spectra excited by 528 nm wavelength of ZnAl2−x−y Tbx Cry O4 samples with various x and y, annealed at 1100 ◦ C It can be seen from Figure 8a that at the high Cr3+ concentrations (y = 0.050, 0.100) the intensity of the lines at 667, 675, 686, 698, 708 and 717 nm considerably reduces Meanwhile, some new lines at 707, 732 739 and 746 nm appear After references [5–7], at the high Cr3+ concentrations the Cr3+ Cr3+ pairs are expected to form, and the line at 707 nm is originated from the nearest Cr3+ -Cr3+ pairs By examining PLE spectrum, the lines at the wavelengths of 732, 739 and 746 nm are believed to be the phonon-sidebands of the line at 707 nm (the PLE spectra not shown here) Figure 8b shows the PL spectra of ZnAl2−x−y Tbx Cry O4 samples, annealed at 1100 ◦ C, with y = 0.005 and various x, excited by the wavelength of 528 nm As seen from the figure, with increasing the Tb3+ ion concentration up to x = 0.050, 0.100, the shape of the spectra does not change, but their intensity is decreased Therefore by selecting the appropriate mole fractions x and y, we can obtain the materials strongly emitting in both blue-green and red regions 10404-p4 T.T Loan et al.: Synthesis and optical properties of ZnAl2 O4 :Cr3+ ,Tb3+ powders Conclusion (a) The ZnAl2 O4 spinel powders co-doped with Tb3+ and Cr3+ ions have been successfully synthesized by a solgel technique The samples undergone heat-treatment at 1100 ◦ C exhibited the best crystallinity The PL spectra of the samples showed two emission regions: the first (blue-green) one was related to the emission of Tb3+ ions and the second (red) one was related to the emission of Cr3+ ions By selecting the appropriate mole fractions of the Tb3+ ions x and Cr3+ ions y, we can obtain the materials strongly emitting in both blue-green and red regions (b) This work is financially supported by VNU University of Science (Project No NT-12-10) and Ministry of Science and Technology of Vietnam (Project No 103.02.51.09 from NAFOSTED) References Fig The PL spectra excited by the wavelength of 528 nm of the ZnAl2−x−y Tbx Cry O4 samples, heat-treated at 1100 ◦ C (a) with x = 0.005 and different y, and (b) with different x and y = 0.005 M Zawadzki, Solid State Sci 8, 14 (2006) X.Y Chen, C Ma, Opt Materials 32, 415 (2010) S Menon, B Dhabekar, E.A Rajaa, S.P More, T.K.G Rao, R.K Kher, J Lumin 128, 1673 (2008) D Zhang, C Wang, Y Liu, Q Shi, W Wang, Y Zhai, J Lumin 132, 1529 (2012) J Derkosch, W.J Mikenda, J Lumin 28, 431 (1983) W Mikenda, J Lumin 26, 85 (1981) W Nie, F.M Michel-Calendini, C Linares, G Boulon, C Daul, J Lumin 46, 177 (1990) 10404-p5 ...T.T Loan et al.: Synthesis and optical properties of ZnAl2 O4 :Cr3+ ,Tb3+ powders Conclusion (a) The ZnAl2 O4 spinel powders co-doped with Tb3+ and Cr3+ ions have been successfully... undergone heat-treatment at 1100 ◦ C exhibited the best crystallinity The PL spectra of the samples showed two emission regions: the first (blue-green) one was related to the emission of Tb3+ ions and. .. was related to the emission of Cr3+ ions By selecting the appropriate mole fractions of the Tb3+ ions x and Cr3+ ions y, we can obtain the materials strongly emitting in both blue-green and red