**3. Role of flux in calcination temperature**

CaAl2O4:Eu3+,R+ (R=Li, Na, K) phosphors were initially prepared by mixing stoichiometric amounts of CaCO3, Al2O3 (A.R.), Eu2O3 (99.99%), and with or without one of Li2CO3, Na2CO3 or K2CO3 (A.R.) flux using solid state reaction technique at high temperature. Then a certain quantity of flux H3BO3 were added. The quantity of the flux H3BO3 is very crucial and dictates the calcination and reduction temperatures. The X-ray diffraction patterns of CaAl2O4:Eu3+, Li+ sample (Eu3+ and Li+ were 3 mol.%) calcined at 1000, 1100, 1200 and 1300 °C for 4 h are shown in **Fig. 4**. After calcined at 1000 and 1100 °C, the precipitated precursors showed some characteristic peaks of Al2O3 and CaO besides the characteristic peaks of CaAl2O4. When the temperature was increased to 1200 °C, only the CaAl2O4 phase was detected (JCPDS card No. 23-1036), and no other products or starting materials were observed. The high intensity of the peaks reveals the high crystallinity of the synthesized

Fig. 4. XRD patterns of CaAl2O4:Eu3+,Li+ phosphors sintered at different temperatures **[2].** 

resulting from each individual of the two fluxes. These powders show different photoluminescence (PL) intensities. To sum up, by selecting distinct individual or binary fluxes, the morphologies, particle size, and the PL intensities of BAM phosphor can be controlled. Beside, both larger crystal size and appropriate aspect ratio play a crucial role on enhancing

CaAl2O4:Eu3+,R+ (R=Li, Na, K) phosphors were initially prepared by mixing stoichiometric amounts of CaCO3, Al2O3 (A.R.), Eu2O3 (99.99%), and with or without one of Li2CO3, Na2CO3 or K2CO3 (A.R.) flux using solid state reaction technique at high temperature. Then a certain quantity of flux H3BO3 were added. The quantity of the flux H3BO3 is very crucial and dictates the calcination and reduction temperatures. The X-ray diffraction patterns of CaAl2O4:Eu3+, Li+ sample (Eu3+ and Li+ were 3 mol.%) calcined at 1000, 1100, 1200 and 1300 °C for 4 h are shown in **Fig. 4**. After calcined at 1000 and 1100 °C, the precipitated precursors showed some characteristic peaks of Al2O3 and CaO besides the characteristic peaks of CaAl2O4. When the temperature was increased to 1200 °C, only the CaAl2O4 phase was detected (JCPDS card No. 23-1036), and no other products or starting materials were observed. The high intensity of the peaks reveals the high crystallinity of the synthesized

Fig. 4. XRD patterns of CaAl2O4:Eu3+,Li+ phosphors sintered at different temperatures **[2].** 

the luminescence of BAM phosphor.

**3. Role of flux in calcination temperature** 

powders. The sintering temperature was optimized to be 1200 °C. The luminescence intensity of CaAl2O4:Eu3+ was significantly enhanced by co-doping with alkali metal ions, probably due to the charge compensation. Furthermore, the emission intensities were gradually enhanced when the radius of R+ became smaller from K+ to Li+ ion. It was probably due to the difference of ionic radii which would give rise to the diversity of sublattice structure around the luminescent center ions. This fundamental work might be important in developing new luminescent devices applicable for tricolor lamps, light emitting diodes and other fields.

In Y2O3:Eu sintered at 700-12000C in air, Li2CO3 flux was found to: (i) enhance the crystalline growth, ii) improved the grain size slightly morphology from a plate like structure to spherical shape, and iii) improved significantly its PL sensitivity. The optimal red PL was achieved when the Y2O3:Eu3+,Li+ phosphor was synthesized using 11 mol% Eu2O3 and 70 mol% Li2CO3 and sintered at 1,200°C for 5 h.

In ZnWO4, the maximum PL intensity was obtained when the sintering temperature was 1,100°C. A significant decrease in PL intensity was measured when the phosphor was sintered at 1,200°C. This decrease was attributed to a change in the crystallinity of the phosphor, in which (020) ZnWO4 was the dominant crystalline phase. Empirically, the change in crystallinity alters the emission mechanisms of the phosphor. The growth of larger phosphor grains was another reason for the decrease in luminescence. Furthermore, the PL spectrum was broadened when the sintering temperature increased. Apparently, oxygen vacancies were involved in the phosphor crystal, and the bluish-green emission was related to electron transitions from the energy levels of the ionized oxygen vacancies to the phosphor valance band. The concentration of oxygen vacancies usually increases with an increase in sintering temperature and a broadened emission is thus observed.

Significantly, on UV illumination, a white-light phosphor could be achieved if the bluishgreen ZnWO4 and red Y2O3:Eu3+,Li+ phosphors were blended.
