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.