Transition metal ions in crystals with the fluorite structure

The presented survey shows that among the transition metal ions the interest was concentrated nearly exclusively on the first-row (3dsup(n)) ions, which were investigated mainly in CaF2 and CdF2. One of the most interesting features discussed are the strong centrosymmetrical trigonal distortions of the cubic environment found for Ni2+, V2+, and Cr3+. As these distortions are connected with strong strain fields around the impurity, an interaction via these fields between like of unlike impurities having orbital triplet ground states can be expected leading to clustering and other effects. Because of the large differences of the ionic radii between 3dsup(n) ions and Sr2+ and Ba2+, SrF2, BaF2, and SrCl2 doped with these ions are candidates for off-centre systems as it was found during the last few years for some of them in the corresponding oxides SrO and BaO. Therefore the tendency of dsup(n) ions for the forming strong covalent complexes as an additional reason for the off-centre position of such ions in cubic crystals should be noted. A further remarkable aspect concerns SrCl2 as host crystal. EPR investigations of Mn2+ and Co2+ in SrCl2 with an applied electric field suggest that these systems can be regarded as intermediate systems between off-centre and normal substitutional ones. A third hint to the occurrence of the off-centre position can be taken from the optical spectra of Co2+ in SrF2 and BaF2. (author)