High-Temperature-Induced Phase Transitions in Honeycomb Layered Oxides A2Ni2TeO6 (A = Na, K)

Phase transitions have been surmised as underlying factors behind the exceptional electrochemical, ionic and magnetic functionalities that have catapulted honeycomb layered oxides as superb functional materials. As such, in this study (short communication), we explore temperature elevation as an avenue for inducing phase transitions in honeycomb layered oxides adopting the composition A2M2TeO6 (A = Li, Na, K; M = (transition) metal). X-ray diffraction analyses indicate structural changes occurring in Na2Ni2TeO6 hexagonal lattice (centrosymmetric P63/mcm → accentric P63(22) space group) with increase in temperature, whilst in the potassium homologue (K2Ni2TeO6), the phase transitions entail multiple changes in the lattice (from the initial hexagonal → monoclinic (pseudo-orthorhombic) lattice at intermediate temperatures) which reverts back to its initial hexagonal lattice with further increase in temperatures. This study opens an alternative channel for generating phase transition beside electrochemical alkali (re)insertion.