Relaxation of Impurity-Defect Complexes in Fluorite Crystals

Abstract

This paper summarizes recent contributions to the understanding of the structure and the reorientation kinetics of various impurity-defect complexes in alkaline earth fluroides. For the most part the relevant investigations involve relaxation of electric polarization as revealed by the ionic thermo-current (ITC) method on CaF2, SrF2 and BaF2 crystals doped with a variety of trivalent rare earth (RE3+) ions. In the absence of contaminating oxygen the extra positive charge is compensated by an interstitial fluoride ion (Fi -) which except at elevated temperature forms a complex with the In CaF2 the (dominant form of the complex as revealed by EPR and ITC is tetragonal with the Fi - located in the nearest neighboring interstitial site (Type I complex). In BaF2 the dominant form is trigonal with the situated in a next nearest site along a ‹111› direction (Type II complex). In SrF2 both Type I and Type II complexes are clearly evident in the ITC relaxation peak structure. In a crystal of CaF2:Gd3+ treated to introduce oxygen the Type I relaxation at 131 K is replaced almost quantitatively by a new peak at 166 K. Comparison with EPR behavior reveals that the new peak is associated with the relaxation of the CaF2:Gd3+O4 complex (Type T1) which had been previously identified by ENDOR.