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Experimental investigation on anisotropic characteristic of YBa3B9O18
Micron 89 (2016) 16–20
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Micron journal homepage: www.elsevier.com/locate/micron
Experimental investigation on anisotropic characteristic of YBa3 B9 O18 Ming He a,∗ , T.Z. Liu b , J. Cai c , Z.H. Zhang b , X.E. Gu a a
Department of Physics, Dalian Jiaotong University, Dalian, China Liaoning Key Materials Laboratory for Railway, Dalian Jiaotong University, Dalian, China c School of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, China b
a r t i c l e
i n f o
Article history: Received 28 March 2016 Received in revised form 12 July 2016 Accepted 12 July 2016 Available online 14 July 2016 Keywords: Electron energy-loss spectra Optical crystal Anisotropy Borate Density of states
a b s t r a c t The energy loss near edge fine structures of the B-K edge and O-K edge have been examined in the optical material YBa3 B9 O18 . The orientation-dependent electron energy-loss spectra (EELS) for both B-K edge and O-K edge were observed. The experimental results were analyzed based on density functional theory calculations. The unoccupied pz features dominated the EELS when it was collected under the energy transfer q along c orientations and pxy states were probed when q was perpendicular to the crystallographic c axis. The results provide direct experimental evidence on the anisotropic characteristic of YBa3 B9 O18 . © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Rare earth and alkaline earth borates have been extensively studied in the past few years (He et al., 2013; Wang et al., 2013; Ingle et al., 2014). The discoveries of a series of isostructural borate compounds, REBa3 B9 O18 (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) have aroused interest in this family of materials (Li et al., 2004). Among them, YBa3 B9 O18 has further demonstrated excellent scintillation properties as revealed by the X-ray excited luminescence (XEL) measurement (He et al., 2007). The relatively low fabrication cost together with the large birefringence, excellent scintillation property and chemical stability make YBa3 B9 O18 an attractive optical crystal. YBa3 B9 O18 adopts a space group P63 /m and each B atom is bonded to three O atoms to form BO3 groups, the three of which further constitute a planar hexagonal B3 O6 ring. These planar rings are stacked along the c axis in the unit cell, with regular YO6 octahedra and irregular BaO6 and BaO9 polyhedra existing in-between the hexagonal B3 O6 rings. The crystal structure of YBa3 B9 O18 was shown in Fig. 1a and the atomic positions were shown in Table 1. There are two types of B and four types of O in the compound. The strong anisotropy is the main structure characteristic, which makes the birefringence of this crystal to be about 0.12, leading it to be especially promising in a number of optical applications such as liquid crystal displays, light modulators, color
∗ Corresponding author. E-mail address: [email protected] (M. He). http://dx.doi.org/10.1016/j.micron.2016.07.003 0968-4328/© 2016 Elsevier Ltd. All rights reserved.
filters, wave plates, and optical axis gratings, etc. The electronic structure of YBa3 B9 O18 has been investigated by both experimental and theoretical ab initial band structure calculations (Zhang et al., 2008; Duan et al., 2007). The strong anisotropic characteristic of the band structure was observed by theoretical calculations (Zhang et al., 2008). The experimental measurements on anisotropic characteristic of YBa3 B9 O18 were needed to understand its electronic structure. Electron-energy-loss-spectroscopy (EELS) combined with a transmission electron microscope (TEM) is a powerful technique to probe the electronic structure at the near atomic level. The energy loss near edge fine structures (ELNES) brings useful information about the chemical bond, electronic structure, the coordination and spin-state of the atoms, etc. (Egerton, 1986; Garvie et al., 1995). In particular, the orientation-dependent ELNES performed on anisotropic materials give additional information on the spatial orientation of electronic states (Browning et al., 1991; Radtke, 2008). The momentum transfer q dependent ELNES has been used to study the orientation dependence of anisotropic crystals (Jiang et al., 2002; Zhu et al., 2002; Ma et al., 2008). In which q is defined as q = k − k0 , where k0 and k are initial and final wave vectors of the high-energy electron beam in TEM, respectively. In this work, we investigated the anisotropy property of electronic structure for YBa3 B9 O18 using q-dependent EELS in a TEM. The unoccupied electronic structures were measured by ELNES of B-K edge and O-K edges. The ELNES involved core-hole effect were calculated based on density functional theory, which gave the theoretical analyze on the experimental results. The results provide
M. He et al. / Micron 89 (2016) 16–20
17
Fig 1. (a) The crystal structure of the YBa3 B9 O18 materials; (b) the electron diffraction pattern under the [001] zone axis, under which the EELS was collected.
Fig. 2. The typical full EELS spectra taken from the specimen including Y, B, O and Ba elements, respectively.
clear experimental evidence for anisotropic characteristic of the electronic structure of YBa3 B9 O18. 2. Experiment and theoretical calculations YBa3 B9 O18 single crystals grown by the Czochralski method were used in our studies. The TEM specimen was prepared in a standard way, first by cutting, gluing and mechanically polishing, and then the Ar-ion milling instrument was used until perforation occurred. EELS experiments were carried out using a Gatan Imaging Filtering (GIF) system attached to the microscope (TEM, JEM-2100F, FEG) with a energy resolution about 1 eV. The energy loss spectra were calibrated by the zero loss peak, followed by a Fourier-ratio deconvolution to eliminate contribution from plural scattering. The EELS measurements were carried out in a conventional TEM diffraction mode with an approximately parallel beam of incident electrons. The crystals were tilted slightly off the zone axes by 1−2◦ to avoid the electron channeling effects close to the Bragg condition. q-dependent EELS were collected under the [001] orientation of the sample and the electron diffraction pattern was shown as Fig. 1b. When the momentum transfer q was along the c axis (qc), the camera length was relatively large (the collection angle was ∼0.3 mrad), the TEM diffraction shift controls were
adjusted to put the transmission spot (that is 000) on the EELS aperture, corresponding to the case of the minimum q. When q was perpendicular to c (q⊥c), the camera length was decreased and thus the collection angle was increased to ∼5.6 mrad. In this study, the smallest collection aperture was used. The calculations were carried out using first-principle method based on density-functional theory (DFT) by CASTEP in the MATERIAL STUDIO program. The Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof scheme (PBE) was adopted for the exchange-correlation potential (Perdew et al., 1996). “Ultrasoft pseudopotentials” were used when calculate the electronic structure, while the “pseudopotentials on the fly” were chosen for the calculations of the core-level spectra. That is, the parameters were provided which govern the generation rather than a file from the database. This approach has a number of advantages; e.g., the same exchange-correlation functional is used in the atomic and solid state calculations; it is possible to generate “softer” or “harder” potentials by changing the core radius; it is possible to study excited configurations with a core hole, etc. The wave functions were expanded in plane waves up to a cutoff energy of 400 eV, and a 2 × 2 × 2 Monkhorst-Pack mesh as k-point sampling was used to sample the irreducible Brillouin zone, ensuring an energy precision of better than 1 meV for the total energy. The test calculations with higher cutoff energies and denser k-point grids were also performed, the overall results remained unchanged. The optimized lattice parameters (a = 7.1761 Å, c = 16.9562 Å) of YBa3 B9 O18 was used in all calculations from the experimental results (a = 7.1841 Å and c = 16.9283 Å). The structure was optimized until the net force on each atom was smaller than 0.01 eV/Å. The simulated nonpolarized ELNES of B-K and O-K edges and the polarized ELNES under [001] and [100] orientations were calculated to understand the experimental results. Note that ELNES includes influences in the core excitation processes beyond the ground state electronic structure, thus core-hole effects were involved in our calculations to give clear analysis of the experimental results. 3. Results and discussions We focused on the study of B and O elements in this work. The boron K-edge or oxygen K-edge ELNES involves the dipole selected electronic transitions from 1 s state of boron atom or oxygen, thus providing a direct detection of the unoccupied p-like DOS of boron or oxygen in YBa3 B9 O18 . So we can probe the unoccupied B-2p and O-2p states by measuring B-K and O-K absorption edges, respectively. Fig. 2 shows the typical full EELS spectra including Y-M4,5 ,
18
M. He et al. / Micron 89 (2016) 16–20
Fig. 3. The experimental B-K edge spectra recorded at two different orientations (a) q⊥c and (b) qc; the experimental O-K edge spectra recorded at two different orientations (c) q⊥c and (d) qc.
Fig. 4. (a) The simulated non-polarized B-K edge; (b) the simulated B-K edge under [001] and [100] polarization directions; (c) the simulated non-polarized O-K edge; (d) the simulated O-K edge under [001] and [100] polarization directions.
B-K, O-K and Ba-M4,5 absorption edges, indicating the specimen we investigated include Y, B, O and Ba elements. The experimental B-K edge spectra recorded at two different orientations (q⊥c and qc) were shown in Fig. 3a and b and the experimental EELS spectra of OK edge collected at q⊥c and qc orientations were shown in Fig. 3c and d, respectively. In general, it can be clearly seen that there is a sharp peak located at 192 eV (marked as a), and a broad peak at 199 eV (marked as b) for B-K edge. While for O-K edge, the peaks are located at 535 eV and 542 eV (marked as peak a and b), respectively. The background was removed according to the power law. For comparison, both spectra were scaled to the post-edge intensity. The differences in the fine structures at different orientations are significant. The intensity of peak a for qc orientations is higher than that of q⊥c orientations for both B-K and O-K edges.
Table 1 The atomic positions of YBa3 B9 O18 . There are two types of B and four types of O in the compound.
Ba1 Y1 Ba2 O1 O2 O3 O4 B1 B2
Site
x
y
Z
4f 2b 2a 6h 12i 6h 12i 6h 12i
0.33333 0.00000 0.00000 0.4578 (8) −0.1227 (5) 0.5870 (8) −0.0026 (5) 0.6208 (11) −0.1640 (8)
0.66667 0.00000 0.00000 0.2908 (8) 0.3795 (5) 0.6679 (8) −0.7506(5) 0.5056 (12) 0.5516 (8)
0.13119 (3) 0.00000 0.25000 0.25000 0.0815 (3) 0.25000 0.0815 (2) 0.25000 0.0824 (4)
The theoretical simulation of ELNES allows a precise interpretation of the core-loss near-edge fine structure. So to clarify the
M. He et al. / Micron 89 (2016) 16–20
19
Fig. 5. (a) (b) The simulated B-K edge under the collection angle ∼5.6 mrad and ∼0.3 mrad, corresponding to q⊥c and qc orientations, respectively; (c) (d) the simulated O-K edge under the collection angle ∼5.6 mrad and ∼0.3 mrad, corresponding to q⊥c and qc orientations, respectively.
the q⊥c component (the weight is lower). In contrast, when q was perpendicular to c axis (relatively small collection angle), the spectra also included the q//c component, while the q⊥c component dominated the spectrum. To address the issue of finite collection angles and the effect of the relative contribution of the parallel and perpendicular components, the calculations were performed by the method provided by the reference (Browning et al., 1993). When the momentum transfer q was along the c axis (the collection in the refangle was ∼0.3mrad, corresponding to the v//c case //
erence), the formula ELF = ε2
2 c
2 2 c +E
+ ε⊥ ln 1 + 2
2 c 2 E
−
2 c
2 2 c +E
was used and the calculated results showed that the parallel component was about 88% and the perpendicular component was about 12%. While for the momentum transfer q was perpendicular to the c axis (the collection angle was ∼5.6mrad, corresponding to the v⊥c case in the reference), the formula ELF = //
(ε⊥ − ε2 ) 2 Fig. 6. The experimental B-K edges taken from different collection angles.
origin of the peaks in the experimental EELS spectra, we have to perform the ab initio calculations to simulate the B-K edge and O-K edge. However, the type of B in YBa3 B9 O18 can not be distinguished from experiment and all types of B are contributed to the B-K edge, and so do for the O-K edges. Thus, we simulated the B-K edge and O-K edge theoretically and involved the “weighting” of each O and B contribution. The simulated non-polarized B-K and O-K ELNES were shown in Fig. 4a and c. They agree well with the experimental results shown in Fig. 3. The B-K and O-K ELNES at different polarization conditions [001] and [100] were also calculated, as shown in Fig. 4b and d. As expected, peak a in both B-K and O-K edges mainly come from the [001] polarization directions (pz states), it can be considered as a * peak, as in graphite and BN (Disko et al., 1982; Liu et al., 2003). Peak b in both the B-K and O-K edges mainly come from the [100] polarization directions (pxy states), it can be considered as an * peak. As seen from Fig. 3, although the EELS was collected under qc (q⊥c) orientations, peak b (peak a) is still quite strong. This is due to the finite size of the EELS entrance aperture. When q was along the c axis (relatively small collection angle), the spectra also included
c2
c2 + 2 E
1 [(ε⊥ 2 2
//
+ ε2 ) ln(1 +
c2 2
)+
E
] was used and the calculated results showed that
the perpendicular component was about 61% and the parallel component was about 39%. The spectra under different experimental conditions can be simulated by the polarized [001] and [100] spectra (shown in Fig. 4b and d). The simulated B-K edge corresponding to the experimental conditions (in Fig. 3a and b) were shown in Fig. 5a and b. The simulated O-K edge corresponding to the experimental conditions (in Fig. 3c and d) were shown in Fig. 5c and d. Fig. 6 shows the q-dependent ELNES series of B-K edge, from which we can see the intensity of peak a increases gradually from lower than peak b to higher than peak b with the decreasing of the collection angle. The orientation dependence of B-K edge and O-K edge gave experimental evidence of anisotropic characteristic of YBa3 B9 O18 . The tendency of the orientation dependent of O-K edge and B-K was very similar which is due to the hybridization between B-p and O-p states of YBa3 B9 O18 . These facts also suggest that the inner-layer interaction in YBa3 B9 O18 is stronger due to the highly anisotropic structure. Note that the orientation effects in the O-K edge were very subtle, this can be explained by the crystal structure of YBa3 B9 O18 (shown in Fig. 1a). There are 4 types of O in the compound (as indicated in Table 1). The YO6 octahedra and irregular BaO6 and BaO9 polyhedra exist in-between the hexagonal B3 O6
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M. He et al. / Micron 89 (2016) 16–20
rings. Thus the O atoms bonded to Y and Ba show less orientation dependence than those bonded to B atoms. 4. Conclusion In summary, we measured the orientation dependence of the BK and O-K edge EELS in YBa3 B9 O18 . The calculation on anisotropy of the electronic structure and the ELNES simulations were performed. The simulation results can fully explain the anisotropy of the B-K and O-K edge. The method presented here requires only one specimen for spectra acquisition and consequently minimizes the influence of specimen individuality on the experimental results. Acknowledgements This work was sponsored by National Natural Science Foundation of China (Nos. 51372027 and 51372026), and the Excellent Talents Foundation of Liaoning Province (No. LR 2014012). References Browning, N.D., Yuan, J., Brown, L.M., 1991. Real-space determination of anisotropic electronic structure by electron energy loss spectroscopy. Ultramicrosocpy 38, 291–298. Browning, N.D., Yuan, J., Brown, L.M., 1993. Theoretical determination of angularly-integrated energy loss functions for anisotropic materials. Philos. Mag. A 67, 261–271. Disko, M.M., Krivanek, O.L., Rez, P., 1982. Orientation-dependent extended fine structure in electron-energy-loss spectra. Phys. Rev. B 25, 4252. Duan, C., Wang, X., Zhao, J., 2007. Luminescent properties and electronic structures of rare earth and alkaline earth borates of REBa3 B9 O18 (RE = Lu, Y). J. Appl. Phys. 101, 023501(1–5). Egerton, R.F., 1986. Electron Energy Loss Spectroscopy in the Electron Microscope. Plenum Press, New York.
Garvie, L.A.J., Craven, A.J., Brydson, R., 1995. Parallel electron energy-loss spectroscopy (PEELS) study of B in minerals; the electron energy-loss near-edge structure (ELNES) of the B K edge. Am. Mineral. 80, 1132–1144. He, Ming, Chen, X.L., Sun, Y.P., Liu, J., Zhao, J.T., Duan, C.J., 2007. YBa3 B9 O18 : a promising scintillation crystal. Cryst. Growth Des. 7, 199–201. He, R., Huang, H.W., Kang, L., Yao, W.J., Jiang, X.X., Lin, Z.S., Qin, J.G., Chen, C.T., 2013. Bandgaps in the deep ultraviolet borate crystals: prediction and improvement. Appl. Phys. Lett. 102, 231904. Ingle, J.T., Gawande, A.B., Sonekar, R.P., Omanwar, S.K., Wang, Y., Zhao, L., 2014. Combustion synthesis and optical properties of oxy-borate phosphors YCa4O(BO3)3:RE3+ (RE = Eu3+, Tb3+) under UV, VUV excitation. J. Alloys Compd. 585, 633–636. Jiang, N., Jiang, B., John, Spence, C.H., 2002. Anisotropic excitons in MgB2 from orientation-dependent electron-energy-loss spectroscopy. Phys. Rev. B 66, 172502. Li, X.Z., Wang, C., Chen, X.L., Li, H., Jia, L.S., Wu, L., Du, Y.X., Xu, Y.P., 2004. Syntheses, thermal stability, and structure determination of the novel isostructural RBa3B9O18 (R = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). Inorg. Chem. 43, 8555–8558. Lei, Liu, Feng, Y.P., Shen, Z.X., 2003. Structural and electronic properties of h-BN. Phys. Rev. B 68, 104102. Ma, C., Xiao, R.J., Geng, H.X., Yang, H.X., Tian, H.F., Che, G.C., Li, J.Q., 2008. Investigation of hole states near the Fermi level in Nb1-x Mgx B2 by electron energy-loss spectroscopy and first-principles calculations. Ultramicroscopy 108, 320–326. Perdew, J.P., Burke, K., Ernzerhof, M., 1996. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865. Radtke, G., 2008. Momentum-resolved energy loss near edge structure in SrCu2 (BO3 )2 . Ultramicroscopy 108, 893–900. Wang, Y., Honma, T., Komatsu, T., 2013. Crystallization and photoluminescence properties of ␣-RE2(WO4)3 (RE: Gd, Eu) in rare-earth tungsten borate glasses. Opt. Mater. 35, 998–1003. Zhang, Z.H., Yang, J.J., Ming, He, Wang, X.F., Li, Q., 2008. Electronic structure of a potential optical crystal YBa3 B9 O18 : experiment and theory. Appl. Phys. Lett. 92, 171903. Zhu, Y., Moodenbaugh, A.R., Schneider, G., Davenport, J.W., Vogt, T., Li, Q., Gu, G., Fischer, D.A., Tafto, J., 2002. Unraveling the symmetry of the hole states near the fermi level in the MgB2 superconductor. Phys. Rev. Lett. 88, 247002.
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Micron journal homepage: www.elsevier.com/locate/micron
Experimental investigation on anisotropic characteristic of YBa3 B9 O18 Ming He a,∗ , T.Z. Liu b , J. Cai c , Z.H. Zhang b , X.E. Gu a a
Department of Physics, Dalian Jiaotong University, Dalian, China Liaoning Key Materials Laboratory for Railway, Dalian Jiaotong University, Dalian, China c School of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, China b
a r t i c l e
i n f o
Article history: Received 28 March 2016 Received in revised form 12 July 2016 Accepted 12 July 2016 Available online 14 July 2016 Keywords: Electron energy-loss spectra Optical crystal Anisotropy Borate Density of states
a b s t r a c t The energy loss near edge fine structures of the B-K edge and O-K edge have been examined in the optical material YBa3 B9 O18 . The orientation-dependent electron energy-loss spectra (EELS) for both B-K edge and O-K edge were observed. The experimental results were analyzed based on density functional theory calculations. The unoccupied pz features dominated the EELS when it was collected under the energy transfer q along c orientations and pxy states were probed when q was perpendicular to the crystallographic c axis. The results provide direct experimental evidence on the anisotropic characteristic of YBa3 B9 O18 . © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Rare earth and alkaline earth borates have been extensively studied in the past few years (He et al., 2013; Wang et al., 2013; Ingle et al., 2014). The discoveries of a series of isostructural borate compounds, REBa3 B9 O18 (RE = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) have aroused interest in this family of materials (Li et al., 2004). Among them, YBa3 B9 O18 has further demonstrated excellent scintillation properties as revealed by the X-ray excited luminescence (XEL) measurement (He et al., 2007). The relatively low fabrication cost together with the large birefringence, excellent scintillation property and chemical stability make YBa3 B9 O18 an attractive optical crystal. YBa3 B9 O18 adopts a space group P63 /m and each B atom is bonded to three O atoms to form BO3 groups, the three of which further constitute a planar hexagonal B3 O6 ring. These planar rings are stacked along the c axis in the unit cell, with regular YO6 octahedra and irregular BaO6 and BaO9 polyhedra existing in-between the hexagonal B3 O6 rings. The crystal structure of YBa3 B9 O18 was shown in Fig. 1a and the atomic positions were shown in Table 1. There are two types of B and four types of O in the compound. The strong anisotropy is the main structure characteristic, which makes the birefringence of this crystal to be about 0.12, leading it to be especially promising in a number of optical applications such as liquid crystal displays, light modulators, color
∗ Corresponding author. E-mail address: [email protected] (M. He). http://dx.doi.org/10.1016/j.micron.2016.07.003 0968-4328/© 2016 Elsevier Ltd. All rights reserved.
filters, wave plates, and optical axis gratings, etc. The electronic structure of YBa3 B9 O18 has been investigated by both experimental and theoretical ab initial band structure calculations (Zhang et al., 2008; Duan et al., 2007). The strong anisotropic characteristic of the band structure was observed by theoretical calculations (Zhang et al., 2008). The experimental measurements on anisotropic characteristic of YBa3 B9 O18 were needed to understand its electronic structure. Electron-energy-loss-spectroscopy (EELS) combined with a transmission electron microscope (TEM) is a powerful technique to probe the electronic structure at the near atomic level. The energy loss near edge fine structures (ELNES) brings useful information about the chemical bond, electronic structure, the coordination and spin-state of the atoms, etc. (Egerton, 1986; Garvie et al., 1995). In particular, the orientation-dependent ELNES performed on anisotropic materials give additional information on the spatial orientation of electronic states (Browning et al., 1991; Radtke, 2008). The momentum transfer q dependent ELNES has been used to study the orientation dependence of anisotropic crystals (Jiang et al., 2002; Zhu et al., 2002; Ma et al., 2008). In which q is defined as q = k − k0 , where k0 and k are initial and final wave vectors of the high-energy electron beam in TEM, respectively. In this work, we investigated the anisotropy property of electronic structure for YBa3 B9 O18 using q-dependent EELS in a TEM. The unoccupied electronic structures were measured by ELNES of B-K edge and O-K edges. The ELNES involved core-hole effect were calculated based on density functional theory, which gave the theoretical analyze on the experimental results. The results provide
M. He et al. / Micron 89 (2016) 16–20
17
Fig 1. (a) The crystal structure of the YBa3 B9 O18 materials; (b) the electron diffraction pattern under the [001] zone axis, under which the EELS was collected.
Fig. 2. The typical full EELS spectra taken from the specimen including Y, B, O and Ba elements, respectively.
clear experimental evidence for anisotropic characteristic of the electronic structure of YBa3 B9 O18. 2. Experiment and theoretical calculations YBa3 B9 O18 single crystals grown by the Czochralski method were used in our studies. The TEM specimen was prepared in a standard way, first by cutting, gluing and mechanically polishing, and then the Ar-ion milling instrument was used until perforation occurred. EELS experiments were carried out using a Gatan Imaging Filtering (GIF) system attached to the microscope (TEM, JEM-2100F, FEG) with a energy resolution about 1 eV. The energy loss spectra were calibrated by the zero loss peak, followed by a Fourier-ratio deconvolution to eliminate contribution from plural scattering. The EELS measurements were carried out in a conventional TEM diffraction mode with an approximately parallel beam of incident electrons. The crystals were tilted slightly off the zone axes by 1−2◦ to avoid the electron channeling effects close to the Bragg condition. q-dependent EELS were collected under the [001] orientation of the sample and the electron diffraction pattern was shown as Fig. 1b. When the momentum transfer q was along the c axis (qc), the camera length was relatively large (the collection angle was ∼0.3 mrad), the TEM diffraction shift controls were
adjusted to put the transmission spot (that is 000) on the EELS aperture, corresponding to the case of the minimum q. When q was perpendicular to c (q⊥c), the camera length was decreased and thus the collection angle was increased to ∼5.6 mrad. In this study, the smallest collection aperture was used. The calculations were carried out using first-principle method based on density-functional theory (DFT) by CASTEP in the MATERIAL STUDIO program. The Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof scheme (PBE) was adopted for the exchange-correlation potential (Perdew et al., 1996). “Ultrasoft pseudopotentials” were used when calculate the electronic structure, while the “pseudopotentials on the fly” were chosen for the calculations of the core-level spectra. That is, the parameters were provided which govern the generation rather than a file from the database. This approach has a number of advantages; e.g., the same exchange-correlation functional is used in the atomic and solid state calculations; it is possible to generate “softer” or “harder” potentials by changing the core radius; it is possible to study excited configurations with a core hole, etc. The wave functions were expanded in plane waves up to a cutoff energy of 400 eV, and a 2 × 2 × 2 Monkhorst-Pack mesh as k-point sampling was used to sample the irreducible Brillouin zone, ensuring an energy precision of better than 1 meV for the total energy. The test calculations with higher cutoff energies and denser k-point grids were also performed, the overall results remained unchanged. The optimized lattice parameters (a = 7.1761 Å, c = 16.9562 Å) of YBa3 B9 O18 was used in all calculations from the experimental results (a = 7.1841 Å and c = 16.9283 Å). The structure was optimized until the net force on each atom was smaller than 0.01 eV/Å. The simulated nonpolarized ELNES of B-K and O-K edges and the polarized ELNES under [001] and [100] orientations were calculated to understand the experimental results. Note that ELNES includes influences in the core excitation processes beyond the ground state electronic structure, thus core-hole effects were involved in our calculations to give clear analysis of the experimental results. 3. Results and discussions We focused on the study of B and O elements in this work. The boron K-edge or oxygen K-edge ELNES involves the dipole selected electronic transitions from 1 s state of boron atom or oxygen, thus providing a direct detection of the unoccupied p-like DOS of boron or oxygen in YBa3 B9 O18 . So we can probe the unoccupied B-2p and O-2p states by measuring B-K and O-K absorption edges, respectively. Fig. 2 shows the typical full EELS spectra including Y-M4,5 ,
18
M. He et al. / Micron 89 (2016) 16–20
Fig. 3. The experimental B-K edge spectra recorded at two different orientations (a) q⊥c and (b) qc; the experimental O-K edge spectra recorded at two different orientations (c) q⊥c and (d) qc.
Fig. 4. (a) The simulated non-polarized B-K edge; (b) the simulated B-K edge under [001] and [100] polarization directions; (c) the simulated non-polarized O-K edge; (d) the simulated O-K edge under [001] and [100] polarization directions.
B-K, O-K and Ba-M4,5 absorption edges, indicating the specimen we investigated include Y, B, O and Ba elements. The experimental B-K edge spectra recorded at two different orientations (q⊥c and qc) were shown in Fig. 3a and b and the experimental EELS spectra of OK edge collected at q⊥c and qc orientations were shown in Fig. 3c and d, respectively. In general, it can be clearly seen that there is a sharp peak located at 192 eV (marked as a), and a broad peak at 199 eV (marked as b) for B-K edge. While for O-K edge, the peaks are located at 535 eV and 542 eV (marked as peak a and b), respectively. The background was removed according to the power law. For comparison, both spectra were scaled to the post-edge intensity. The differences in the fine structures at different orientations are significant. The intensity of peak a for qc orientations is higher than that of q⊥c orientations for both B-K and O-K edges.
Table 1 The atomic positions of YBa3 B9 O18 . There are two types of B and four types of O in the compound.
Ba1 Y1 Ba2 O1 O2 O3 O4 B1 B2
Site
x
y
Z
4f 2b 2a 6h 12i 6h 12i 6h 12i
0.33333 0.00000 0.00000 0.4578 (8) −0.1227 (5) 0.5870 (8) −0.0026 (5) 0.6208 (11) −0.1640 (8)
0.66667 0.00000 0.00000 0.2908 (8) 0.3795 (5) 0.6679 (8) −0.7506(5) 0.5056 (12) 0.5516 (8)
0.13119 (3) 0.00000 0.25000 0.25000 0.0815 (3) 0.25000 0.0815 (2) 0.25000 0.0824 (4)
The theoretical simulation of ELNES allows a precise interpretation of the core-loss near-edge fine structure. So to clarify the
M. He et al. / Micron 89 (2016) 16–20
19
Fig. 5. (a) (b) The simulated B-K edge under the collection angle ∼5.6 mrad and ∼0.3 mrad, corresponding to q⊥c and qc orientations, respectively; (c) (d) the simulated O-K edge under the collection angle ∼5.6 mrad and ∼0.3 mrad, corresponding to q⊥c and qc orientations, respectively.
the q⊥c component (the weight is lower). In contrast, when q was perpendicular to c axis (relatively small collection angle), the spectra also included the q//c component, while the q⊥c component dominated the spectrum. To address the issue of finite collection angles and the effect of the relative contribution of the parallel and perpendicular components, the calculations were performed by the method provided by the reference (Browning et al., 1993). When the momentum transfer q was along the c axis (the collection in the refangle was ∼0.3mrad, corresponding to the v//c case //
erence), the formula ELF = ε2
2 c
2 2 c +E
+ ε⊥ ln 1 + 2
2 c 2 E
−
2 c
2 2 c +E
was used and the calculated results showed that the parallel component was about 88% and the perpendicular component was about 12%. While for the momentum transfer q was perpendicular to the c axis (the collection angle was ∼5.6mrad, corresponding to the v⊥c case in the reference), the formula ELF = //
(ε⊥ − ε2 ) 2 Fig. 6. The experimental B-K edges taken from different collection angles.
origin of the peaks in the experimental EELS spectra, we have to perform the ab initio calculations to simulate the B-K edge and O-K edge. However, the type of B in YBa3 B9 O18 can not be distinguished from experiment and all types of B are contributed to the B-K edge, and so do for the O-K edges. Thus, we simulated the B-K edge and O-K edge theoretically and involved the “weighting” of each O and B contribution. The simulated non-polarized B-K and O-K ELNES were shown in Fig. 4a and c. They agree well with the experimental results shown in Fig. 3. The B-K and O-K ELNES at different polarization conditions [001] and [100] were also calculated, as shown in Fig. 4b and d. As expected, peak a in both B-K and O-K edges mainly come from the [001] polarization directions (pz states), it can be considered as a * peak, as in graphite and BN (Disko et al., 1982; Liu et al., 2003). Peak b in both the B-K and O-K edges mainly come from the [100] polarization directions (pxy states), it can be considered as an * peak. As seen from Fig. 3, although the EELS was collected under qc (q⊥c) orientations, peak b (peak a) is still quite strong. This is due to the finite size of the EELS entrance aperture. When q was along the c axis (relatively small collection angle), the spectra also included
c2
c2 + 2 E
1 [(ε⊥ 2 2
//
+ ε2 ) ln(1 +
c2 2
)+
E
] was used and the calculated results showed that
the perpendicular component was about 61% and the parallel component was about 39%. The spectra under different experimental conditions can be simulated by the polarized [001] and [100] spectra (shown in Fig. 4b and d). The simulated B-K edge corresponding to the experimental conditions (in Fig. 3a and b) were shown in Fig. 5a and b. The simulated O-K edge corresponding to the experimental conditions (in Fig. 3c and d) were shown in Fig. 5c and d. Fig. 6 shows the q-dependent ELNES series of B-K edge, from which we can see the intensity of peak a increases gradually from lower than peak b to higher than peak b with the decreasing of the collection angle. The orientation dependence of B-K edge and O-K edge gave experimental evidence of anisotropic characteristic of YBa3 B9 O18 . The tendency of the orientation dependent of O-K edge and B-K was very similar which is due to the hybridization between B-p and O-p states of YBa3 B9 O18 . These facts also suggest that the inner-layer interaction in YBa3 B9 O18 is stronger due to the highly anisotropic structure. Note that the orientation effects in the O-K edge were very subtle, this can be explained by the crystal structure of YBa3 B9 O18 (shown in Fig. 1a). There are 4 types of O in the compound (as indicated in Table 1). The YO6 octahedra and irregular BaO6 and BaO9 polyhedra exist in-between the hexagonal B3 O6
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M. He et al. / Micron 89 (2016) 16–20
rings. Thus the O atoms bonded to Y and Ba show less orientation dependence than those bonded to B atoms. 4. Conclusion In summary, we measured the orientation dependence of the BK and O-K edge EELS in YBa3 B9 O18 . The calculation on anisotropy of the electronic structure and the ELNES simulations were performed. The simulation results can fully explain the anisotropy of the B-K and O-K edge. The method presented here requires only one specimen for spectra acquisition and consequently minimizes the influence of specimen individuality on the experimental results. Acknowledgements This work was sponsored by National Natural Science Foundation of China (Nos. 51372027 and 51372026), and the Excellent Talents Foundation of Liaoning Province (No. LR 2014012). References Browning, N.D., Yuan, J., Brown, L.M., 1991. Real-space determination of anisotropic electronic structure by electron energy loss spectroscopy. Ultramicrosocpy 38, 291–298. Browning, N.D., Yuan, J., Brown, L.M., 1993. Theoretical determination of angularly-integrated energy loss functions for anisotropic materials. Philos. Mag. A 67, 261–271. Disko, M.M., Krivanek, O.L., Rez, P., 1982. Orientation-dependent extended fine structure in electron-energy-loss spectra. Phys. Rev. B 25, 4252. Duan, C., Wang, X., Zhao, J., 2007. Luminescent properties and electronic structures of rare earth and alkaline earth borates of REBa3 B9 O18 (RE = Lu, Y). J. Appl. Phys. 101, 023501(1–5). Egerton, R.F., 1986. Electron Energy Loss Spectroscopy in the Electron Microscope. Plenum Press, New York.
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