Studies of high temperature ionic thermocurrents observed in CaF 2 and CaF 2:Eu

Abstract

A strong and thermally stable ionic thermocurrent (ITC) band peaked at 316 K was observed in europium-free CaF 2 crystals. The experimental ITC spectrum can be described by an ionic conduction kinetic expression of the type τ = τ 0 exp( E a kT ) , with τ 0 = 1.90 × 10 −13 s and E a = 0.93 eV. This new band was very sensitive to europium doping, the ITC peak intensity being drastically reduced with the addition of only 0.002 mol.% europium. At the same time, a high-temperature typical ionic thermocurrent (HT ITC) band due to mobile interstitial fluorine ions appeared at 416 K. The decrease of the new band at 316 K is explained by assuming that mobile fluorine vacancies are compensated with fluorine ions introduced into CaF 2 as charge compensators for the trivalent europium ions. The dependence of the temperature T m on the ITC peak in the higher-temperature ITC band on europium concentration is determined and the kinetic parameters are estimated.