Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (epsilon') and imaginary (epsilon'') parts of the dielectric constant of polycrystalline hematite (alpha-Fe2O3) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (alpha-dispersion) in the temperature regions T < 233 K and T > 338 K. In the intermediate temperature range 233 K < T < 338 K a charge carrier mechanism takes place (electron jump from the O2- ion into one of the magnetic ions Fe3+) which gives rise to the low frequency conductivity and to the Omega-dispersion. The temperature dependence of relaxation time (tau) in the -ln tau vs 10(3)/T plot shows two linear regions. In the first, T < 238 K, tau increases with increasing T implying a negative activation energy -0.01 eV, and in the second region T > 318 K tau decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T-Mo=284 +/- 1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains. (c) 2004 Elsevier Ltd. All rights reserved.