ELEMENTARY FORMULATION OF THE VERWEY TRANSITION IN MAGNETITE VIA THE ORDER-DISORDER FORMALISM

A review of experimental data relating to the Verwey transformation in Fe3(1-δ)O4 and in Fe3-xZnxO4 is presented. The transition from first order to higher order with increasing δ or x is rationalized by representing the host crystal in terms of octahedral site pairs that share electrons between the two sites. The individual octahedral interstices can remain unoccupied or become occupied by localized or itinerant electrons. The application of the order-disorder formalism leads to the construction of a free energy expression which must be optimized. If the resulting relations are simplified by adopting energy parameters associated with several limiting cases, one automatically obtains the model of Strässler and Kittel. This theory had previously been demonstrated to be very useful in rationalizing both the thermodynamic and the transport properties of magnetite. © 1992 Academic Press, Inc.