Formation of honeycomb ordered monoclinic Li₂M ₂TeO₆ (M = Cu, Ni) and disordered orthorhombic Li ₂Ni₂TeO₆ oxides

Ion-exchange reactions of Na₂Cu₂TeO₆ with excess of lithium nitrate at low temperature (300 °C) readily resulted in an isostructural honeycomb ordered monoclinic layered structure Li ₂Cu₂TeO₆, otherwise inaccessible by direct solid state high temperature reactions. Similarly, Li₂Ni ₂TeO₆(i) stabilized approximately in equal amounts in both of the known P2-type polymorphs (P6₃/mcm and P6₃22) was synthesized as an ion-exchange reaction product from Na₂Ni ₂TeO₆ using a melt of lithium nitrate. Additionally, a unique tellurium containing oxide Li₂Ni₂TeO ₆(ii) without the honeycomb ordering of Ni²⁺/Te ⁶⁺ ions has been obtained for the first time by the direct high temperature (800-900 °C) synthesis. The oxides were investigated by refinement of powder X-ray diffraction patterns, room temperature magnetization experiments along with Raman spectroscopy and photoluminescence measurements. A structural model has been suggested for the metastable Li₂Ni ₂TeO₆(ii) and the presence of structural disorder was evidenced in the broadening of the Raman bands and the intense broad photoluminescence (PL) spectra obtained for Li₂Ni₂TeO ₆(ii).