Abstract
The origins of N-lines in the luminescence spectra of the three chromium-doped spinels MgAl 2O 4, ZnAl 2O 4 and ZnGa 2O 4 are investigated by a detailed study of their intensity dependences on the chromium concentration and on the thermal history of the samples. By thermal treatment experiments (varying the cooling rate after a high-temperature treatment; annealing at increasing temperature) it is shown that, with the exception of lines due to the nearest-neighbour Cr 3+ pairs, the N-lines in the frequency range v(R 1)− v<250 c/m −1 are caused by the defects of the spinel host lattices. These “structure-dependent” N-lines can be interpreted to arise from different classes of Cr 3+ ions with slightly different short-range orders, the differences being due to cation inversion effects.
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