Study of the Morin transition in pseudocubic alpha-Fe2O3 particles

摘要

We have studied the Morin transition in nanostructured pseudocubic alpha-Fe2O3 particles of about 1.8 mu m side. The preparation was carefully chosen to obtain a system with a very narrow crystallite size distribution and particles of homogeneous morphology. Two samples were studied: one without thermal treatment (alpha-Fe2O3(ap)) and another annealed at 673 K in air for 12 h alpha-Fe2O3(an)). Both were characterized by XRD, SEM, TGA and Mossbauer spectroscopy. The results indicate that the Morin transition is suppressed for alpha-Fe2O3(ap), however, alpha-Fe2O3(an) has a T-M approximate to 230 K and the transition is completed over a very narrow temperature range. These results are discussed in connection with the crystallite size, the cell parameters, and the presence of OH- groups (hydrohematite) or incorporated water (protohematite).