Studies of defect clustering in CaF2: Y3+ by ionic conductivity and thermal depolarization

Measurements of the ionic conductivity σ of yttrium-doped calcium fluoride reveal a complex and highly unusual dependence of logσT 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_s³⁺), Y_s³⁺?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 CaF₂: Y³⁺ is very complicated and reveals six different relaxations. One of these is associated with the type I (nn) and type II (nnn) Y_s³⁺?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 CaF₂: Y³⁺.