Effect of Sodium Excess and Cobalt Deficiency on the Structural and Magnetic Properties of Honeycomb Na₂Co₂TeO₆

Na₂Co₂TeO₆ is a layered honeycomb magnet crystallizing in the monoclinic C2/m space group. It has attracted attention as a candidate system for realizing bond-dependent Kitaev magnetic interactions. In this work, we investigate how variations in sodium content and cobalt deficiency influence its structural and magnetic properties. By introducing Na excess and reducing Co occupancy, we observe a systematic decrease in the lattice parameter along the c-axis, indicating a contraction of the interlayer spacing. Magnetic susceptibility measurements reveal that the characteristic peak temperature associated with antiferromagnetic ordering shifts to lower values with increasing Na content. These results suggest that Na stoichiometry and Co-site vacancies play a key role in tuning both the crystal structure and magnetic interactions in Na₂Co₂TeO₆, providing a pathway to control exchange couplings in honeycomb cobaltates.