Structure and properties of the KNbW2O9 hexagonal bronze doped with Eu3+ ions as an optically active probe

Luminescence properties of KNbW2O9 crystal doped with Eu3+, investigated at room and liquid nitrogen temperatures, showed that the active ions occupy two crystallographically different sites. The structural and IR spectra investigations were also carried for better understanding of Eu3+ ion behavior. The luminescence decay profiles were slightly non-exponential. The luminescence lifetimes of Eu3+ were approximated by biexponential decays. Both were ascribed to two different Eu(111) sites. The longer component was determined to be ca. 1 ms, whereas the shorter one was about 0.5 ms. It was found that the Eu sites characterized by longer decay times were not affected by temperature, whereas the sites characterized by shorter decay times revealed strong decrease of decay times with temperature. Infrared investigation demonstrated the presence of water molecules occupying hexagonal tunnels in the KNbW2O9 crystal. The UV irradiated crystal showed the presence of blue coloration. The observed color was attributed to small W5+ polarons or intervalence charge transfer between W5+ and W6+ states. The electrons, necessary for the formation of the W5+ states, originate most likely from UV photodissociation of water molecules. (C) 2004 Elsevier B.V. All rights reserved.