Heat capacity of Fe(3-alpha)M(alpha)O(4) (M=Zn, Ti, 0<=alpha<=0.04)

Systematic studies of the specific-heat anomaly and of the entropy changes at the Verwey transition in the Fe3-xTixO4 and Fe3-yZnyO4 series are reported. These investigations clearly demonstrate that in Fe3-xTixO4 and Fe3-yZnyO4, as in magnetite, there occurs only one, well defined phase transition. In the composition range O less than or equal to x,y<0.012 the phase transition is of first order, as manifested by extremely sharp spikes in the specific-heat curve at the transition temperature T-v; for 0.012<x,y<0.04 the transition is of second or higher order, consistent with the broadened specific-heat anomalies near T-v. Deviations from the undoped Fe3O4 state result in a lowering of the transition temperature and in a concomitant diminution of the entropy of the transition in a manner very similar to nonstoichiometric magnetite. The difference in heat capacity below T-v for the first- and second-order samples is analyzed and the lattice properties are identified as the principal factor responsible for this difference.