Spatial energy transfer: Cr 3+ clustering around vacancies in MgO crystal

The optical energy transfer between Cr 3+ ions from cubic centres to tetragonal centres is measured. Transfer is observed only when the donor is very close to the acceptor and the measured transfer times are relatively short (~ 100 ?s). The tetragonal acceptors are Cr 3+-ion pairs which form different triads with the donor. It is deduced that transfer to rhombic sites is also rapid only from very close cubic centres. The shortness of the transfer times explains why a spin memory is observed during the energy transfer from cubic to rhombic centres. The localisation of optical excitation in triads is due to the presence of charge-compensating Mg 2+ vacancies and the inefficiency of the dipole-dipole interaction.