to be only due to Na+ ion ordering in the interlayer prisms.
► On heating, high Na+ ion conductivity is observed, e.g., 4 S/m at 300 °C.
► With Na+ ion movement, distortion decreases and vanishes at ca. 400 °C.
► 10 other variants of M and/or Te substitutions in Na2M2TeO6 family were unsuccessful.