DIELECTRIC-RELAXATION OF CALCITE-TYPE CARBONATE SALTS - DEFECT STRUCTURE AND DEFECT DIPOLE DYNAMICS IN POLYCRYSTALLINE MAGNESITE

The low-temperature thermal depolarization spectrum of polycrystalline magnesite MgCO3 reveals a unique broad dipolar relaxation peak, which reaches a maximum around 140 K. The relaxation mechanism is probably related to a defect dipole population, with distribution in the relaxa tion time. By employing the partial heating scheme, we found the activ ation energy values distributing from 0.19 to 0.30 eV. Assuming a Gaus sian distribution in the activation energy values, the full curve fitt ing led to the evaluation of the relaxation parameters E-0=0.220 eV, s igma=0.023 eV, and tau(0)=4.787x10(-6) s. Our results are extensively discussed in relation to those reported for the other calcite-type car bonate members; i.e., calcite (CaCO3) and dolomite [CaMg(CO3)(2)]. The overall view of the dielectric relaxation in the calcite family mater ials shaw that each sublattice type (the calcium and the magnesium one s) favors a certain defect structure. There is strong evidence that Mn 2+ and Sr2+ impurities as well as the water molecules or the hydroxyl ions are the most probable participants to the defect dipole configura tions.