Abstract
The ionic conductivity of Sr0.667R0.333F2.333 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 Sr0.667R0.333F2.333 alloys yields strong ordering (equilibrium for the annealing temperatures) of the fluorite structure (CaF2 type, sp. gr. Fm3m, 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 Sr0.667R0.333F2.333 alloys with the initial (nonequilibrium) ordering stage of t-Sr2RF7 phases with almost complete (equilibrium) ordering decreases by a factor of 3–4.5.
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