Evolution of Structure and Magnetism in FeCl₂ and FeCl₃: From Clusters to Monolayers

In this work, we address one of the most fundamental questions in cluster science─how do the structure and properties evolve from clusters to crystals? Using density functional theory (DFT), we focus our study on the evolution of structure and magnetism in iron-chloride systems, from clusters to monolayers. The choice of this system is motivated by the recent experimental confirmation of one of the author's earlier theoretical prediction that the FeCl₂ cluster is magnetic with a spin magnetic moment of 4 μ_B localized at the Fe site, while its dimer, Fe₂Cl₄, is antiferromagnetic. Similarly, FeCl₃ cluster is magnetic with a total spin magnetic moment of 5 μ_B, with 4 μ_B localized at the Fe site and 1 μ_B distributed over the Cl sites. The dimer clusters Fe₂Cl₄ and Fe₂Cl₆ have an antiferromagnetic ground state, and upon Li-functionalization, both can be magnetically transformed from antiferromagnetic to ferromagnetic states. In contrast, FeCl₂ and FeCl₃ monolayers exhibit different magnetic ground states in their periodic forms: FeCl₂ is ferromagnetic (FM), but in FeCl₃, the antiferromagnetic (AFM) and FM states are energetically nearly degenerate. Such a difference arises due to the different chemical coordination of the Fe atoms with the Cl atoms, caused by their different oxidation states, which is +2 in FeCl₂ and +3 in FeCl₃, respectively. Interestingly, Li-functionalization allows both FeCl₃ and FeCl₂ monolayers to be ferromagnetic. Our study highlights that several, but not all, electronic and magnetic characteristics of isolated clusters are preserved in the extended periodic structures. This systematic investigation of iron-halide clusters is expected to inspire further experimental and theoretical exploration into the magnetism of other transition metal halides.