Clustering in thermally treated BaF2:Y3+crystals

Abstract

Ba${\mathrm{F}}_2$ crystals doped with yttrium with molar concentrations ranging from ${10}^{-5\mathrm{}}$ to ${10}^{-3\mathrm{}}$ were studied by ionic thermal-current techniques. Two low-temperature peaks $A$ and $B$ were observed for all the concentrations located at 161 and 183.5 K. The relaxation parameters obtained for peak $A$ were $E_A=0.45\mathrm{}$ eV and ${\tau }_{0A}=5.7\mathrm{}\times {10}^{-13\mathrm{}}$ s. For peak $B$ a model assuming a Gaussian distribution of the reorientation energy around a mean value $E_0$ and with a width $\sigma$ was assumed and gave $E_{0B}\left(\sigma \right)=0.56\mathrm{}$ eV, ${\sigma }_B=0.009\mathrm{}$ eV, and ${\tau }_{0B}=3.3\mathrm{}\times {10}^{-14\mathrm{}}$ s. Peaks $A$ and $B$ were attributed to the relaxation of nearest-neighbor and next-nearest-neighbor dipoles, respectively. Thermal treatments were made on these crystals in order to study the clustering of defects and the dissolution of the nonpolarizable clusters existing in the samples. After quenchings from increasing temperatures, from 700 to 1200 K, the number of free dipoles was determined. Assuming the nonpolarizable clusters to be the 2:2:2 dimer, the dissolution energy was estimated and compared to the theoretical calculations. However, the dissolution mechanism could be better understood if the cluster were the linear ${\mathrm{Y}}^{3+}{\mathrm{F}}_{i2\mathrm{}}^{-}$ and this possibility is discussed herein.