Tetragonal distortions of some tetragonal Cr3+ centers in fluoroperovskite ABF3 crystals

The tetragonal distortions [characterized by the displacement ΔR of the F− ion intervening in Cr3+ and VB (B2+ vacancy) or M+ ion in B2+ site] for Cr3+-VB and Cr3+-Li+ centers in Cr3+-doped fluoroperovskites KBF3 (B = Mg, Zn) and ACdF3 (A = K, Rb, Cs) and also for Cr3+-Na+ center in KZnF3 have been studied by calculating the EPR parameters D, g|| and g⊥ from the crystal-field theory. From the studies, an interesting trend is suggested, i.e., when the size of the co-doped monovalent impurity M+ in the nearest neighbor B2+ site is comparable to that of the replaced B2+ ion, the displacement ΔR of the intervening F− ion in Cr3+-M+ center is smaller than that in Cr3+-VB center and if the M+ iron is much smaller or much larger than that of the replaced B2+ ion, the displacement ΔR in Cr3+-M+ center is greater than that in Cr3+-VB center. The causes of the trend are discussed.