Raman study of magnetite (Fe3O4): laser-induced thermal effects and oxidation

Natural magnetite (Fe3O4) in the form of single crystal and powder was studied by laser Raman spectroscopy at various laser powers. The correlations between the power of the excitation laser, the temperature of the sampled spot and the degree of oxidation of magnetite were accurately established. In the course of the oxidation of the single crystal of magnetite, the first characteristic features of hematite appear at about 300 and 410 cm(-1), at a temperature close to 240degreesC. This may explain the erroneous assignment of these modes to the intrinsic Raman modes of magnetite in some studies. For the finely powdered magnetite, which is much more easily prone to oxidation, the reaction mechanism proceeds via a metastable maghemite (gamma-Fe2O3) before the final product hematite is formed. Three independent methods based on the quasiharmonic approximation and on the ratio of the Stokes to anti-Stokes intensities were used to calculate the local temperature of the laser-heated spot. Temperature-induced shifts of phonon bands were also evaluated. The phonons shift at the following rates: A(1g) - 0.023(1) cm(-1) K-1, T-2g(2) - 0.030(2) cm(-1) K-1, E-g -0.019(5) cm(-1) K-1 and T-2g(1) - 0.00(1) cm(-1) K-1. Copyright (C) 2003 John Wiley Sons, Ltd.