Powder XRD pattern of pure, xCe3+ doped BaAl2O4 (x = 0.1, 0.5 and 1 M.W %) and Li+ co-activated BaAl2O4: 0.5Ce3+, y Li+ (y = 0.1, 0.5 and 1 M.W %) is shown in the Fig. 1. The XRD pattern is in well agreement with standard JCPDS pattern data No.17–0306 with space group P6322, which exhibit hexagonal structure along with minor impurities peak around 24°, due to the presence of BaCO3 [8, 13]. In spite of Ce and Li doping, no change in the XRD pattern is observed. Figure.2 represents the Rietveld refinement of pure BAO using full proof suite and BaAl2O4 stuffed tridymite structure in the AlO4 tetrahedral site. In the refinement at low angle 24° there was no Bragg position (green line), which indicates the presence of minor impurity phase existing in the sample. The lattice parameters obtained by Rietveld refinement are listed in table 1, which are in good agreement with previous reports. .
Figure 3. shows the Raman spectra of the synthesized BaAl2O4. The Raman spectra shows 4 distinct peaks at 244 cm− 1, 415 cm− 1, 591 cm− 1 and 691 cm− 1 corresponding to F2g (1), Eg, F2g (2) and F2g (3) modes. The wavenumbers and the sequence of these modes (F2g (1) < Eg < F2g (2) < F2g (3)) conform to the characteristic modes exhibited by aluminate spinels .
The F2g(1) Raman mode observed at ~ 244 cm− 1 along with a shoulder observed at 199 cm− 1 are assigned to the translation of the tetrahedron in the crystal structure, which is typically occupied by Ba2+ ions . The Eg mode observed at ~ 415 cm− 1 has been attributed to the movement of oxygen atoms along the T-O bonds, (i.e.) bonds between the tetrahedral sites, occupied by Ba2+ ions, and oxygen . A shoulder can be observed on the Eg mode at ~ 395 cm− 1. This can be attributed to the cation disordering caused by the bending mode of Al ions on the tetrahedral sites . There isn’t much work discussing the origins of the F2g (2) peak observed at ~ 591 cm− 1. It has been assigned to the symmetric bending of oxygen with respect to the cations at the tetrahedral sites . Shoulders to the left and right of the F2g (2) mode can be observed in Fig. 3. Further examination would yield information about the origin of the disorders causing the breakdown of Raman selection rules. The nature of the octahedral cation dictates the position of the F2g (3) mode, observed at 691 cm− 1 . However, few authors have assigned this mode to the asymmetric bending or stretching of oxygen with respect to the tetrahedral cation . Group theory analysis predicts the occurrence of an acoustic mode A1g and its wavenumber is likely to be > F2g (3) . Chromates and ferrites are known to exhibit intense A1g modes while in contrast, aluminates (BaAl2O4) are known to exhibit weak A1g peaks. A week peak can be observed at ~ 760 cm− 1 from Fig. 3 corresponding to A1g mode.
Figure 4. shows the FTIR spectra of pure, 0.5% Ce3+doped BAO and Li+ co-activated BaAl2O4: 0.5Ce3+, 0.1% Li+. It can be observed that all the synthesized samples exhibit absorptions peaks at 662, 732, 803, 856, 1021 and 1450 cm− 1. The four absorption peaks centered around frequency range of 600–1000 cm− 1 can be designated to the stretching vibrations of Al–O, Ba–O and Ba–O–Al in the synthesis BaAl2O4 phosphors, respectively. The absorption peak at 1450 cm− 1 can be designated to the stretching vibration of AlO4 in a unit of the tetrahedral BaAl2O4 phosphors, which confirms the formation of the BaAl2O4 phase .
Compositional analysis of BAO: 0.5 Ce3+ phosphors, performed using X-ray photoelectron spectroscopy (XPS) is shown in Fig. 5. Ba 3d, Al 2p, O 1s and Ce 3d core levels of the BAO: 0.5Ce3+ are from the photoelectron signals. Two peaks present at approximately 790 eV (3d5/2) and 795.4 eV (3d3/2) confirm the presence of Ba 3d, which matches well with the reported data. Al 2p spectrum consists of two peaks at 72.3 eV (2P3/2), 73.5 eV (2P1/2), which affirms the presence of Al metal and Al-O, respectively. Photoelectron signal from O 1s, observed at 530.4 eV and 531.3 eV can be attributed to the formation of metal oxides and metal carbonates . Ce 3d composed of two multiplets to spin-orbit split 3d5/2 and 3d5/2, respectively. The binding energy peaks at 906 eV, 901.54 eV (Ce4+ 3d5/2) and peaks at 896.49 eV, 890.14 eV (Ce4+ 3d3/2) is assigned to Ce 3d. Four photoelectron signal peaks at 898.4 eV, 904.2 eV and 884.4 eV, 887.5 eV is assigned to Ce3+ 3d5/2 and Ce3+ 3d3/2 [13, 19, 20].
Figure 6. shows the photoluminescence excitation spectra of BAO: xCe3+ (x = 0.1, 0.5 and 1 M. W %) phosphor. The spectrum was recorded in the range of 260 to 350 nm for the emission wavelength 589 nm. Excitation spectrum at 320 nm (2F5/2 - excited 5D states) corresponds to the transition to 4f-5d from the ground state of Ce3+ ion. Generally, Ce3+ has two ground states of 2F5/2 and 2F7/2 due to the spin–orbit interaction .
Figure 7. shows the emission spectra of phosphors recorded in the range between 400 to 700 nm. It shows the photoluminescence emission spectra of BAO: xCe3+ (x = 0.1, 0.5 and 1 M. W %) and BAO: 0.5% Ce3+ co-doped with Li+ phosphor under excitation wavelength of 320 nm. The comparison of PL emission spectra of only Ce3+ doped and Ce3+ doped with Li+ co-doped is shown in the inset of Fig. 7. The phosphor exhibits broad peak centered at 589 nm which signifies a transition from excited level 5D to 2F5/2 . The transitions of the yellow light emission of the Ce3+ rare-earth element belongs to the electric dipole transition which is tuned by Ce3+ ion site .
The critical energy distance (Rc) can be calculated using Blasse Eq. (1) .
Here N represents the number of Z cations in the unit cell (i.e. 8)
V is the Volume of unit cell (Å3) = 830.4 Å3
Xc is the critical concentration = 0.5
The interaction between Ce3+ ions is stimulated by increasing doping concentration but non radiative energy transfer between the doping ions can lead to either electric multipolar or exchange interactions. Generally, exchange interactions take place a smaller value of Rc (< 5 Å), otherwise electric multipolar interactions dominate. For BAO: Ce3+ phosphors the value of Rc was found to be 15.83 Å, herein the electric multipolar interactions take place [13, 25]. To enhance the luminescence intensity of barium aluminates, different concentration of Li+ ions are doped into highest luminescence of BAO: 0.5Ce3+ host matrix. Researchers reported the successful enhancement of luminous intensity by co doping Li+ with CaO: Ce3+, CaTiO: Dy, and Ca2SrAl2O6: Ce3+, Mn3+, SrAl2O4:Eu2+, Ce3+ matrix [26, 27, 28, 29]. It is noted that the luminescence intensity can be further increased at suitable Li+ ion concentration. Initially the luminescence intensity increases and then decreases with the increase of Li+ concentration. Li+ ion doped into BAO: xCe3+ host lattice, the Li+ ion radii (76 pm) in turn lead to occupy the defect site due the smaller ionic radius, which changes the symmetry of local environment of Ce3+ in crystal lattice and prevents the relaxation between Ce3+ ion. As a result, the luminescence intensity is enhanced by co-activator Li+ ion [10, 30].
Exact emission color of phosphors for both BAO: x Ce3+ (x = 0.1, 0.5 and 1 M.W %) and BAO: 0.5 Ce3+, y Li+ (y = 0.1, 0.5 and 1 M.W %) was identified using the Commission International del’Eclairage (CIE) chromaticity coordinates (X, Y) graph as shown in Figure. 8, 9, respectively. It can be seen that the emission is located in yellow region . From the above results, BAO: 0.5Ce3+, 0.1Li+ phosphor can be considered as a proper source for emitting yellow light under 320 nm excitation wavelength. Calculated CIE chromaticity coordinates of the phosphors is shown in inset of Figs. 8 & 9. The color purity of the synthesized phosphors is found by given expression. (2). 
x, y is the chromaticity coordinate 0.5% Ce3+ doped BAO (x = 0.4864, y = 0.4412) and BAO: 0.5%Ce3+, 0.1%Li+ of the sample (x = 0.49868, y = 0.44171)
xi, yi are the coordinates of the illuminate point (xi= 0.3101, yi =0.3162)
xd, yd are the coordinate of dominant emission wavelength of 0.5% Ce3+ doped BAO (xd =0.5550, yd=0.4423) and BAO: 0.5%Ce3+, 0.1%Li+ of the sample (xd =0.5568, yd=0.4455)
In this Present work, the synthesized phosphors displayed a color purity 78.4 % and 81.3 % for 0.5% Ce3+ doped BAO and BAO: 0.5% Ce3+, 0.1% Li+, respectively. Correlated color temperature (CCT) value of the phosphors is calculated using McCamy empirical formulae (3) and (4). In the current work CCT value of 0.5% Ce3+ doped BAO phosphor and BAO: 0.5% Ce3+, 0.1% Li+ are 2583 K and 2450 K, respectively. Results indicate that both doped phosphors are close to value of warm yellow light (< 4000 K) .
CCT = − 449n3 + 3525n2 − 6823.3n + 5520.33---------------- (3)
Where, n = ------------ (4)
Here, xe = 0.332, ye = 0.186, is the epicenter
Figure 10 and 11 shows the lifetime decay spectra of BAO: xCe3+ (x = 0.1, 0.5, 1 MW %) and BAO: 0.5% Ce3+, xLi+ (x = 0.1, 0.5, 0.1 MW%) phosphors excited under 320 nm. 0.5 MW% Ce3+ ion doped BAO phosphor exhibited high decay lifetime values when compared to other percentage of Ce3+ dopant. Figure 11 results show a decrease lifetime with increase in Li+ ion concentration. The curves are fitted with biexpotential function indicated with red line in the spectra.
Fitted by a biexponential function is given Eq. (5) .
y = y0 + A1 exp + A2 exp ------ (5)
A1, A2 - weighting parameters,
τ1, τ2 - components of lifetime decay.
The average lifetime is calculated using the Eq. (6) given below:
The calculated average lifetimes of BAO: 0.5Ce3+ and BAO: 0.5Ce3+, 0.1Li+ phosphors are 4.333 and 5.63 ns respectively. The result indicate that the Li+ doping results in the enhancement of lifetime in Ce3+ doped BAO phosphor. When the Li+ ion concentration increases in BAO: 0.5 Ce3+ phosphor, the life time decreases due to non-radiative transition 5d→4f between the Ce3+ ion.
From Luminescence decay spectra, the lifetimes of 0.5% Ce3+ ions increase with increase of Li+ ion, which is due to the energy transfer from the Ce3+ to Li+ ions. The energy transfers efficiency (ηET) was found using Eq. (7) .
τ 0.5ce - lifetimes of BAO: 0.5Ce3+
τ xLi - lifetimes of BAO: 0.5% Ce3+, xLi+ (x = 0.1, 0.5, 1 MW%)
The calculated values of energy transfer efficiency are 23.09%, 13.40% and 09.41% for BAO: 0.5% Ce3+, xLi+ (x = 0.1, 0.5, 1 MW%), respectively.