A percolation model for the metal-insulator transition in pyrite Ni(S,Se)2

A percolation model is proposed to explain the metal-insulator transition (MIT) in the pyrite-structured nickel dichalcogenide NiS2-xSex. In our model, we assume that a NiX6 octahedron (X=S or Se) connected with Se- Se dimers to another is conductive while one with S-Se/Se-S dimers is probabilistically conductive and one with S-S dimers is insulating. Then, the insulator-to-metal phase transition occurs at a percolation threshold of the conductive dimers network. We have performed a Monte-Carlo simulation to obtain the appropriate condition to explain the critical Se concentration x(c) similar or equal to 0.55 in the MIT. The simulation result indicates that S-Se/Se-S dimers are conductive with a probability of 0.33. The physical meaning of our toy model is discussed with respect to the electronic structure of this system.