Metal-insulator and order-disorder phase transitions: Magnetite

A thermodynamic model is presented for a system which exhibits both metal-insulator and order-disorder phase transitions. It is found that transition temperatures could be much lower than those predicted by pure order-disorder -only considerations. A variety of behaviors, including first-order and second-order transitions and critical points is found as the parameters change. The model explains qualitatively the experimental results for magnetite and suggests other possible behaviors for closely related systems.