An investigation of the phase transitions in bornite (Cu5FeS4) using neutron diffraction and differential scanning calorimetry

Bornite (Cu5FeS4) exists in three polymorphic forms related by superstructu ring, with structural transitions at 200 and 265 degrees C. The phase trans itions and structural behavior in two natural bornite samples were investig ated as a function of temperature using differential scanning calorimetry ( DSC) to characterize the thermal anomalies associated with each transition and in-situ high-resolution neutron powder diffraction to determine the var iation in superlattice intensity and lattice parameters. These two methods, carried out over the temperature range 50-350 degrees C, provided insight into the short- and long-range interactions, respectively, and a comparison of measurements taken up-T and down-T enabled thermal hysteresis effects t o be quantified. The high to intermediate transition at 265 degrees C invol ves long-range cation ordering and vacancy clustering, resulting in a doubl ing of the high-temperature cubic unit cell (m3m). The square of both the s pontaneous strain and the 2a superlattice intensity varied linearly with te mperature, indicating that the transition is tricritical in character. Loss of long- and short-range order occurred simultaneously during heating, whe reas during cooling, the reappearance of 2a superlattice reflections was de pressed some 50 degrees C below the short-range transition recorded by DSC due to the formation of antiphase domains. The presence of antiphase domain s also caused the discontinuity in the strain associated with the transitio n to occur at a different temperature to the appearance of superlattice int ensity. The intermediate-low transition associated with the development of a 2a4a2a orthorhombic superlattice (Pbca) is strongly first-order and exhib its a large intrinsic hysteresis (38 degrees C). The down-T transition temp erature of the intermediate-low transition is shown to be dependent on the degree of order attained in the intermediate phase.