Studies of defect clustering in CaF 2: Y3+ by ionic conductivity and thermal depolarization

Abstract

Measurements of the ionic conductivity σ of yttrium-doped calcium fluoride reveal a complex and highly unusual dependence of logσ T on T −1, where T denotes the temperature. This conductivity curve has been interpreted in terms of a model that assumes frozen-in defects on the cation sub-lattice together with charge-compensating interstitial fluoride ions and anion vacancies. At low temperatures the conductivity is dominated by mobile interstitial fluoride ions ( F i −) in equilibrium with trapping centres that may include substitutional yttrium ions ( Y s 3+), Y s 3+− F i − complexes (monomers) and larger clusters. At intermediate temperatures the charge-carrying F i − are considered to arise by ionization from dimers (the 2:2:2 complexes) and at higher temperatures still from larger clusters which are probably tetramers, although the possibility that the mobile F i − in these two temperature regions are those that had formerly been trapped by dimers and tetramers cannot be excluded. The thermal depolarization spectrum of CaF 2: Y 3+ is very complicated and reveals six different relaxations. One of these is associated with the type I ( nn) and type II ( nnn) Y s 3+− F i − complexes and a second with 2:2:2 clusters. The remaining four relaxations were not identified but almost certainly provide evidence for the existence of polarizable defect clusters in CaF 2: Y 3+.