The parameters of the crystal field of the tetragonal O center associated with a Yb3+ ion in this material, previously found during a study of optical and electron spin resonance spectra, were used to analyze lattice distortions in the vicinity of the impurity ion and the O2– ion which compensated for the excess positive charge. Using the superposition model, it was deduced that the Yb3+ ion and neighboring ions of F and O, on the axis of the center, shifted appreciably along the direction from the O2– ion to the Yb3+ ion during formation of the tetragonal O center. As this occurred, the distances of the F and O ions from the impurity ion increased. The four F– ions in the nearest octahedral neighborhood of Yb3+, which were arranged symmetrically in a plane perpendicular to the axis of the center, receded slightly from the axis.

Analysis of the Structure of Tetragonal Oxygen Centers Associated with Yb3+ Ions in the KMgF3 Crystal. A.M.Leushin: Physics of the Solid State, 2003, 45[12], 2281-6