Thermally stimulated polarisation currents in barium fluoride

Current peaks which appear in unirradiated and UV-irradiated BaF ₂:Tb³⁺ crystals during heating from 80 to 400K were investigated. A peak which appeared at 195K without any previous optical excitation, when the crystal was heated in an external field, is attributed to a thermally stimulated polarisation current (TSPC), caused the the rotation of dipoles. After cooling to 80K in an external field and heating without an external field, two ionic thermocurrent (ITC) peaks were observed at 194 and 340K. It is shown that the ITC peak at 194K is due to the reorientation of dipoles, and that at 340K is attributable to the displacement of free interstitial fluorine ions. The effective polarisation temperature for the ITC measurements was found to be approximately 200K, even when the electrical field was applied at higher temperatures.