The dual luminescence of M2MgTeO6 (M= Sr, Ba) double perovskites

Sr2MgTeO6 (SMT) and Ba2MgTeO6 (BMT) ceramics with the double perovskite structure have been synthesized by solid-state route and by thermal decomposition of nitrate solution precursor. Both compounds possess interesting and unexpected emission properties in the visible and NIR regions, never reported before for this type of materials. These properties have been measured in different experimental conditions in order to characterize their nature. The literature information on tellurium containing luminescent materials is not rich and the data are somewhat contradictory: nevertheless, they agree in assigning the emission properties to Te4+ in octahedral coordination, whereas the Te6+ ion is considered not luminescent. The presence of Te4+ ions has then been verified by XPS measurements. On the basis of the experimental evidences, the visible luminescence has been assigned to a transition between the electronic levels of the Te4+ ion. The origin of the NIR emission appears more difficult to assess: among the explored hypotheses, the most convincing one, at the present level of the information, involves the possible interactions between the Te4+ and Te6+ ions with consequent formation of an optically active charge transfer state, as already observed for other ns2-type ions (Bi3+, Sb3+).