Effect of structural packing on the luminescence properties in tungsten bronze compounds M2KNb5O15 (M=Ca, Sr, Ba)

Tungsten bronze compounds M2KNb5O15 (M=Ca, Sr, Ba) were successfully synthesized, and the luminescence properties were investigated. Among the three compounds, Ca2KNb5O15 showed an obviously broad band of host luminescence at 460 nm with exciting at 269 nm. By doping Eu3+ into the M sites, Ca2KNb5O15:Eu3+ displayed strong red emission from Eu3+ ions characteristic transitions, nearly four times higher than Sr2KNb5O15:Eu3+ and seven times higher than Ba2KNb5O15:Eu3+. Crystal packing factor (PF) was introduced to account for this luminescence difference, lower PF being correlated to higher luminescence intensity for perovskite-related structure. Both the as-prepared compounds and the literature examples were proved to fit this correlation. This can be explained through the influence of the structural packing on the environment distortion and crystal field splitting of the doping site.