Extended Honeycomb Metal Chloride with Tunable Antiferromagnetic Correlations

In recent years, the exploration of quantum triangular lattice antiferromagnets has become the subject of intensive research. Within this realm, bilayer triangular lattice antiferromagnets with diverse magnetic exchange interactions have garnered significant interest. In this work, we reported two new bilayer triangular antiferromagnets, Cs3M^IFeMnCl9, where M = Na⁺ and Ag⁺. The structures of these compounds are derived from the known Cs3Fe2Cl9, featuring a buckled honeycomb structure, where half of Fe³⁺ are substituted by Mn²⁺ and the bilayer triangular lattice layers are connected with monovalent Na⁺ or Ag⁺ ions. Surprisingly, we found that the band gaps are substantially reduced compared to Cs3Fe2Cl9. Density functional theory (DFT) results attribute this trend to a strong involvement of Mn²⁺ d-orbitals in the valence band maximum of Cs3M^IFeMnCl9. Moreover, magnetic susceptibility measurements demonstrate a stronger magnetic exchange interaction in Cs3M^IFeMnCl9, which is modulated by bridging Na⁺ or Ag⁺ ions. These findings illustrate an interesting chemical approach for manipulating the electronic and magnetic properties of buckled honeycomb antiferromagnets.