Thermodynamics of Fe(II)Fe(III) oxide systems II. Zinc- and cadmium-doped Fe3O4 and crystalline magnetite

Abstract

Four doped samples of composition MzFe3 - xO4, in which for M = cadmium, x = 0.005 and 0.010, and for M = zinc, x = 0.005 and 0.066, were prepared using a high-temperature solid-state ceramic technique. X-ray, magneto-t.g.a., chemical analysis, and comparative thermophysical data confirmed the quality of the samples. The heat capacities of the doped samples were measured from 5 to 350 K using adiabatic calorimetry; magnetite crystals were studied over the range 65 to 350 K by the same technique. Temperature elevation of the higher-temperature transition was observed for the doped sample Zn0.005Fe2.995O4. Elimination of the double anomaly is occasioned by high mole fraction of dopant and crystal-structure expansion as demonstrated by the doped samples Cd0.005Fe2.995O4, Cd0.010Fe2.990O4, and Zn0.066Fe2.934O4. Displacements of the lower-temperature transition to higher and lower temperatures were observed. Systematic variations in the transitional thermal properties have been related also to dopant type and mole fraction. Tentative assignment of the two lambda-type transitions to interdependent Wigner and structural order-disorder mechanisms is proposed. Although adjuvant measurements would be desirable to confirm this analysis, the pre-eminence of the heat capacity as a sensitive detector and indicator of dopant level has been demonstrated.