Coexistence phenomenon of Ce3+-Ce4+ and Eu2+-Eu3+ in Ce/Eu co-doped LiBaB9O15 phosphor: luminescence and energy transfer.

Ce/Eu-doped LiBaB9O15 (LBB) samples were prepared via conventional high temperature solid state reactions. The XRD patterns, crystal structures, luminescence properties, and decay times were investigated systematically. Ce3+ ions exist in LBB:xCe3+ that were synthesized in a reducing atmosphere and in an air atmosphere. However, we observed Eu2+ ions in LBB:yEu2+ in a reducing atmosphere and Eu3+ ions in LBB:zEu3+ in an air atmosphere. LBB:0.05Ce3+,yEu2+ phosphors synthesized in a reducing atmosphere only possess Ce3+ and Eu2+ and exhibit a broad excitation band ranging from 350 to 425 nm. A reduction phenomenon of Eu3+ → Eu2+ and coexistence of Ce4+, Ce3+, Eu2+ and Eu3+ were observed when LBB:0.05Ce,wEu phosphors were synthesized in an air atmosphere. There are three processes in LBB:0.05Ce,wEu, i.e., energy transfers from Ce3+ to Eu2+ and from Eu2+ to Eu3+, and metal-metal charge transfer (MMCT) between Ce3+ and Eu3+. Moreover, the MMCT process is dominant in LBB:0.05Ce,wEu due to less efficient energy transfer from Ce3+ to Eu2+. Moreover, the CIE coordinates of LBB:0.05Ce,wEu vary systematically from light blue (0.313, 0.129) to red (0.589, 0.315) for LBB:0.05Ce3+,wEu synthesized in air with the changes in Eu ion concentration. Thus, we can control the color by controlling the synthesis atmospheres.