Crystal and Electronic Structures, Photoluminescence Properties of Eu2+-Doped Novel Oxynitride Ba4Si6O16-3x/2Nx

The crystal structure and the photoluminescence properties of novel green Ba4-yEuySi6O16-3x/2Nx phosphors were investigated. The electronic structures of the Ba4Si6O16 host were calculated by first principles pseudopotential method based on density functional theory. The results reveal that the top of the valence bands are dominated by O-2p states hybridized with Ba-6s and Si-3p states, while the conduction bands are mainly determined by Ba-6s states for the host, which is an insulator with a direct energy gap of 4.6 eV at Γ. A small amount of nitrogen can be incorporated into the host to replace oxygen and forms Ba4-yEuySi6O16-3x/2Nx solid solutions crystallized in a monoclinic (space group P21/c, Z = 2) having the lattice parameters a = 12.4663(5) Å, b = 4.6829(2) Å, c = 13.9236(6) Å, and β = 93.61(1)°, with a maximum solubility of nitrogen at about x = 0.1. Ba4Si6O16-3x/2Nx:Eu2+ exhibits efficient green emission centered at 515–525 nm varying with the Eu2+ concentration when excited under UV to 400 nm. Furthermore, the incorporation of nitrogen can slightly enhance the photoluminescence intensity. Excitation in the UV-blue spectral range (λexc = 375 nm), the absorption and quantum efficiency of Ba4-yEuySi6O16-3x/2Nx (x = 0.1, y = 0.2) reach about 80% and 46%, respectively. Through further improvement of the thermal stability, novel green phosphor of Ba4-yEuySi6O16-3x/2Nx is promising for application in white UV-LEDs.


Introduction
Recently, in the exploration of novel phosphors for applications in white LED lighting, several oxynitride based phosphors with improved properties have been created by partial cross-substitution of Si-N for Al-O in the oxide based host lattices, like alkaline earth aluminates of MAl 2 O 4 :Eu 2+ (M = Ca, Sr, Ba) [1] and alkaline earth aluminosilicates, viz., Sr 2 Al 2 SiO 7 :Eu 2+ [2] and Sr 3 Al 10 SiO 20 :Eu 2+ [3,4]. By this approach, the absorption and excitation bands of Eu 2+ can be extended to the longer wavelength [1,2] and the emission intensity can be enhanced [3,4] due to the incorporation of more covalent bond of Si-N, providing a stronger reducing environment around the Eu 2+ ions in the oxide based host lattices [5]. On the other hand, it is also highly interesting to know whether or not a single nitrogen can be incorporated into the oxide based lattices occupied on the oxygen sites, which could be more flexible than the cross-substitution of Si-N Al-O and increase the possibility of inventing novel oxynitride phosphor materials to meet the requirements of the development of white LEDs.
As far as we know, no related such reports have been found, and the correlations of the crystal and electronic structure with the luminescence properties also have not yet been studied in the barium silicate system of Ba 4 Si 6 O 16 . As one of our exploration investigations, in the present work Ba 4 Si 6 O 16 was used as the host lattice to check the possibility of the incorporation of single nitrogen by the general formula of Ba 4-y Eu y Si 6 O 16-3x/2 N x . For a better understanding of the photoluminescence properties, firstly we calculated the electronic structures of the host lattice of Ba 4 Si 6 O 16 by first principles method, then we characterized the crystal structure, photoluminescence properties, and thermal stability of Eu 2+ -doped Ba 4 Si 6 O 16-3x/2 N x .

Computational Details
The density of states (DOS) and band structure calculation for Ba 4 Si 6 O 16 were performed by first principles method using pseudopotentials and a plane wave basis set [18] within the density functional theory (DFT) performed by VASP package [19][20][21]. The initial structural parameters were adopted from the single crystal data [22]. The projector augmented wave pseudopotentials were adapted for Ba, Si and O atoms. Exchange correlations were treated with the generalized gradient approximation (GGA) with a Perdew-91 functional form [23]. The numerical integration of the Brillouin zone (BZ) was performed using a discrete 4 × 6 × 4 Monkhorst-Pack k-point sampling, and the plane wave cutoff energy was fixed at 500 eV. The Wigner-Seitz radius employed in the calculations is about 1.979 Å for Ba, 1.312 Å for Si, and 0.82 Å for O. The Fermi energy level was set at zero energy for the calculations.

Synthetic approaches
The oxynitride phosphors with the composition of Ba 4-y Eu y Si 6 O 16-3x/2 N x (x < 1, and y = 0.04-0.4) were synthesized by a solid state reaction approach using BaCO 3 (Sigma-Aldrich, 99%), SiO 2 (High Purity Chemical Co., Ltd, 99.9%), Si 3 N 4 (UBE, SN-E10), and Eu 2 O 3 (Shin-Etsu Chemical Co. Ltd., 99.99%) as the starting materials. The appropriate amount of the raw materials were weighted out and then mixed by ball milling in hexane with silicon nitride balls for about 4 h. Subsequently, the dried powder mixtures were diverted in a BN boat and then fired within a tube furnace at 1200-1300 °C for 6 h under a N 2 -H 2 (5%) atmosphere.

Characterization
The X-ray diffraction (XRD) patterns of the prepared materials were recorded by the X-ray powder diffraction (Rigaku, RINT Ultima-III) with the graphite monochromator using Cu-K α radiation (λ = 1.54056 Å), operating at 40 kV and 40 mA. For the structure analysis, the XRD data were collected in the range of 10-100° in 2θ using a step-scan mode with a step size of 0.02 and a count time of 5 s per step. The Rietveld refinements were carried out by the GSAS package [24,25]. The structural parameters of Ba 4 Si 6 O 16 [22] were used as an initial model for the refinement of the crystal structure of Ba 4-y Eu y Si 6 O 16-3x/2 N x , and the Eu and N ions are supposed to be randomly occupied on Ba and O sites, respectively, in the course of structural refinements.
The photoluminescence spectra were measured by a fluorescent spectrophotometer (F-4500, Hitachi Ltd., Japan) at room temperature with a 150 W xenon short arc lamp. The emission spectrum was corrected for the spectral response of a monochrometer and Hamamatsu R928P photomultiplier tube by a light diffuser and tungsten lamp. The excitation spectrum was also corrected for the spectral distribution of the xenon lamp intensity by measuring rhodamine-B as the reference. The quantum efficiency and the temperature-dependent luminescence properties were recorded on an intensified multichannel spectrophotometer (MCPD-7000, Otsuka Electrics, Japan) with a 200 W Xe lamp as an excitation source. A white BaSO 4 plate was employed as a standard reference for the quantum efficiency measurement. With regard to the thermal stability measurement, the powder samples within a quartz container were heated from room temperature to 200 °C in air with a heating rate at 100 °C/min, and the time duration was set for 5 min at each recorded temperature.     Figure 3 shows the X-ray powder diffraction patterns of Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x . It was evidently found that single nitrogen could be incorporated into the host lattice of Ba 4 Si 6 O 16 through partial replacement of the oxygen atoms to form the limited solid solutions in a single phase form. The maximum solubility of nitrogen is only about x = 0.1 based on the fact that the Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x phosphors are single phase products at x ≤ 0.1, nevertheless an unindexed second-phase occurs when the x value surpasses 0.1. The O/N ratio measured by chemical analysis was also given the similar result (16/0.12). As compared to the cross-substitution of Si-N Al-O, the lower solubility of nitrogen within metal silicates could be mainly related to the type of the crystal structure, as well as the composition, e.g., the Ba/Si ratio.   Figure 4, the observed XRD pattern perfectly matches with that of the calculated one, confirming that small amounts of the nitrogen atoms can partially replace the oxygen atoms in the Ba 4 Si 6 O 16 host lattice and form the defected (i.e., oxygen vacancy) solid solution of Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x . The perspective view of the crystal structure of Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x is shown in Figure 5 along with the local coordination of the Ba/Eu atoms with the O/N atoms (Figure 5c).

Formation and crystal structure of Ba 4-y Eu y Si 6 O 16-3x/2 N x
In comparison with oxide based Ba 3.88 Eu 0.12 Si 6 O 16 , the lattice parameters of Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x (x = 0.1) are slightly expanded by introducing the nitrogen atom because the ionic size of N 3-(1.46 Å) is larger than that of O 2-(1.38 Å) in the four-fold coordination [26]. As a consequence, both the unit cell volume and the average bond distance of Ba/Eu-O/N show slight increase for the obtained oxynitride phosphor due to the size effect. In addition, the unit cell volumes of Eu 2+ -doped Ba 4 [22] crystallized in a monoclinic system with the space group P2 1 /c (No. 14), having the lattice parameters a = 12.4663 (5) Figure 6a shows the diffuse reflection spectra of Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x of x = 0 and x = 0.1. There is a broad absorption band centered at about 360 nm for the Eu 2+ concentration of 3 mol % in oxide (x = 0) and oxynitride (x = 0.1) based phosphors, which is associated with the 4f 5d transition of Eu 2+ . It is clearly seen that the absorption edge of Eu 2+ shifts to long wavelength (i.e., low energy) from about 443 to 447 nm by introducing nitrogen, suggesting that a small amount of nitrogen indeed can be incorporated into the oxide lattice of Ba 4 Si 6 O 16 by partial substitution oxygen because nitrogen can narrow the band gap energy of the host through introduction of impurity levels close the bottom of the conduction bands [29]. On the other hand, as shown in Figure 6b, the absorption edge of Ba 4-y Eu y Si 6 O 16-3x/2 N x (x = 0.1) also significantly shifts to longer wavelengths with an increase of the doping Eu 2+ concentration due to the reabsorption between the Eu 2+ ions [28]. Figure 7 represents the excitation and emission spectra of Ba 4-y Eu y Si 6 O 16-3x/2 N x as a function of the doping nitrogen content. The excitation spectra consist of two major bands centered at about 276 and 340 nm accompanying with a strong shoulder at about 386 nm, in fair agreement with the observed absorption band of Eu 2+ (~365 nm) in the reflection spectra ( Figure 6). Corresponding to the incorporation of nitrogen into the lattice, a weak shoulder also appears at about 440 nm, which can be enhanced at high nitrogen content. When excited at UV and UV/blue (i.e., 360-400 nm), Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x (x = 0-0.1) shows bright green emission with a broad emission band peaking at about 520 nm, arising from the 4f 6 5d 1 4f 7 transition of the Eu 2+ ion, whose position show a slight red shift (1 ~ 2 nm) in comparison with that of oxide based one (x = 0) due to the presence of nitrogen that results in an increase of covalent bonding in the lattice. It is worth noting that the position of the emission band of Eu 2+ obtained in this work for x = 0 is significantly different with the studies on Ba 2 Si 3 O 8 :Eu 2+ [7], where the emission band was found to be located at about 485 nm or 500 nm, giving bluish green light. As mentioned above (see section 4.2), since the two luminescence centers of Eu 2+ are so similar in structures that the two emission bands of Eu 2+ can hardly distinguished, namely they are almost completely overlapping together (Figure 7). Additionally, both the excitation and emission intensity of Ba 4-y Eu y Si 6 O 16-3x/2 N x exhibit a slight increased tendency with the nitrogen content increasing in the x range of 0-0.1.   intensity reaches a maximum value at the Eu 2+ concentration y ≈ 0.2. While when y surpasses 0.2, due to concentration quenching within the Eu 2+ ions, the PL intensity shows a decrease tendency ( Figure  8b). Similar to the reflection spectra, the right wing of the excitation spectra also shows a slight red shift originated from the reabsorption of Eu 2+ [28]. As usual, while the position of the emission band of Ba 4-y Eu y Si 6 O 15.85 N 0.1 linearly shifts to longer wavelengths from 515 to 526 nm (Figure 8b) mainly caused by the energy transfer and/or reabsorption within the Eu 2+ ions, as well as the increased Stokes shift, for example the Stokes shift is estimated to be 6600 and 6700 cm -1 for y = 0.04 and y = 0.2, respectively.

Photoluminescence properties
The relationships between the absorption and quantum efficiency of Ba 4-y Eu y Si 6 O 15.85 N 0.1 with the Eu 2+ concentration are given in Figure 9. The preliminary results showed that the highest absorption and quantum efficiency could be achieved with 80% and 46%, respectively, when y = 0.2 in Ba 4-y Eu y Si 6 O 15.85 N 0.1 under a monitoring wavelength at 375 nm.
The thermal stability of the Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x phosphors at high temperature is given in Figure 10. Surprisingly, the thermal quenching rate is very high and the relative emission intensity shows a nearly linear decrease with the temperature rising. The thermal quenching temperature T 1/2 is at about 100 °C for Ba 4-y Eu y Si 6 O 15.85 N 0.1 (y = 0.1). This behavior may be mainly related to the crystal structure and composition. As compared to oxide based phosphors (y = 0), with the incorporation of nitrogen, e.g., y = 0.1, the thermal stability of oxynitride based Ba 3.88 Eu 0.12 Si 6 O 16-3x/2 N x phosphor has been slightly improved. The incorporation of nitrogen into Ba 4 Si 6 O 16 :Eu 2+ can increase the rigidity of the host lattice because Si-(O, N) has high covalent bond than that of Si-O as expected. On the other hand, due to the presence of nitrogen the Eu 2+ ions are more stable in air at high temperature. Those two reasons may be responsible for an increase of the thermal stability. Furthermore, it has been found that the thermal stability of Ba 4-y Eu y Si 6 O 15.85 N 0.1 could be further increased by the modification the chemical composition with alkaline earth cation that results will be reported elsewhere. As a whole, with improved temperature dependence, novel oxynitride Ba 4-y Eu y Si 6 O 16-3x/2 N x could be a potential candidate green conversion phosphor for application in white-light UV-LEDs.

Conclusions
Novel Ba 4-y Eu y Si 6 O 16-3x/2 N x phosphor has been synthesized by a solid state reaction at 1200-1300 °C in a N 2 -H 2 (5%) atmosphere. First-principles calculations indicate that the host of peaking at about 520 nm associated with the transition of 4f 6 5d 1 4f 7 of Eu 2+ , and the position of the emission bands can be modified by varying the Eu 2+ concentration. The quantum efficiency of Ba 4-y Eu y Si 6 O 15.85 N 0.1 is about 46% for y = 0.2 under excitation wavelength at 375 nm. The thermal quenching temperature is about 100 °C. With improved thermal stability oxynitride based Ba 4-y Eu y Si 6 O 16-3x/2 N x phosphor is promising white UV-LED applications.