Ionic thermocurrents in natural calcium fluoride

Natural calcium fluoride from Criciuma, Santa Catarina is studied. Ionic Thermocurrents (ITC) has been used to determine the properties of dipole defects present in this crystal. The ITC spectrum of this material shows three bands in the temperature range of 80K to 450K. The first one, at 130K, is due to the dipoles formed by a trivalent impurity and an interstitial fluorine ion in a neighbour site (nnRs3+-Fi-). The second one, at 202K, is due to the presence of small aggregates of dipoles (like dimers). The last band, at 360K, is due to the formation of large clusters. The best for these bands was obtained with a continuous distribution model with mean activation energies of 0.41eV, 0.595eV and 1.02eV for the first, second and third band respectively. Thermal treatments can modify the number of dipoles, dimers and clusters present in the crystal. Thermal treatments lasting from 15 minutes to 10 hours at temperatures between 473K(2000C) and 773K(5000C) were used. For thermal treatments at 573K(3000C), the dipoles and the dimers are created while the clusters are destroyed as the time of thermal treatment increases. At 673K(4000C) the clusters are created while the dipoles and the dimers are destroyed. At 773K(5000C) the three kinds of defects are destroyed. An equilibrium concentration is observed after 2 to 3 hours of thermal treatment. This equilibrium concentration is a function of the thermal treatment temperature showing a maximum in the investigated temperature range, around 573K, for the dipoles and dimers, and 623K for the clusters. (author)