Luminescence in the system Mg 4Ta 2- xNb xO 9

Detailed emission and excitation spectra and decay measurements of the luminescence of the system Mg ₄Ta _{2- x}Nb _xO ₉ are reported. In Mg ₄Ta ₂O ₉ a new emission and excitation band at relatively low energy has been found. In the samples with a high niobium concentration ( x ⩾ 1) a new weak orange emission has been observed upon long-wavelength uv excitation. A new interpretation of the results, based on the occurrence of different niobate or tantalate centers, is presented.