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First-principles calculation of influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ
Computational Materials Science 163 (2019) 256–261
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First-principles calculation of influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ
T
⁎
Meng Zhanga,b, Ting Songb,c, , Xinyang Zhangd a
Faculty of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China School of Science, Shandong Jiaotong University, 5001 Haitang Road, Jinan 250357, China c School of Physics, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China d Xinjiang Laboratory of Phase Transitions and Microstructures in Condensed Matters, College of Physical Science and Technology, Yili Normal University, Yining 835000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen oxide Phosphors White pc-LED
In this work, influence of the nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) materials had been investigated by the first-principles calculation. For the initial Sr3MgSi2O8 without nitrogen substitution, the band gap is about 5.889 eV between the conduction band and valence band. Analysis of its density of states (DOS) indicates that the conduction band is mainly constituted by Sr-d orbit. The valence band is mainly constituted by O-p orbit. After the nitrogen substituting for oxygen occurs, the band gap gradually decreases to 4.421 eV in the case σ = 0.250. Top of the valence band is still locating near 0 eV while bottom of the conduction band drops down to 4.421 eV. On the other hand, it can be observed that the N-p orbit hybridized into the top of the valence band which induced a new band above the O-p orbit. For Sr3MgSi2O8-σNσ materials with rare earth ions or transition metal ions serving as luminescence centres, this condition would influence the charge transfer process between the host materials and the luminescence centres. In addition, the energy needed for the substitution of one nitrogen anion for one oxygen anion is calculated to 89.50 eV. This illuminates clearly that the preparation condition of nitrogen oxide compounds is harsh. This work is helpful for better understanding the influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials.
1. Introduction Nowadays, lighting equipment plays an important role in our daily life in fields such as lighting, display, medical image, high energy ray detection and so on [1–6]. For example, white phosphor converting light emitting diode (white pc-LED) gradually become dominant in daily lighting due to the advantages of high brightness, low energy consumption and long service life. As an important component, the synthesis and design of novel phosphors always attract the researchers’ attentions [7–13]. However, tradition rare earth ions or transition metal ions doped fluoride-base phosphors usually break down easily. On the other hand, phonon energy of the oxide-based phosphors is generally high which leads to low luminous efficiency. Recently, large amounts of reports on nitrogen oxides-based phosphors appeared and their excellent properties such as fine chemical stability, good thermal stability and high fluorescence conversion efficiency significantly improved performance of white emitting LED devices. Increasing quantity
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of nitrogen oxide compound-based phosphors have been developed from the traditional oxide-based phosphors [14–21]. Among the traditional phosphors, silicate-based materials doped with rare-earth ions dopants (typically Eu2+ or Ce3+) had been deeply explored and exhibited excellent performances. This is all because that they have advantages in aspects such as ease of synthesis, high fluorescence efficiency, excellent thermal quenching characteristics, chemical stability and environmentally friendly [14,16,22–25]. Among the silicate-based materials, rare earth or transition metal ions doped Sr3MgSi2O8, which could exhibit efficient emissions when excited by UV-light, have attracted much attention as the promising host materials [25–30]. Lattice of Sr3MgSi2O8 belongs to the P 21/c space group and has a monoclinic structure of which the β gets close to 90°. Although there have been many literatures on reporting the luminescence properties of rare earth ion doped Sr3MgSi2O8 phosphor, however, there has been few researches concentrating on the preparing of Sr3MgSi2O8based nitrogen oxide phosphors and influence of nitrogen anion
Corresponding author. E-mail address: [email protected] (T. Song).
https://doi.org/10.1016/j.commatsci.2019.03.043 Received 30 November 2018; Received in revised form 23 February 2019; Accepted 23 March 2019 0927-0256/ © 2019 Elsevier B.V. All rights reserved.
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M. Zhang, et al.
substituting for oxygen anion on the crystal structure and electronic band structure is still unrevealed. Therefore, a theoretical calculation is essential to give a description on the changes of crystal structure and electronic band structure when substitution of nitrogen anion for oxygen anion occurs. In this work, the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials were investigated by using the first-principles calculation. Band gap of Sr3MgSi2O8 without nitrogen substitution is about 5.889 eV. It can be observed that the band gap gets decreasing along with the increasing nitrogen substitution concentrations. For both the Sr3MgSi2O8-σNσ materials before and after nitrogen substituted for oxygen occurred, the conduction band is mainly constituted by Sr-d orbit while the valence band is mainly constituted by O-p orbit. Nevertheless, the p orbit of the substituting nitrogen would hybridize into the top of valence band. In addition, the energy needed for substitution of one nitrogen anion for one oxygen anion is about 89.50 eV. This illuminate clearly that the preparation condition of nitrogen oxide compounds is harsh. This work is helpful for better understanding the influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials.
Fig. 1. (a) Crystal structure of Sr3MgSi2O8 materials. (b)–(g) Structure of a SiO4 group, a Si-O3-N group, a Si-O2-N2 group, a Si-O2-N group, a Si-O-N2 group and a Si-O3 structure, respectively.
Sr3MgSi2O8 without nitrogen substitution, the lattice constant a = 14.14 Å, b = 5.48 Å and c = 9.69 Å with the β gets close to 90°. The obtained parameters matched well with the experimental data. For the crystal structures of Sr3MgSi2O8-σNσ, the crystal structures have been geometry optimized and the related data have been listed in Table 1. It can be observed that there is little change on the lattice constants. Nevertheless, during the nitrogen substituted for oxygen process, it is supposed that Si-O3-N, Si-O2-N2, Si-O2-N, Si-O-N2 and Si-O3 as shown in Fig. 1(c)–(g) will form and make up the new Sr3MgSi2O8-σNσ (σ = 0.125 and 0.250) structures. To investigate the influence of nitrogen substitution for oxygen on the crystal structure of Sr3MgSi2O8, the calculated lattice constants and bond lengths are listed in Table 1. It can be found that the substitution of one oxygen atom by nitrogen would result in increasing Si-ligands (O or N) bond lengths. Usually, Eu2+ and Ce3+ are the most commonly used luminescence centres with d electron configurations. Chang et al. reported that the effective ionic radii of Eu2+ ions and Ce3+ ions are similar to that of Sr2+ ions under the same coordination environments [39]. Therefore, the doped Eu3+ or Ce3+ ions tend to occupy the Sr2+ sites. Due to the decreased lattice constants and the enlarged Si-ligands (O or N) bond lengths, it can be concluded that the sizes of the adjacent SrO12, SrO7 and MgO6 would be reduced. It has been reported by Song et al. that the reduced polyhedron size would lead to increased crystal field strength around the luminescence centres [40]. Emissions of luminescence centres with d electron configurations such as Eu2+, Ce3+ or Cr3+ ions would be significantly changed by the crystal field strength [41]. Therefore, luminescence properties modulation can be realised for Sr3MgSi2O8 based phosphors with rare earth ions or transition metal ions serving as luminescence centres. Due to the charge mismatch of nitrogen anion and oxygen anion, the chemical formula of nitrogen substituted Sr3MgSi2O8 with stable structures may be written as either Sr3MgSi2O8-σNσ or Sr3MgSi2O8-2σ/ 3Nσ. To confirm the chemical formula of nitrogen substituted Sr3MgSi2O8, total energies of the aforementioned samples have been
2. Theoretical calculations ABINIT package based on the density functional theory (DFT) was employed to calculate the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials [31–33]. GGA-PEB functional was adopt to carry out the geometry optimization and HSE06 was used to obtain the band gap values in the calculation [34–37]. In order to investigate the influence of nitrogen substituting for oxygen on the electronic band structure, a 1 × 2 × 1 Sr3MgSi2O8 supercell containing 24 Sr anions, 8 Mg anions, 16 Si anions and 64 O anions was created. The Sr3MgSi2O7.875N0.125 structure is obtained by substituting one O anion by one N anion. For the calculation process of band gap values, the k-point samplings were set at the gamma point. For both the geometry optimization and the properties calculations, the tolerance of self-consistent field calculations (SCF) was 2.0e–6 eV/anion and the energy cut-offs of plane waves were set as 800 eV. These could guarantee the accuracy of the work. The energy needed for Sr3MgSi2O8 lattice to substitute one oxygen anion by one nitrogen anion can be expressed as follow [38]: p N EfN (T ) = Etol (T ) − [Etol (T ) + μN − μO ]
(1)
EfN (T )
are the energy need for one nitrogen substituting for while the N (T ) is the total energy of the structure after the one oxygen at T K. Etol substitution of one oxygen anion by one nitrogen anion occurred at T K. The μN and μO are the chemical potentials of nitrogen anion and oxygen anion, respectively. 3. Results and discussion Fig. 1a gives the crystal structure of Sr3MgSi2O8. The Sr3MgSi2O8 lattice is crystallized into the P 21/c space group and possesses a monoclinic structure of which the β gets closed to 90°. There exist three kinds crystallographic sites in this lattice. Two of them have been occupied by Sr ions and one of the Sr is twelve-fold coordinated by O anions forming SrO12 polyhedrons while the other is seven-fold coordinated by O anions forming SrO7 polyhedrons. Meanwhile, for the Mg ions, only one distinct crystallographic site is available and the Mg ion is six-fold coordinated by O anions forming MgO6 octahedron. The Si ion is four-fold coordinated by oxygen anions and forming SiO4 tetrahedron. In addition, the SiO4 tetrahedron are connecting to the aforementioned SrO12, SrO7 and MgO6 polyhedrons by the O anions. Therefore, the changed SiO4 structures would lead to the altered SrO12, SrO7 and MgO6 polyhedrons nearby. The lattice parameters are obtained after performing geometric optimization. For the lattice of
Table 1 The calculated lattice constants of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) by geometry optimization. σ
0 0.125 0.250
257
Lattice Constants (Å)
Bond length (Å)
a
b
c
Si-O1/N1
Si-O2
Si-O3
Si-O4
14.14 13.87 13.77
5.48 5.41 5.38
9.69 9.45 9.41
1.66 1.65 1.68 1.65
1.65 1.69 1.68 1.69
1.64 1.65 1.66 1.68
1.63 1.64 1.65 1.65
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4 represents the Sr3MgSi2O8-2σ/3Nσ structure possessing one Si-O-N2 group as shown in Fig. 1(f). 5 represents the Sr3MgSi2O8-2σ/3Nσ structure possessing one Si-O2-N2 group and one Si-O3 group as shown in Fig. 1(d) and (g), respectively. 6 represents the Sr3MgSi2O8-2σ/3Nσ structure possessing two Si-O3-N groups and one Si-O3 group as shown in Fig. 1(c) and (g), respectively. It can be observed that the total energy of 1 is the lowest when σ = 0.250. This indicates Sr3MgSi2O8-σNσ structure possessing two Si-O3-N groups and bearing two negative charges would stay stable when two oxygen anions have been substituted by two nitrogen anions. In addition, when nitrogen substituted for oxygen occurred, the chemical formula is confirmed to be written as Sr3MgSi2O8-σNσ by comparing different crystal structures when nitrogen substituting for oxygen occurred. Meanwhile, the appearance of two Si-O3-N groups instead of one Si-O2-N group when σ = 0.250 suggests that the nitrogen anions tend to be spread over the Sr3MgSi2O8-σNσ lattice rather than flock together. Fig. 2(c) shows the total energy of the Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) with stable structures. Linear fitting of the obtained data indicates that the oxygen anions can be totally substituted by the nitrogen anions. However, according to the calculated results by equation (1), the energy needed for one nitrogen anion substituted for one oxygen anion (σ = 0.125) is calculated to be 89.50 eV. This indicates that the preparation process of Sr3MgSi2O8-σNσ nitride oxide structure is really harsh. Electronic band structures of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) have been investigated by employing a 1 × 2 × 1 supercell and shown in Fig. 3. For all the aforementioned samples, the bottoms of the conduction band occurred at the Γ point and the top branches of the valence band are flat. It can be observed that band gap of the initial Sr3MgSi2O8 crystal is about 5.889 eV as shown in Fig. 3(a). When nitrogen anions substituting for oxygen anions occur, the band gap of Sr3MgSi2O7.875N0.125 decreases to 4.629 eV, meanwhile the band gap of Sr3MgSi2O7.750N0.250 decreases to 4.421 eV. It can be found that the nitrogen substituting for oxygen in Sr3MgSi2O8 would lead to the gradually decreased band gap. To further investigate the electronic band structures of Sr3MgSi2O8σNσ crystals, their total and partial densities of states are presented in Fig. 4. For the initial Sr3MgSi2O8 crystal, it can be observed in Fig. 4a that the conduction band is mainly constituted of d orbit of Sr. In addition, it shows a hybridizing character with s and p orbits of Mg and s and p orbits of Si. The valence bands could be mainly attributed to s orbit of O hybridizing with s/p orbits of Mg/Si, respectively. For the Sr3MgSi2O8-σNσ (σ = 0.125 and 0.250) lattice with the substitution of nitrogen anion for oxygen anion, the total and partial densities of states are given Fig. 3b and c, respectively. There is a little difference that a new isolate band near 0 eV appeared. The newly-presented band can be mainly attributed to the p orbit of nitrogen. This could be corresponding to the condition in Fig. 3b and c that new bands appeared. In addition, it can be observed that the bottom of the conduction band gets closer to the valence band compared to that of Sr3MgSi2O8 and its band gap decreased. Usually, for oxide-based phosphors, the charge transition process often occurred between oxygen anion and luminescence centers. Generally, for the oxide-based phosphors, the change transfer band located in the deep UV-region below 300 nm. With the presence of oxynitride-based phosphors, the charge transition state extended to the n-UV region, which vastly optimized the matching degree between phosphors and n-UV LED chips. The changing charge transfer state can be attributed to the incorporation of N p orbit into the top of the valence band. To serving as the host materials for lighting applications, strong absorption in the UV region is essential for effective energy transfer process from the host to the luminescence centers. To investigate the influence of nitrogen substituting for oxygen on the optical properties of Sr3MgSi2O8, the calculated absorption spectra have been obtained and shown in Fig. 5. For the initial Sr3MgSi2O8 material, it can be observed that it possesses a strong absorption in the UV region below
Fig. 2. (a) The calculated total energy of Sr3MgSi2O8-σNσ structure when σ = 0.125. (b) The calculated total energy of Sr3MgSi2O8-σNσ structure when σ = 0.250. (c) The calculated total energy of the Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) materials.
calculated. For the case that one oxygen anion was substituted by one nitrogen anion, due to the charge imbalance between one nitrogen anion and one oxygen atom, the Sr3MgSi2O8-σNσ or Sr3MgSi2O8-2σ/3Nσ supercells might either bear a negative charge or lose one oxygen anion and bear a positive charge. The calculated total energies of the aforementioned structures have been shown in Fig. 2(a). 1 represents the Sr3MgSi2O8-σNσ structure possessing one Si-O3-N group as shown in Fig. 1(c) and bearing a negative charge. 2 and 3 represent the Sr3MgSi2O8-2σ/3Nσ structure losing one oxygen anion and bearing a positive charge. 2 possesses one Si-O2-N group as shown in Fig. 1(e) meanwhile 3 possesses one Si-O3-N group and one Si-O3 group as shown in Fig. 1(c) and (g). It can be found that the total energy of 1 is lower than 2 or 3, which indicates that the Sr3MgSi2O8-σNσ structure possessing one Si-O3-N group and bearing a negative charge would stay stable when one oxygen anion has been substituted by one nitrogen anion. For the case that two oxygen anions were substituted by two nitrogen anions, due to the charge imbalance between nitrogen anions and oxygen atoms, the Sr3MgSi2O8-σNσ or Sr3MgSi2O8-2σ/3Nσ supercells might either bear two negative charges or lose one oxygen anion. The calculated total energies of the aforementioned structures have been shown in Fig. 2(b). 1 represents the Sr3MgSi2O8-σNσ structure possessing two Si-O3-N groups as shown in Fig. 1(c) and bearing two negative charges. 2 represents the Sr3MgSi2O8-σNσ structure possessing one SiO2-N2 group as shown in Fig. 1(d) and bearing two negative charges. 3 represents the Sr3MgSi2O8-2σ/3Nσ structure possessing one Si-O3-N group and one Si-O2-N group as shown in Fig. 1(c) and (e), respectively. 258
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Fig. 3. Calculated band structures of (a) Sr3MgSi2O8, (b) Sr3MgSi2O7.875N0.125 and (c) Sr3MgSi2O7.750N0.250.
4. Conclusion
270 nm. However, this degree of coverage in the UV region can't live up to the needs of UV-pumped phosphors for the emissions of now available chips are in the excess of 350 nm. Fortunately, it can be observed from Fig. 5 that the nitrogen substituting for oxygen in Sr3MgSi2O8 could extend the absorption band from deep UV region to near UV region. For the Sr3MgSi2O8-σNσ (σ = 0.125 and 0.250), there appear two strong absorption peaks in the 230–375 nm region. In addition, it can be found that the coverage of absorption gradually tends to expand to the visible region as the increasing N concentrations. This indicates that the absorption of Sr3MgSi2O8 can be expanded by substitution of nitrogen for oxygen and Sr3MgSi2O8-σNσ is suited for serving as host material of UV-pumped phosphors.
The electronic band structures of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) with various nitrogen substituting concentrations were investigated by using the first-principles calculation. Their band structures, total densities of states (TDOS) and partial densities of states (PDOS) were obtained. For the initial Sr3MgSi2O8 material, the band gap is about 5.889 eV. For the Sr3MgSi2O8-σNσ lattice, the band gap gradually decreased to 4.421 eV due to the substitution of nitrogen anion for oxygen anion in Sr3MgSi2O8 lattice. For the Sr3MgSi2O8-σNσ (σ = 0.125 and 0.250) with nitrogen substitution for oxygen, it can be observed that p orbit of nitrogen hybridized into top of the valence band, which would change the charge transfer process between the oxygen and the luminescence centers. The energy needed for substitution of one nitrogen anion for one oxygen anion is about 89.50 eV. In
Fig. 4. Calculated total and partial densities of state for (a) Sr3MgSi2O8 (b) Sr3MgSi2O7.875N0.125 and (c) Sr3MgSi2O7.750N0.250. 259
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[9]
[10]
[11] [12]
[13] [14]
[15]
[16]
[17]
[18]
[19]
Fig. 5. The calculated absorption spectra of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250). [20]
addition, the calculated absorption spectra showed that the absorption of Sr3MgSi2O8 can be expanded from the deep UV region to the near UV region, which makes it suitable for serving as host material of UVpumped phosphors. This investigation is helpful for better understanding the crystal structure and electronic band structure changes caused by the substitution of nitrogen anion for oxygen anion in Sr3MgSi2O8-Sr3MgSi2O8-σNσ systems.
[21]
[22]
[23]
CRediT authorship contribution statement
[24]
Meng Zhang: Conceptualization, Investigation, Software, Data curation, Writing - original draft, Writing - review & editing. Ting Song: Conceptualization, Software, Resources, Writing - original draft, Writing - review & editing. Xinyang Zhang: Software.
[25] [26]
Acknowledgements
[27]
The authors would like to thank Prof. Changshan Xu (School of Physics, Northeast Normal University) for providing guides and helps on finishing this work. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
[28]
[29]
[30]
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First-principles calculation of influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ
T
⁎
Meng Zhanga,b, Ting Songb,c, , Xinyang Zhangd a
Faculty of Chemistry, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China School of Science, Shandong Jiaotong University, 5001 Haitang Road, Jinan 250357, China c School of Physics, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China d Xinjiang Laboratory of Phase Transitions and Microstructures in Condensed Matters, College of Physical Science and Technology, Yili Normal University, Yining 835000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nitrogen oxide Phosphors White pc-LED
In this work, influence of the nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) materials had been investigated by the first-principles calculation. For the initial Sr3MgSi2O8 without nitrogen substitution, the band gap is about 5.889 eV between the conduction band and valence band. Analysis of its density of states (DOS) indicates that the conduction band is mainly constituted by Sr-d orbit. The valence band is mainly constituted by O-p orbit. After the nitrogen substituting for oxygen occurs, the band gap gradually decreases to 4.421 eV in the case σ = 0.250. Top of the valence band is still locating near 0 eV while bottom of the conduction band drops down to 4.421 eV. On the other hand, it can be observed that the N-p orbit hybridized into the top of the valence band which induced a new band above the O-p orbit. For Sr3MgSi2O8-σNσ materials with rare earth ions or transition metal ions serving as luminescence centres, this condition would influence the charge transfer process between the host materials and the luminescence centres. In addition, the energy needed for the substitution of one nitrogen anion for one oxygen anion is calculated to 89.50 eV. This illuminates clearly that the preparation condition of nitrogen oxide compounds is harsh. This work is helpful for better understanding the influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials.
1. Introduction Nowadays, lighting equipment plays an important role in our daily life in fields such as lighting, display, medical image, high energy ray detection and so on [1–6]. For example, white phosphor converting light emitting diode (white pc-LED) gradually become dominant in daily lighting due to the advantages of high brightness, low energy consumption and long service life. As an important component, the synthesis and design of novel phosphors always attract the researchers’ attentions [7–13]. However, tradition rare earth ions or transition metal ions doped fluoride-base phosphors usually break down easily. On the other hand, phonon energy of the oxide-based phosphors is generally high which leads to low luminous efficiency. Recently, large amounts of reports on nitrogen oxides-based phosphors appeared and their excellent properties such as fine chemical stability, good thermal stability and high fluorescence conversion efficiency significantly improved performance of white emitting LED devices. Increasing quantity
⁎
of nitrogen oxide compound-based phosphors have been developed from the traditional oxide-based phosphors [14–21]. Among the traditional phosphors, silicate-based materials doped with rare-earth ions dopants (typically Eu2+ or Ce3+) had been deeply explored and exhibited excellent performances. This is all because that they have advantages in aspects such as ease of synthesis, high fluorescence efficiency, excellent thermal quenching characteristics, chemical stability and environmentally friendly [14,16,22–25]. Among the silicate-based materials, rare earth or transition metal ions doped Sr3MgSi2O8, which could exhibit efficient emissions when excited by UV-light, have attracted much attention as the promising host materials [25–30]. Lattice of Sr3MgSi2O8 belongs to the P 21/c space group and has a monoclinic structure of which the β gets close to 90°. Although there have been many literatures on reporting the luminescence properties of rare earth ion doped Sr3MgSi2O8 phosphor, however, there has been few researches concentrating on the preparing of Sr3MgSi2O8based nitrogen oxide phosphors and influence of nitrogen anion
Corresponding author. E-mail address: [email protected] (T. Song).
https://doi.org/10.1016/j.commatsci.2019.03.043 Received 30 November 2018; Received in revised form 23 February 2019; Accepted 23 March 2019 0927-0256/ © 2019 Elsevier B.V. All rights reserved.
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substituting for oxygen anion on the crystal structure and electronic band structure is still unrevealed. Therefore, a theoretical calculation is essential to give a description on the changes of crystal structure and electronic band structure when substitution of nitrogen anion for oxygen anion occurs. In this work, the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials were investigated by using the first-principles calculation. Band gap of Sr3MgSi2O8 without nitrogen substitution is about 5.889 eV. It can be observed that the band gap gets decreasing along with the increasing nitrogen substitution concentrations. For both the Sr3MgSi2O8-σNσ materials before and after nitrogen substituted for oxygen occurred, the conduction band is mainly constituted by Sr-d orbit while the valence band is mainly constituted by O-p orbit. Nevertheless, the p orbit of the substituting nitrogen would hybridize into the top of valence band. In addition, the energy needed for substitution of one nitrogen anion for one oxygen anion is about 89.50 eV. This illuminate clearly that the preparation condition of nitrogen oxide compounds is harsh. This work is helpful for better understanding the influence of nitrogen substituting for oxygen on the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials.
Fig. 1. (a) Crystal structure of Sr3MgSi2O8 materials. (b)–(g) Structure of a SiO4 group, a Si-O3-N group, a Si-O2-N2 group, a Si-O2-N group, a Si-O-N2 group and a Si-O3 structure, respectively.
Sr3MgSi2O8 without nitrogen substitution, the lattice constant a = 14.14 Å, b = 5.48 Å and c = 9.69 Å with the β gets close to 90°. The obtained parameters matched well with the experimental data. For the crystal structures of Sr3MgSi2O8-σNσ, the crystal structures have been geometry optimized and the related data have been listed in Table 1. It can be observed that there is little change on the lattice constants. Nevertheless, during the nitrogen substituted for oxygen process, it is supposed that Si-O3-N, Si-O2-N2, Si-O2-N, Si-O-N2 and Si-O3 as shown in Fig. 1(c)–(g) will form and make up the new Sr3MgSi2O8-σNσ (σ = 0.125 and 0.250) structures. To investigate the influence of nitrogen substitution for oxygen on the crystal structure of Sr3MgSi2O8, the calculated lattice constants and bond lengths are listed in Table 1. It can be found that the substitution of one oxygen atom by nitrogen would result in increasing Si-ligands (O or N) bond lengths. Usually, Eu2+ and Ce3+ are the most commonly used luminescence centres with d electron configurations. Chang et al. reported that the effective ionic radii of Eu2+ ions and Ce3+ ions are similar to that of Sr2+ ions under the same coordination environments [39]. Therefore, the doped Eu3+ or Ce3+ ions tend to occupy the Sr2+ sites. Due to the decreased lattice constants and the enlarged Si-ligands (O or N) bond lengths, it can be concluded that the sizes of the adjacent SrO12, SrO7 and MgO6 would be reduced. It has been reported by Song et al. that the reduced polyhedron size would lead to increased crystal field strength around the luminescence centres [40]. Emissions of luminescence centres with d electron configurations such as Eu2+, Ce3+ or Cr3+ ions would be significantly changed by the crystal field strength [41]. Therefore, luminescence properties modulation can be realised for Sr3MgSi2O8 based phosphors with rare earth ions or transition metal ions serving as luminescence centres. Due to the charge mismatch of nitrogen anion and oxygen anion, the chemical formula of nitrogen substituted Sr3MgSi2O8 with stable structures may be written as either Sr3MgSi2O8-σNσ or Sr3MgSi2O8-2σ/ 3Nσ. To confirm the chemical formula of nitrogen substituted Sr3MgSi2O8, total energies of the aforementioned samples have been
2. Theoretical calculations ABINIT package based on the density functional theory (DFT) was employed to calculate the crystal structures and electronic band structures of Sr3MgSi2O8-σNσ materials [31–33]. GGA-PEB functional was adopt to carry out the geometry optimization and HSE06 was used to obtain the band gap values in the calculation [34–37]. In order to investigate the influence of nitrogen substituting for oxygen on the electronic band structure, a 1 × 2 × 1 Sr3MgSi2O8 supercell containing 24 Sr anions, 8 Mg anions, 16 Si anions and 64 O anions was created. The Sr3MgSi2O7.875N0.125 structure is obtained by substituting one O anion by one N anion. For the calculation process of band gap values, the k-point samplings were set at the gamma point. For both the geometry optimization and the properties calculations, the tolerance of self-consistent field calculations (SCF) was 2.0e–6 eV/anion and the energy cut-offs of plane waves were set as 800 eV. These could guarantee the accuracy of the work. The energy needed for Sr3MgSi2O8 lattice to substitute one oxygen anion by one nitrogen anion can be expressed as follow [38]: p N EfN (T ) = Etol (T ) − [Etol (T ) + μN − μO ]
(1)
EfN (T )
are the energy need for one nitrogen substituting for while the N (T ) is the total energy of the structure after the one oxygen at T K. Etol substitution of one oxygen anion by one nitrogen anion occurred at T K. The μN and μO are the chemical potentials of nitrogen anion and oxygen anion, respectively. 3. Results and discussion Fig. 1a gives the crystal structure of Sr3MgSi2O8. The Sr3MgSi2O8 lattice is crystallized into the P 21/c space group and possesses a monoclinic structure of which the β gets closed to 90°. There exist three kinds crystallographic sites in this lattice. Two of them have been occupied by Sr ions and one of the Sr is twelve-fold coordinated by O anions forming SrO12 polyhedrons while the other is seven-fold coordinated by O anions forming SrO7 polyhedrons. Meanwhile, for the Mg ions, only one distinct crystallographic site is available and the Mg ion is six-fold coordinated by O anions forming MgO6 octahedron. The Si ion is four-fold coordinated by oxygen anions and forming SiO4 tetrahedron. In addition, the SiO4 tetrahedron are connecting to the aforementioned SrO12, SrO7 and MgO6 polyhedrons by the O anions. Therefore, the changed SiO4 structures would lead to the altered SrO12, SrO7 and MgO6 polyhedrons nearby. The lattice parameters are obtained after performing geometric optimization. For the lattice of
Table 1 The calculated lattice constants of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) by geometry optimization. σ
0 0.125 0.250
257
Lattice Constants (Å)
Bond length (Å)
a
b
c
Si-O1/N1
Si-O2
Si-O3
Si-O4
14.14 13.87 13.77
5.48 5.41 5.38
9.69 9.45 9.41
1.66 1.65 1.68 1.65
1.65 1.69 1.68 1.69
1.64 1.65 1.66 1.68
1.63 1.64 1.65 1.65
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4 represents the Sr3MgSi2O8-2σ/3Nσ structure possessing one Si-O-N2 group as shown in Fig. 1(f). 5 represents the Sr3MgSi2O8-2σ/3Nσ structure possessing one Si-O2-N2 group and one Si-O3 group as shown in Fig. 1(d) and (g), respectively. 6 represents the Sr3MgSi2O8-2σ/3Nσ structure possessing two Si-O3-N groups and one Si-O3 group as shown in Fig. 1(c) and (g), respectively. It can be observed that the total energy of 1 is the lowest when σ = 0.250. This indicates Sr3MgSi2O8-σNσ structure possessing two Si-O3-N groups and bearing two negative charges would stay stable when two oxygen anions have been substituted by two nitrogen anions. In addition, when nitrogen substituted for oxygen occurred, the chemical formula is confirmed to be written as Sr3MgSi2O8-σNσ by comparing different crystal structures when nitrogen substituting for oxygen occurred. Meanwhile, the appearance of two Si-O3-N groups instead of one Si-O2-N group when σ = 0.250 suggests that the nitrogen anions tend to be spread over the Sr3MgSi2O8-σNσ lattice rather than flock together. Fig. 2(c) shows the total energy of the Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) with stable structures. Linear fitting of the obtained data indicates that the oxygen anions can be totally substituted by the nitrogen anions. However, according to the calculated results by equation (1), the energy needed for one nitrogen anion substituted for one oxygen anion (σ = 0.125) is calculated to be 89.50 eV. This indicates that the preparation process of Sr3MgSi2O8-σNσ nitride oxide structure is really harsh. Electronic band structures of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) have been investigated by employing a 1 × 2 × 1 supercell and shown in Fig. 3. For all the aforementioned samples, the bottoms of the conduction band occurred at the Γ point and the top branches of the valence band are flat. It can be observed that band gap of the initial Sr3MgSi2O8 crystal is about 5.889 eV as shown in Fig. 3(a). When nitrogen anions substituting for oxygen anions occur, the band gap of Sr3MgSi2O7.875N0.125 decreases to 4.629 eV, meanwhile the band gap of Sr3MgSi2O7.750N0.250 decreases to 4.421 eV. It can be found that the nitrogen substituting for oxygen in Sr3MgSi2O8 would lead to the gradually decreased band gap. To further investigate the electronic band structures of Sr3MgSi2O8σNσ crystals, their total and partial densities of states are presented in Fig. 4. For the initial Sr3MgSi2O8 crystal, it can be observed in Fig. 4a that the conduction band is mainly constituted of d orbit of Sr. In addition, it shows a hybridizing character with s and p orbits of Mg and s and p orbits of Si. The valence bands could be mainly attributed to s orbit of O hybridizing with s/p orbits of Mg/Si, respectively. For the Sr3MgSi2O8-σNσ (σ = 0.125 and 0.250) lattice with the substitution of nitrogen anion for oxygen anion, the total and partial densities of states are given Fig. 3b and c, respectively. There is a little difference that a new isolate band near 0 eV appeared. The newly-presented band can be mainly attributed to the p orbit of nitrogen. This could be corresponding to the condition in Fig. 3b and c that new bands appeared. In addition, it can be observed that the bottom of the conduction band gets closer to the valence band compared to that of Sr3MgSi2O8 and its band gap decreased. Usually, for oxide-based phosphors, the charge transition process often occurred between oxygen anion and luminescence centers. Generally, for the oxide-based phosphors, the change transfer band located in the deep UV-region below 300 nm. With the presence of oxynitride-based phosphors, the charge transition state extended to the n-UV region, which vastly optimized the matching degree between phosphors and n-UV LED chips. The changing charge transfer state can be attributed to the incorporation of N p orbit into the top of the valence band. To serving as the host materials for lighting applications, strong absorption in the UV region is essential for effective energy transfer process from the host to the luminescence centers. To investigate the influence of nitrogen substituting for oxygen on the optical properties of Sr3MgSi2O8, the calculated absorption spectra have been obtained and shown in Fig. 5. For the initial Sr3MgSi2O8 material, it can be observed that it possesses a strong absorption in the UV region below
Fig. 2. (a) The calculated total energy of Sr3MgSi2O8-σNσ structure when σ = 0.125. (b) The calculated total energy of Sr3MgSi2O8-σNσ structure when σ = 0.250. (c) The calculated total energy of the Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) materials.
calculated. For the case that one oxygen anion was substituted by one nitrogen anion, due to the charge imbalance between one nitrogen anion and one oxygen atom, the Sr3MgSi2O8-σNσ or Sr3MgSi2O8-2σ/3Nσ supercells might either bear a negative charge or lose one oxygen anion and bear a positive charge. The calculated total energies of the aforementioned structures have been shown in Fig. 2(a). 1 represents the Sr3MgSi2O8-σNσ structure possessing one Si-O3-N group as shown in Fig. 1(c) and bearing a negative charge. 2 and 3 represent the Sr3MgSi2O8-2σ/3Nσ structure losing one oxygen anion and bearing a positive charge. 2 possesses one Si-O2-N group as shown in Fig. 1(e) meanwhile 3 possesses one Si-O3-N group and one Si-O3 group as shown in Fig. 1(c) and (g). It can be found that the total energy of 1 is lower than 2 or 3, which indicates that the Sr3MgSi2O8-σNσ structure possessing one Si-O3-N group and bearing a negative charge would stay stable when one oxygen anion has been substituted by one nitrogen anion. For the case that two oxygen anions were substituted by two nitrogen anions, due to the charge imbalance between nitrogen anions and oxygen atoms, the Sr3MgSi2O8-σNσ or Sr3MgSi2O8-2σ/3Nσ supercells might either bear two negative charges or lose one oxygen anion. The calculated total energies of the aforementioned structures have been shown in Fig. 2(b). 1 represents the Sr3MgSi2O8-σNσ structure possessing two Si-O3-N groups as shown in Fig. 1(c) and bearing two negative charges. 2 represents the Sr3MgSi2O8-σNσ structure possessing one SiO2-N2 group as shown in Fig. 1(d) and bearing two negative charges. 3 represents the Sr3MgSi2O8-2σ/3Nσ structure possessing one Si-O3-N group and one Si-O2-N group as shown in Fig. 1(c) and (e), respectively. 258
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Fig. 3. Calculated band structures of (a) Sr3MgSi2O8, (b) Sr3MgSi2O7.875N0.125 and (c) Sr3MgSi2O7.750N0.250.
4. Conclusion
270 nm. However, this degree of coverage in the UV region can't live up to the needs of UV-pumped phosphors for the emissions of now available chips are in the excess of 350 nm. Fortunately, it can be observed from Fig. 5 that the nitrogen substituting for oxygen in Sr3MgSi2O8 could extend the absorption band from deep UV region to near UV region. For the Sr3MgSi2O8-σNσ (σ = 0.125 and 0.250), there appear two strong absorption peaks in the 230–375 nm region. In addition, it can be found that the coverage of absorption gradually tends to expand to the visible region as the increasing N concentrations. This indicates that the absorption of Sr3MgSi2O8 can be expanded by substitution of nitrogen for oxygen and Sr3MgSi2O8-σNσ is suited for serving as host material of UV-pumped phosphors.
The electronic band structures of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250) with various nitrogen substituting concentrations were investigated by using the first-principles calculation. Their band structures, total densities of states (TDOS) and partial densities of states (PDOS) were obtained. For the initial Sr3MgSi2O8 material, the band gap is about 5.889 eV. For the Sr3MgSi2O8-σNσ lattice, the band gap gradually decreased to 4.421 eV due to the substitution of nitrogen anion for oxygen anion in Sr3MgSi2O8 lattice. For the Sr3MgSi2O8-σNσ (σ = 0.125 and 0.250) with nitrogen substitution for oxygen, it can be observed that p orbit of nitrogen hybridized into top of the valence band, which would change the charge transfer process between the oxygen and the luminescence centers. The energy needed for substitution of one nitrogen anion for one oxygen anion is about 89.50 eV. In
Fig. 4. Calculated total and partial densities of state for (a) Sr3MgSi2O8 (b) Sr3MgSi2O7.875N0.125 and (c) Sr3MgSi2O7.750N0.250. 259
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[9]
[10]
[11] [12]
[13] [14]
[15]
[16]
[17]
[18]
[19]
Fig. 5. The calculated absorption spectra of Sr3MgSi2O8-σNσ (σ = 0, 0.125 and 0.250). [20]
addition, the calculated absorption spectra showed that the absorption of Sr3MgSi2O8 can be expanded from the deep UV region to the near UV region, which makes it suitable for serving as host material of UVpumped phosphors. This investigation is helpful for better understanding the crystal structure and electronic band structure changes caused by the substitution of nitrogen anion for oxygen anion in Sr3MgSi2O8-Sr3MgSi2O8-σNσ systems.
[21]
[22]
[23]
CRediT authorship contribution statement
[24]
Meng Zhang: Conceptualization, Investigation, Software, Data curation, Writing - original draft, Writing - review & editing. Ting Song: Conceptualization, Software, Resources, Writing - original draft, Writing - review & editing. Xinyang Zhang: Software.
[25] [26]
Acknowledgements
[27]
The authors would like to thank Prof. Changshan Xu (School of Physics, Northeast Normal University) for providing guides and helps on finishing this work. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
[28]
[29]
[30]
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