HEAT-CAPACITY OF FE(3-ALPHA)M(ALPHA)O(4) (M=ZN, TI, THAN-OR-EQUAL-TO-ALPHA-LESS-THAN-OR-EQUAL-TO-0.04)

Systematic studies of the specific-heat anomaly and of the entropy cha nges at the Verwey transition in the Fe3-xTixO4 and Fe3-yZnyO4 series are reported. These investigations clearly demonstrate that in Fe3-xTi xO4 and Fe3-yZnyO4, as in magnetite, there occurs only one, well defin ed 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 ext remely sharp spikes in the specific-heat curve at the transition tempe rature T-v; for 0.012<x,y<0.04 the transition is of second or higher o rder, consistent with the broadened specific-heat anomalies near T-v. Deviations from the undoped Fe3O4 state result in a lowering of the tr ansition temperature and in a concomitant diminution of the entropy of the transition in a manner very similar to nonstoichiometric magnetit e. The difference in heat capacity below T-v for the first- and second -order samples is analyzed and the lattice properties are identified a s the principal factor responsible for this difference.