Effect of multiple cationic substitutions on structure and magnetism of honeycomb-layered hexagonal tellurates Na2M2TeO6 (M = Co, Ni, Cu, Zn)

Hexagonal layered Na2T2TeO6 (T = Co1/3Ni1/3Cu1/3) and Na2Z2TeO6 (Z = Co1/4Ni1/4Cu1/4Zn1/4) have been prepared by solid-state reactions. According to the X-ray Rietveld refinement, Na2Z2TeO6 is isostructural with its honeycomb-ordered constituents Na2M2TeO6 (M = Co, Zn), space group P6322. For Na2T2TeO6, however, only subcell (ignoring T/Te ordering) could be successfully refined despite presence of weak superstructure reflection. This is attributed to intergrowth of two packing modes with similar lattice parameters: P63/mcm (characteristic of Na2Ni2TeO6) and P6322. According to magnetic susceptibility and heat capacity measurements, both materials undergo antiferromagnetic ordering at low temperatures with negative Weiss temperatures of −38 and −26 K for the T- and Z-compositions, respectively. The Néel point of Na2T2TeO6, 16.9 K, is considerably lower than those of its Ni and Co constituents (both being about 27 K), in contrast to the sister system, monoclinic Na3T2SbO6, where TN is intermediate between those of Ni and Co constituents. Further lowering of the Néel point in Na2Z2TeO6, 8.6 K, is attributed to the diamagnetic dilution with Zn2+.