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

The temperature and composition dependencies of the anionic conductivity in the temperature range from ambient to 1073 K were studied for single cryst als of Ca1-xGdxF2+x (x = 1 x 10(-4), 1 x 10(-3), 1 x 10(-2) and 1 x 10(-1)) and of Ca0.8R0.2F2.2 (R = La, Ce, Pr, Nd, Gd, Dy, Er, Tm, Yb, Lu and Y), h aving the fluorite structure. The conductivity plots for the concentrated C a0.8R0.2F2.2 solid solutions display the low-temperature and high-temperatu re linear (Arrhenius) regions with the knee temperature T-k similar to 770 K. The values of the conductivity activation enthalpy obey the relation Del ta H-HT(T > T-k) > Delta H-LT(T < T-k). The conductivity mechanism in heavi ly doped Ca1-xRxF2+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 ef fect of the dimensional factor (doped cation radii) on the anionic transpor t in these crystals. A correlation between anionic transport and atomic str ucture of the studied crystals is discussed. (C) 1999 Published by Elsevier Science B.V. All rights reserved.