Abstract
The ac electrical response of cell systems composed of single crystals of the concentrated solid solutions M 1− x− y U x Ce y F 2+2 x+ y ( M = Ca, Sr, Ba and 2.7 < 2 x + y < 26.5 m/o), and ionically blocking electrodes has been studied as a function of frequency and temperature. At elevated temperatures the crystals react with traces of oxygen or water vapor. Complex admittance analysis reveals the formation of low-conducting surface layers, contrary to diluted solid solutions which under similar conditions react to form high-conducting surface layers ( 2). The activation enthalpy for the layer conductivity is substantially larger than that for the bulk conductivity, and equals that for interstitial fluoride ion motion in dilute solid solutions. A mechanism of charge compensation in the layers is presented. After reaction the solid solutions based on CaF 2 show also a surface electronic conductivity. Scanning electron micrographs clearly reveal the surface degradation.
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