ITC spectra of impurity aggregate in CaF2 crystals doped with trivalent rare-earth ions

The two most prominent ionic thermoconductivity (ITC) peaks in CaF2 doped with Gd3+ are studied as a function of Gd3+ concentration nc, thermal annealing, and exposure to ionizing radiation. Peak 1 which occurs at 166 K is identified with the thermal reorientation of a nearest neighbor Gd3+-fluoride interstitial dipolar complex (type I) and peak 2 is attributed to the next-nearest neighboring complex (type II). Whereas the peak I identification is firmly based, results presented here indicate that peak 2 is not associated with a type II complex. Rather, the rapid increase of peak 2 relative to peak 1 with increasing nc together with the observation that room temperature γ-irradiation of CaF2:Gd3+ crystals cause a nine-fold greater decrease of peak 2 as compared to peak 1 suggests that peak 2 has its origin in a small complex or cluster of dipoles with a residual moment. Both peaks go through maxima with increasing nc which is analogous to observations by Tallant et al. who studied the various defect complexes using tunable laser spectroscopy. Annealing at elevated temperature reveals that heating above 600°C enhances peak 1 at the expense of peak 2 which is consistent with the conclusion that peak 2 is associated with a complex which is thermally decomposed at elevated temperature. Similar studies on CaF2:Sm3+ reveal ITC peaks which behave in an analogous fashion.