The Eu3+ ion occupies two different crystallographic sites in (Yr _XEu,),03 and (Gdt_ XEu,),03, with site symmetry S, and C,. Energy transfer over more than 7 A occurs from Eu3+ (S,) ions to Eu3+ (C,) ions. This is shown to be a direct one-phonon assisted process, in combination with a one-site resonant two-phonon assisted process at higher temperatures. For x = 1 there is energy migration over the Eu 3+ (C,) sublattice to quenching impurities. The presence of cooperative absorption points to superexchange interaction between the Eu3+ ions.

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