Isomorphous substitutions and Stability of Solid Solutions in La 1-x Ln x F 3 , Ln = Ce-Ho Systems

Within the framework of the crystal–energy theory of isomorphous substitutions, the mixing energies and critical temperatures of decomposition (stability) of solid solutions with the tizonite structure in the La 1-x Ln x F 3 , Ln = Ce–Ho systems are calculated. A diagram of the thermodynamic stability of solid solutions is presented, which makes it possible to predict the limits of substitutions depending on the temperature or the temperature of decomposition according to the given limits of substitutions. The regions of thermodynamic stability, instability and metastability of solid solutions are determined. The calculation results in a number of systems that do not contradict the experimental data described earlier in the literature. They can be useful in choosing the ratio of components in “mixed” matrices, the amount of activator in luminescent, laser and other practically important materials, as well as for the immobilization of toxic and radioactive waste.