Vibrational modeling of the thermodynamic properties of magnetite (Fe3O4) at high pressure from Raman spectroscopic study

A Raman study of natural magnetite has been performed at high pressure up to 20 GPa. Upon compression, the vibrational spectra do not exhibit any changes, except for the continuous shift to higher frequencies. Vibrational modeling of the specific heat and entropy based on the modified Kieffer's model allowed the estimation of the magnetic contribution to the thermodynamic properties of magnetite by comparison with published calorimetric measurements. High-pressure data provided information on the volume dependence of phonon modes of magnetite expressed by the mode Gruneisen parameters gamma(iT). The volume dependence of vibrational modes of magnetite is described by the averaged value (gamma) over bar =1.33, weighted by the contribution of each mode to the total lattice specific heat. This value was employed for the calculation of the pressure-temperature dependence of heat capacity and entropy. The effect of the uniaxial stress on the compressional behavior and related thermodynamic parameters of magnetite was inferred from experiments conducted in hydrostatic and quasi-hydrostatic environments. (C) 2003 American Institute of Physics.