Formation of honeycomb ordered monoclinic Li2M2TeO6 (M = Cu, Ni) and disordered orthorhombic Li2Ni2TeO6 oxides

Ion-exchange reactions of Na2Cu2TeO6 with excess of lithium nitrate at low temperature (300 degrees C) readily resulted in an isostructural honeycomb ordered monoclinic layered structure Li2Cu2TeO6, otherwise in-accessible by direct solid state high temperature reactions. Similarly, Li2Ni2TeO6(I) stabilized approximately in equal amounts in both of the known P2-type polymorphs (P6(3)/mcm and P6(3)22) was synthesized as an ion-exchange reaction product from Na2Ni2TeO6 using a melt of lithium nitrate. Additionally, a unique tellurium containing oxide Li2Ni2TeO6(II) without the honeycomb ordering of Ni2+/Te6+ ions has been obtained for the first time by the direct high temperature (800-900 degrees 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 Li2Ni2TeO6(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 Li2Ni2TeO6(II).