Abstract
The effects of impurity ions that enter octahedral a sites on the spectra of ions in dodecahedral c sites of garnets are investigated. Two types of such impurities have been studied: nonstoichiometric defects and transition ions . Based on the new data on in YAG it is shown that a nonstoichiometric defect such as determines four different perturbations of the nearby ions (P centres). Due to the low symmetry at the c site, the four directions of perturbation from the nearby centres are nonequivalent with respect to the local symmetry axes and determine particular compositions of perturbed crystal fields. If the ion is a non-Kramers one such as or , the lowering in symmetry determined by octahedral impurities is clearly observed by additional lines (mainly for one of the P centres) forbidden in . A splitting of for one of the P centres in is assigned to the rise of Kramers degeneracy due to a short-range magnetic order determined by an anisotropic superexchange interaction. Other arguments in favour of anisotropic a-c superexchange interaction are the high energy transfer rates measured for one centre and one centre in YAG.
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