HEAT CAPACITY STUDIES OF IRON(3(1-DELTA))OXIDE(4)

Heat capacity measurements have been used to systematically study single crystal, annealed Fe(,3(1-(delta)))O(,4) in the temperature range 5 - 350 K. The single crystals of magnetite were grown and annealed under a controlled oxygen atmosphere to produce homogeneous, single phase samples. The heat capacity was measured by the relaxation method; heating and cooling curves were used to investigate the order of the Verwey transition. For closely stoichiometric Fe(,3)O(,4)((delta) = 0), heat capacity studies revealed a single, sharp first order heat capacity anomaly at the Verwey transition, T(,v) (TURNEQ) 121 K, no anomaly at 10 K, no evidence for short range order, and an entropy of transition of (TRIANGLE)S(,V) (TURNEQ) R In 2 per mole of Fe(,3)O(,4). Heating and cooling curves at the transition revealed the thermal arrests characteristic of a first order transition. For Fe(,3(1-(delta)))O(,4), heat capacity measurements by relaxation calorimetry methods revealed a drastic decrease in the transition temperature, T(,V), and the entropy of transition, (TRIANGLE)S(,V), with increasing deviation from stoichiometry, (delta). For (delta) \u3c 0.0035, the transition was of first order, as confirmed by thermal arrests in the heating and cooling curves. For (delta) \u3e 0.0049, the Verwey transition appeared to be of second order; alternatively, this could be an indication of a nearby critical point. This is the first experimental evidence for a critical or tricritical point in magnetite