Thermodynamic model of the insulator-metal transition in nickel iodide

A model calculation of the phase diagram for the isostructural insulator-metal phase transition of nickel iodide is presented. The system is modeled by the Falicov-Kimball Hamiltonian with the calculated band structure of ${\mathrm{NiI}}_2$ and (Ising) spin-spin interactions, but neglects all hybridization effects between nickel 3d and iodine 5p bands. Many-body interactions and spin superexchange are both treated in mean-field theory. The results include second-order antiferromagnetic-paramagnetic insulator transitions, first-order insulator-metal transitions, and a (as yet unobserved and probably unobservable) classical critical point. The calculated phase diagram and transport properties agree well with the recent experimental results.