Nanostructured Crystals of Fluorite Phases Sr1 – xRxF2 + x and Their Ordering: 12. Influence of Structural Ordering on the Fluorine-Ion Conductivity of Sr0.667R0.333F2.333 Alloys (R = Tb or Tm) at Their Annealing

Abstract

The ionic conductivity of Sr_0.667R_0.333F_2.333 alloys (rational Sr₂RF₇ compositions) in SrF₂–RF₃ systems (R = Tb or Tm), prepared by spontaneous crystallization, has been investigated for the “as-grown” state and after annealing in CF₄ at 900 ± 20°C for 96 h. As-grown samples of both compositions, prepared by fast (200°C/min) melt crystallization, exhibit partial (nonequilibrium) ordering, which increases from Tb to Tm. Annealing of Sr_0.667R_0.333F_2.333 alloys yields strong ordering (equilibrium for the annealing temperatures) of the fluorite structure (CaF₂ type, sp. gr. Fm3̅m, Z = 4) at the formation of t-Sr₂RF₇ tetragonal compound (sp. gr. I4/m, Z = 30). It is established that ordering of the alloy fluorite structure reduces the fluorine-ion conductivity. After the annealing, the conductivity of Sr_0.667R_0.333F_2.333 alloys with the initial (nonequilibrium) ordering stage of t-Sr₂RF₇ phases with almost complete (equilibrium) ordering decreases by a factor of 3–4.5.