ORDERING, INCOMMENSURATION, AND PHASE TRANSITIONS IN PYRRHOTITE .2. A HIGH-TEMPERATURE X-RAY POWDER DIFFRACTION AND THERMOMAGNETIC STUDY

摘要

High-temperature X-ray diffraction (HTXRD) has been combined with thermomagnetic measurements to study synthetic and natural pyrrhotites. The temperature dependence of the properties observed in DTA, thermomagnetic and diffraction measurements indicates that transitions consistently occur upon heating and cooling so long as the sample is kept below 400 degrees C, but if the ultimate heating temperature is higher than 550 degrees C the transitions are altered, probably due to the loss of sulfur, The previously proposed transition sequence for Fe7S8 was supported by HTXRD observation, especially on natural monoclinic pyrrhotite. For intermediate pyrrhotite, the principal ordering was found to be the formation of Kagome nets, and it was found that the vertical stacking sequences that are commensurate along the c axis yield antiferromagnetic characteristics. Based on the changes observed in the HTXRD patterns it is proposed that the mechanism for an observed magnetic lambda-transition between the temperatures 150 and 250 degrees C is the transformation of a commensurate structure with ABCD layering of Kagome nets into a vacancy-disordered structure with the same c modulation. (C) 1996 Academic Press, Inc. [References: 27]