An experimental and theoretical study of crystals of calcium fluoride doped with alkali metal cations

Abstract

Experimental measurements of thermal depolarization in crystals of CaF 2, grown from the melt containing LiF, NaF, KF, or RbF, reveal a common relaxation (designated M2) with an activation energy of 0.51 eV. In addition, the Li +- and K +-doped crystals exhibit a relaxation ( M1) with an activation energy of 0.34eV. A similar relaxation has been found in CaF 2:Rb + by J. Fontanella private communication) and in CaF 2:Na + by R. D. Shelley and G. R. Miller ( J. Solid State Chem., 1, 218, 1970). Theoretical calculations on M +-doped CaF 2 (where M = Li,Na,K,Rb) are in agreement with the hypothesis that the M2 relaxation is due to Na + in all four systems studied and is associated with the jump of a nearest-neighbor (nn) anion vacancy (F v −) around the substitutional Na + ion (Na s +). The assignment of M1 is less certain, but it appears to be associated with similar Li s +&.z.sbnd;F v − dipoles resulting from Li + impurity present because of the lower volatility of LiF compared to that of KF and RbF. When LiF dissolves in CaF 2 the Li + ion also forms quadrupoles consisting of a cation vacancy and two Li + interstitials and the reorientation of these quadrupoles has also been studied theoretically.