Site selective laser spectroscopy of defects in PbF2, a superionic conductor

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Abstract
Site selective laser spectroscopy has been used to identify the defect sites in PbF2:Er3+ and follow their distribution as a function of dopant concentration and annealing temperature. Three sites labeled S1, S2, and S3 are found to correspond to single Er3+ ions with nearby F′i charge compensation, three sites labeled D1, D2, and D3 are found to correspond to clusters (dimer or higher) of Er3+ ions with nearby F′i charge compensation, and a cubic site labeled C where the Er3+ ion is compensated distantly. The cubic site is found to continually increase relative to S1 as the Er3+ concentration is raised. This behavior is contrary to that expected from simple mass action relationships, but is similar to that found in other fluorites. As the annealing temperature is raised, the cubic site increases markedly while the other sites decrease markedly over a small temperature range around the superionic transition temperature. This change is direct evidence for the sudden increase in charge screening effects which have been suggested as the driving force for the transition. It is suggested that the additional interactions caused by strain fields exaggerate the Debye–Hückel nonideality corrections and make the screening effects more effective.

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