Anionic conductivity and thermal stability of single crystals of solid solutions based on calcium fluoride

Abstract

The temperature and composition dependencies of the anionic conductivity in the temperature range from ambient to 1073 K were studied for single crystals of Ca 1− x Gd x F 2+ x ( x=1×10 −4, 1×10 −3, 1×10 −2 and 1×10 −1) and of Ca 0.8R 0.2F 2.2 (R=La, Ce, Pr, Nd, Gd, Dy, Er, Tm, Yb, Lu and Y), having the fluorite structure. The conductivity plots for the concentrated Ca 0.8R 0.2F 2.2 solid solutions display the low-temperature and high-temperature linear (Arrhenius) regions with the knee temperature T k∼770 K. The values of the conductivity activation enthalpy obey the relation Δ H HT( T> T k)>Δ H LT( T< T k). The conductivity mechanism in heavily doped Ca 1− x R x F 2+ x crystals is associated with the clusters of the point defects which decrease the potential barriers for fluoride anions moving by hops over the structural sites of the anion sub-lattice. We studied the effect of the dimensional factor (doped cation radii) on the anionic transport in these crystals. A correlation between anionic transport and atomic structure of the studied crystals is discussed.