d–f luminescence of Ce3+ and Eu2+ ions in BaAl2O4, SrAl2O4 and CaAl2O4 phosphors

Ce3+ and Eu2+ doped alkaline earth aluminates MAl2O4 (M = Ca, Sr, Ba) were prepared by single-step combustion synthesis at low temperature (600 °C). X-ray diffraction (XRD) analysis confirmed the formation of BaAl2O4, CaAl2O4, and SrAl2O4. Photoluminescence spectra and optimal luminescent properties of Ce3+ and Eu2+ doped MAl2O4 phosphors were studied. Relation between Eu2+ and Ce3+ f–d transitions was explained. Spectroscopic properties known for Ce3+ were used to predict those of Eu2+ by using Dorenbos’ method. The values thus calculated were in excellent agreement with the experimental results. The preferential substitution of Ce3+ and Eu2+ at different Ba2+, Sr2+, Ca2+ crystallographic sites was discussed. The dependence of emission wavelengths of Ce3+ and Eu2+ on local symmetry of different crystallographic sites was also studied by using Van Uitert’s empirical relation. Experimental results matched excellently with the predictions of Dorenbos’ and Van Uitert’s models.


Introduction
Sulfide-based phosphors have been used for flat panel displays, cathode ray tubes, and fluorescent lamps. But at high current density, sulfide-based phosphor components degrade rapidly, while oxide-based phosphors doped with rare earth ions are thermally and chemically more stable. They also have good heat resistance in display applications. Hence, recently there is growing interest in oxide-based phosphors.
Aluminate-based phosphors doped with Ce 3+ have received remarkable attention owing to their wide applications in flat panel displays, X-ray imaging, and tri-colour lamps [1]. Ce 3+ doped phosphors are used for fast scintillators and also as long lasting phosphorescent materials. Jia and co-workers [2] reported luminescence in BaAl 2 O 4 :Ce 3+ and BaAl 2 O 4 :Ce 3+ ,Dy 3+ . The photoluminescence in BaAl 2 O 4 :Mn 2+ ,Ce 3+ was reported by Suriyamurthy and Panigrahi [3]. In recent, all kinds of lamps and display devices are being replaced by light emitting diodes (LEDs). The first commercial white LEDs have been produced by the combination of blue LED with cerium doped yettrium aluminum garnet (YAG:Ce) phosphor. Divalent europium doped oxide-based phosphors are highly useful owing to their high brightness, tunable emission wavelength from UV to red, low toxicity, and increased chemical and thermal stability. Therefore,  these phosphors are important for industrial and technological applications in fluorescent lamps, LEDs, and emissive displays for computers and mobile telephones [4,5]. The phosphorescence of Eu 2+ in most hosts is caused by the 4f→5d transition [6,7]. BaAl 2 O 4 :Eu 2+ ,Dy 3+ [8] and BaAl 2 O 4 :Eu 2+ ,Nd 3+ [9] also exhibit long lasting phosphorescence (LLP) properties. Eu 2+ doped BaAl 2 O 4 and SrAl 2 O 4 could be promising phosphor materials for plasma display panel (PDP) application [10,11]. SrAl 2 O 4 is one of the foremost promising host materials for fluorescent lamp, light emitting diodes, and persistent luminescent materials [12][13][14][15][16]. The effects of various doping compositions and impurities on the phosphorescence of green-emitting alkaline earth aluminate phosphor (SrAl 2 O 4 :Eu 2+ ,Dy 3+ ) have been reported by Kim et al. [17]. CaAl 2 O 4 :Eu 2+ is the most important blue phosphor material among the phosphor group which is useful in LLP devices.
Synthesis technique highly affects the quality of luminescent material. Use of conventional processes like solid-state reaction and sol-gel method for synthesis of phosphorescence materials involves unavoidable problems such as extremely high temperature and quite long reaction time at high temperature [18]. In comparison, combustion method is relatively simple. Moreover, there are many merits of the combustion synthesis. It is energy-saving and safe. It gets completed only in few minutes (5 min). This synthesis technique avoids steps such as washing, filtration, drying, etc. It has been extensively used to prepare various oxide materials at a relatively low temperature. Therefore, combustion method is a promising technique for synthesis of complex oxide ceramics such as aluminates.
In this study, we have prepared Ce 3+ and Eu 2+ doped BaAl 2 O 4 , SrAl 2 O 4 , CaAl 2 O 4 phosphors using single-step combustion synthesis at an initiating temperature of 600 ℃ with urea as a fuel. The prepared samples were characterized using X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, and scanning electron microscopy (SEM). In this paper, we have reported the concentration quenching behavior of Ce 3+ and Eu 2+ luminescence in these hosts. Systematic relationship between emission wavelengths of Ce 3+ and Eu 2+ ions at the same crystallographic site was studied. The spectroscopic properties, crystal field splitting, centroid shift, red shift, and Stokes shift were estimated. Spectroscopic properties known for Ce 3+ were used to predict those of Eu 2+ by using Dorenbos' formula [19]. Experimental results matched with the theoretical predictions. The preferential substitution of Ce 3+ and Eu 2+ ions at different Ba 2+ , Sr 2+ , Ca 2+ crystallographic sites was explained by using Van Uitert's empirical relation [20]. The dependence of emission wavelengths of Ce 3+ and Eu 2+ on the local symmetry of crystallographic site was studied.

Experimental
The samples were prepared by mixing nitrates of barium (Ba(NO 3 ) 2 ), strontium (Sr(NO 3 ) 2 ), or calcium (Ca(NO 3 ) 2 ) and aluminum (Al(NO 3 ) 3 ·9H 2 O), with cerium nitrate or europium nitrate, and urea (CO(NH 2 ) 2 ) in stoichiometric ratio using a mortar and pestle. The resulting paste for each mixture was then heated in a muffle furnace at an initiating combustion temperature of 600 ℃. The paste underwent dehydration and finally decomposed with the evolution of gases (oxides of nitrogen and ammonia). The mixture frothed and swelled, forming the foam that ruptured with a flame. The entire combustion process was completed in less than 5 min. The voluminous combustion ashes of combustion synthesized phosphors were ground using the pestle and mortar to make fine white powders.
Reducing atmosphere was needed for Eu 2+ . Eu doped sample as described above was taken in alumina crucible and placed in closed stainless still box filled with charcoal. The box was heated at 800 ℃ for 1.30 h. Incomplete burning of charcoal provided reducing atmosphere. To avoid the contamination from burning charcoal, the crucible was covered by piece of ceramic fiber blanket followed by alumina lid. This method is low cost and simple. Various concentrations of Eu 2+ (0.05%-3%) and Ce 3+ (0.02%-2%) dopants were tried. The fine powders were characterized further. XRD measurements were performed using Philips PANalytical X'pert Pro diffractometer. XRD pattern of reduced sample was not different from that of the sample without reduction. PL measurements in the spectral range of 220-700 nm were made on Hitachi F-4000 spectro-fluriometer at room temperature. The morphology was studied using EVO 18 scanning electron microscope with an accelerating voltage of 20 kV and working distance of 8.5 mm.
Formation of oxides by the combustion process is represented by following equations: 6Al(NO 3 ) 3

1 X-ray diffraction
The phase formation of samples was checked by XRD. Figures 1-3 Table 1.

2 Scanning electron microscopy
Surface morphology of prepared powders was analyzed using SEM. Figure 4 represents the SEM micrographs of the host without any doping. In all three samples, particles tend to agglomerate forming small clusters with non-uniform shapes and sizes. The morphologies of the phosphors are irregular with diameter varying from two to several microns. In addition, there are plate like structures having cracks and pores. A large number of pores are formed in the combustion derived powders due to large quantity of escaping gases. The plate like morphology has an advantage in light out-coupling. The irregularity in shapes, sizes, and porosity is due to irregular mass flow during combustion and non-uniform distribution of temperature. Finally, the aluminates derived from the combustion process reflect inherent foamy nature.

3 Crystal structure of MAl 2 O 4 (M = Ca, Sr, Ba)
The compounds MAl 2 O 4 (M = Ca, Sr, Ba) belong to the family of stuffed tridymite structure [21]. The structure has corner-sharing AlO 4 tetrahedron in three-dimensional framework. Each tetrahedron has one net negative charge as each oxygen atom in the tetrahedron is shared with two aluminum atoms. Divalent cations Ba 2+ , Ca 2+ , Sr 2+ occupy interstitial sites within the tetrahedral framework and achieve the charge balance. The tetrahedral framework is isostructural within the tridymite structure [22]. SrAl 2 O 4 undergoes a phase transition from a low temperature monoclinic distorted structure to the hexagonal tridymite structure at 650 ℃ [23]. CaAl 2 O 4 has a stuffed tridymite structure but transforms to at least three other polymorphs at high pressure [24]. Figure 5 shows three-dimensional sketch of hexagonal BaAl 2 O 4 crystal structure showing two Ba 2+ sites in the BaAl 2 O 4 structure, each with 9-fold coordination [25]. According to the crystal structure, the first Ba 2+ site (2a) has the multiplicity of two and site symmetry of C3 while the second one (6c) has the multiplicity of six and site symmetry of C1. The sites are similar in average size (d(Ba-O) ave = 2.9162 Å). Ionic radius of 9-coordinated Ba 2+ is 1.47 Å [26].
The SrAl 2 O 4 host crystallizes in the stuffed tridymite type of structure. SrAl 2 O 4 belongs to monoclinic structure with space group P2 1 (4). The three-dimensional network consists of corner-sharing AlO 4 tetrahedron containing large voids, in which the Sr 2+ ions locate on two types of 9-fold coordinated sites which differ only due to slight distortion of their square planes [27] (Fig. 6). Average distances of these 9 oxygen ions are 2.8776 and 2.8359 Å for Sr1 and Sr2 respectively. Ionic radius of 9-coordinated Sr 2+ is 1.31 Å [26].
Distance of each neighboring oxygen anion from the alkaline earth ion at the different sites along with the calculated distances using the formula given by Dorenbos [19] are shown in Table 2 and discussed in the next section.   The ground state (4f 1 configuration) of Ce 3+ ion splits due to spin orbit coupling into two states namely 2 F 5/2 and 2 F 7/2 with separation of nearly 2000 cm 1 . The excited state (5d configuration) of Ce 3+ ion is split into two to five components due to crystal field effect. The excitation bands in these hosts are due to the transition from typical 4f 1 state to crystal field split 4f 0 5d 1 state of Ce 3+ . The emission of these phosphors is due to transitions from the lowest crystal field split components of the 2 D state to the 2 F 5/2 and 2 F 7/2 levels of ground state. The transition in both the cases is fully allowed, as 5d→4f transition is parity allowed. The 5d level of free Ce 3+ ion is at 6.35 eV above the 4f ground state. The luminescence is very strongly dependent on the host lattice. It varies from UV to the visible region.

4 Ce 3+ luminescence
The differences in the photoluminescence spectra of these samples are discussed in terms of effect of crystal structures on Ce 3+ energy levels. From the emission spectra of different compounds, Dorenbos [19,29,30,] derived the semi-empirical relation for predicting PL spectra of Ce 3+ . He used the terms CFS  which is defined as the crystal field splitting (CFS) that is the energy difference between the maxima of the highest and lowest 5d band in the spectra, and C  which is defined as the centroid shift that is the shift of the average of the 5d configuration [19]. D(n+) is the red shift of the lowest 5d excitation level as compared to that of free n-valent ion. S  is the Stokes shift between excitation ( abs E ) and emission ( em E ) bands. The relation between the red shift D(3+) with crystalline environment is as follows [31,32]: av R is the average distance from ion to ligand and R  is the correction for lattice relaxation when dopant enters the sites. The Stokes shift can be calculated as abs em Above equations are used for Ce 3+ doped 9-coordinated Ba 2+ , Sr 2+ , and Ca 2+ sites in these phosphors. All the evaluated data are listed in Table 3.
In the next section, these spectroscopic properties are used to predict those of divalent lanthanide Eu 2+ in the same host at the same site.

5 Prediction of Eu 2+ levels from Ce 3+
Dorenbos [32] established the relation between red shift (D) and Stokes shift ( S  ) of divalent and trivalent lanthanides as    (2 ) 0.61 The energy difference between the lowest 4f 7 ( 8 S 7/2 ) level and the first 4f 6 ( 7 F 0 )5d 1 level is lowered from the free electron value when the Eu 2+ ion is brought into a crystal environment. The effect of the host crystal on this energy difference is expressed by the red shift D and the Stokes shift S  . The energy of f →d absorption and that of the d→f emission can be written respectively, as [33]: The free ion value of Eu 2+ is free E = 4.2 eV [34]. We determined red shift (Eq. (7)) and Stokes shift (Eq. (8)) of divalent lanthanide (Eu 2+ ) using the free ion value of Eu 2+ , red shift and Stokes shift of trivalent lanthanide (Ce 3+ ) ( Table 3). Using the data from Eqs. (7) and (8) in Eq. (10), we calculated emission energy in europium doped BaAl 2 O 4 , SrAl 2 O 4 , and CaAl 2 O 4 . Emission bands are obtained at 474, 451, and 448 nm respectively for 9-coordinated site. We notice that the calculated values of emission energies are in good agreement with experimental emission spectrum (485, 498, and 440 nm, respectively for BaAl 2 O 4 , SrAl 2 O 4 , and CaAl 2 O 4 ) as shown in Figs. 11-13. This confirms that the Ce 3+ and Eu 2+ preferentially occupy 9-coordinated Ba 2+ , Sr 2+ , and Ca 2+ sites.  Figure 11(a) illustrates the excitation spectrum of BaAl 2 O 4 :Eu 2+ , which consists of three peaks at 270, 328, and 397 nm. The intensity of 328 nm peak is 5 times greater than that of 270 nm peak. It is the strongest excitation peak in BaAl 2 O 4 :Eu 2+ . The excitation spectrum is ascribed to 4f 7 →4f 6 5d transition of Eu 2+ . The emission spectrum is the same for different excitation wavelengths centered at 270, 328, and 397 nm. This shows that emission spectrum does not depend on the excitation wavelengths. The emission spectrum has two well resolved peaks at 485  and 433 nm, which confirms the presence of two Ba 2+ crystallographic sites in BaAl 2 O 4 . The emission spectrum has maximum centered at 485 nm, showing blue luminescence. The emission spectrum is due to 4f 6 5d→4f 7 transition of Eu 2+ . The PL spectra are in good agreement with the results reported in the literature [11,35,36].

6 Eu 2+ luminescence
The excitation spectrum of SrAl 2 O 4 :Eu 2+ is broader consisting of two peaks at 230 and 350 nm. The bluish green emission has maximum at 498 nm which is in good agreement with measurement of Palilla et al. [11] who found the band at 500 nm. Excitation spectrum of CaAl 2 O 4 :Eu 2+ shows two peaks at 275 and 329 nm. It shows blue luminescence having peak at 440 nm. This is exactly the same as that reported by Palilla et al. [11] and close to the result of Kim et al. [37]. Observation of single emission band in CaAl 2 O 4 and SrAl 2 O 4 indicates emission from one site only. This seems to be in contrast to the crystallographic data [38,39], that there are three Ca 2+ and two Sr 2+ sites of very low symmetry, respectively. In order to obtain optimal luminescent properties of Eu 2+ doped BaAl 2 O 4 , SrAl 2 O 4 , and CaAl 2 O 4 , a series of these phosphors were synthesized. The concentration quenching occurs when Eu 2+ is doped above 2%, 1%, and 2% in these hosts respectively.
The maximum acceptable percentage difference ( r D ) in ionic radii between doped and substituted ions must not exceed 30% [40,41]. The calculations of the radius percentage difference ( r D ) between the doped ions (Ce 3+ , Eu 2+ ) and the possible substituted ions (Ba 2+ , Sr 2+ , Ca 2+ , Al 3+ ) are summarized in Table 4. The values are based on the formula, where CN is the coordination number, M (CN) R is the radius of host cation, and d (CN) R is the radius of doped ion. The value of r D between Ce 3+ and Ba 2+ , Eu 2+ and Ba 2+ on 9-coordinated sites is 18.63% and 11.56% respectively, while the value of r D between Ce 3+ and Al 3+ , Eu 2+ and Al 3+ is 206.66% and 233.33% respectively. Thus, doped ions Ce 3+ and Eu 2+ will preferentially substitute the 9-coordinated barium sites. Similar arguments hold for doping of Ce 3+ and Eu 2+ ions in strontium and calcium sites.

7 Occupancy of Ce 3+ and Eu 2+ at different crystallographic sites from emission wavelengths
To understand the relationship between emission wavelength and sites of Ba 2+ , Ca 2+ , Sr 2+ occupied by Ce 3+ and Eu 2+ , we used Van Uitert's empirical formula [20]. Van Uitert calculated the positions of the lower d-band edge for Ce 3+ and Eu 2+ ions using the following empirical formula: where E is the position in energy of the lower d-band edge for Ce 3+ and Eu 2+ in three-dimensional structure, Q is the energy edge of the lower d-band for the free ions (Q = 50000 cm −1 for Ce 3+ ion and 34000 cm 1 for Eu 2+ ion), V is the valence of the active cation (here V is +3 for Ce 3+ and +2 for Eu 2+ ) , n is the number of anions in the immediate shell about this ion, ea is the electron affinity of the coordination radial, and r is the radius of the host cation replaced by the activator.   (1.30 Å) is relatively too large compared to that of Ca 2+ so the crystallographic distortions will influence the crystal field and emission spectra when Eu 2+ enters the Ca 2+ site. In addition experimental results are near to 9-fold coordination geometry of Ca 2+ ion than the 6-fold coordination. Thus Ce 3+ and Eu 2+ will occupy tri-capped trigonal anti-prism polyhedron 9-coordinated Ca3 site in crystal lattice [28].