SPECTRAL AND DYNAMICAL EFFECTS OF OCTAHEDRAL IMPURITIES ON RE3+ IN GARNETS

The effects of impurity ions that enter octahedral a sites on the spec tra of RE3+ ions in dodecahedral c sites of garnets are investigated. Two types of such impurities have been studied: nonstoichiometric defe cts and transition ions (Cr3+). Based on the new data on EP3+ in YAG i t is shown that a nonstoichiometric defect such as Y3+(a) determines f our different perturbations of the nearby ES3+(c) ions (P centres). Du e to the low symmetry D-2 at the c site, the four directions of pertur bation from the nearby Y3+(a) centres are nonequivalent with respect t o the local symmetry axes and determine particular compositions of per turbed crystal fields. If the RE3+ ion is a non-Kramers one such as Tm 3+ or Pr3+, the lowering in symmetry determined by octahedral impuriti es is clearly observed by additional lines (mainly for one of the P ce ntres) forbidden in Da. A splitting of similar to 2 cm(-1) for one of the P centres in Er3Al5O12 is assigned to the rise of Kramers degenera cy due to a short-range magnetic order determined by an anisotropic Er 3+(c)-Er3+(a) superexchange interaction. Other arguments in favour of anisotropic a-c superexchange interaction are the high energy transfer rates measured for one Er3+(a)-Er3+(c) centre and one Cr3+(a)-Tm3+(c) centre in YAG.