Nonstoichiometry in Inorganic Fluorides: 2. Ionic Conductivity of Nonstoichiometric M1-xRxF2+x and R1-yMyF3-y Crystals (M = Ca, Sr, Ba; R Are Rare Earth Elements)

The peak manifestation of nonstoichiometry in fluoride systems in the number of phases with valuable properties and wide homogeneity ranges is 45 MF2-RF3 systems, where M = Ca, Sr, Ba and R are 15 rare earth elements from La to Lu and Y (with Pm and Sc excluded). A deviation from stoichiometry in crystals of the M1 - xRxF2 + x (CaF2 fluorite type) and R1 - yMyF3 - y (LaF3 tysonite type) phases is responsible for the fluorine superionic conductivity sigma. The range of variation in sigma with changes in the qualitative (M, R) and quantitative (x, y) compositions in both structure types is very wide. The sigma value changes by a factor of 10(8) in the M1 - xRxF2 + x phases (at 500 K) and by a factor of 10(6) in the R1 - yMyF3 - y phases (at 293 K). Changing compositions, one can also obtain crystals with sigma values large enough for their use as fluorine-conducting solid electrolytes. Phases promising for solid electrolytes were revealed in the MFm-RFn systems (m < n <= 4), which were studied within the program of searching for new multicomponent fluoride materials at the Institute of Crystallography, Russian Academy of Sciences (IC RAS). Superionic conductivity is one of the peak manifestations of the influence of defect structure of nonstoichiometric crystals on their properties. The subject of this review is the results of the studies performed at the IC RAS on the ionic conductivity of single crystals of the M1 - xRxF2 + x and R1 - yMyF3 - y nonstoichiometric phases.