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J. Phys. Chem. B 2001, 105, 7908-7912
ARTICLES Neutron Diffraction and X-ray Absorption Spectroscopic Analyses for Lithiated Aurivillius-Type Layered Perovskite Oxide, Li2Bi4Ti3O12 Jin-Ho Choy,* Jong-Young Kim, and In Chung National Nanohybrid Materials Laboratory, School of Chemistry and Molecular Engineering, College of Natural Sciences, Seoul National UniVersity, Seoul 151-747, Korea ReceiVed: March 8, 2001; In Final Form: May 21, 2001
The chemical lithiation of Aurivillius-type layered perovskite oxide, Bi4Ti3O12 (BTO), was demonstrated for the first time. The crystal and electronic structures of the lithiated BTO were characterized by neutron diffaction and X-ray absorption spectroscopic analyses. According to the neutron diffaction studies, it has been found that the lithium atoms are positioned between a perovskite block and a Bi2O2 layer. From the Bi LIII- and Ti K-edge XANES/EXAFS analyses, it has been found that significant amounts of Bi and Ti cations are reduced due to the Li-intercalation, and that the local structure around Ti becomes more distorted.
Introduction Since the discovery of fatigue-free behavior of strontium bismuth tantalate (SrBi2Ta2O9; SBT) in a nonvolatile ferroelectric random access memory (NVFRAM) device,1 considerable attention has been focused on the Aurivillius-type layered perovskite oxides. As shown in Figure 1, the Aurivillius phases can be described as intergrowth structures composed of alternating layers of fluorite-like (Bi2O2)2+ and perovskite units (An-1BnO3n+1)2-; the typical examples are Bi2WO6 (n ) 1), Bi2SrTa2O9 (n ) 2), and Bi4Ti3O12 (n ) 3). In this work, we attempted to prepare a new Li-intercalated phase of Aurivillius oxide, Bi4Ti3O12 (BTO), by a soft chemical method. Until now, there has been no report on the intercalation compound of the Aurivillius-type layered oxides, even though a number of studies on the lithium intercalation reaction into the lamellar transition metal oxides and sulfides such as LixMoO32 and LixMS2 (M ) Mo, Ta, Nb, and W)3 were carried out. Moreover, such Li-intercalation compounds can be delaminated into colloidal nanoplatelets which have been used as basic components of multilayered architectures constructed by various techniques such as layer-by-layer (LBL) assembly,4a LangmuirBlodgett technique,4b and intercalation reaction.4c Our research strategy is to synthesize colloidal nanoplatelets of Aurivilliustype layered oxides by the exfoliation reaction of their Liintercalated phases, which can be potentially applied to ferroelectric ultrathin film or multilayered assembly. In the present study, we carried out neutron powder diffraction analysis to investigate the crystal structure of the Li-intercalated BTO since the contribution of lithium and oxygen to the overall neutron scattering is fairly significant compared to the X-ray scattering. Complementarily, XANES/EXAFS spectroscopic analyses were performed to investigate the evolution of electronic and geometric structures with respect to the Li-intercalation. * Author to whom correspondence should be addressed. Tel: +82-2880-6658. Fax: +82-2-872-9864. E-mail:
[email protected].
Experimental Section Li-Intercalation of BTO. The pristine BTO was prepared by conventional solid-state reaction. A stoichiometric amount of Bi2O3 and TiO2 were mixed, pelleted, and fired at 1000 °C for 24 h in air. The Li-intercalated BTO (LiBTO) was obtained by reacting the bulk powder of BTO with 3 moles excess of n-BuLi in hexane for at least 3 days. The resulting black powder was washed with hexane more than 5 times and then dried in Vacuo. Elemental Analysis. Elemental analyses of Bi, Ti, and Li for the Li-intercalated BTO phase were performed by inductive coupled plasma (ICP) and atomic absorption (AA) spectroscopic methods. The chemical formula of LiBTO powder was determined as Li2.1(1)Bi4.0Ti3.1(1)O12. Neutron Diffraction Analysis. Powder neutron diffraction data for the Li-intercalated BTO were collected at 297 K on the high-resolution powder diffractometer, HRPD at the HANARO facility, Taejon, Korea. A wavelength of 1.8347 Å was selected from a Ge monochromator. The counting time was 4 h and 10 g of sample in a vanadium can were used. Data analysis was carried out by the Rietveld method using the FULLPROF program suite.5 A pseudo-Voigt function was chosen to generate the line shape of diffraction peaks. The coherent scattering lengths for Bi, Ti, O, and Li were 8.531, -3.438, 5.803, and -1.90 fm. The lithium atoms were located by Fourier synthesis using FOURIER and GFOURIER programs. In the final run, the following parameters were refined: background coefficients, zero-point, half-width, and pseudo voigt parameters for the peak shape, scale factor, unit cell parameters, isotropic thermal parameters, and positional parameters for Li, Bi, Ti, and O atoms. Crystallographic data for Li2Bi4Ti3O12 are as follows; B2cb, a ) 5.4428(5) Å, b ) 5.4091(5) Å, c ) 32.785(3) Å, V ) 965.2 Å3, Fcalcd ) 8.16 g cm-3, Z ) 4, Rwp ) 0.0399, Rp ) 0.0306, Rexp ) 0.0164, the measurement range 10 e θ e 155°, 2900 data points, 351 observed reflections, 48 parameters refined (38 crystallographic ones).
10.1021/jp010895h CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001
Analyses for Lithiated Aurivillius-Type Li2Bi4Ti3O12
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Figure 1. Crystal structures of Aurivillius-type perovskite oxides Bi4Ti3O12 and Li-intercalate Li2Bi4Ti3O12.
X-ray Absorption Measurements. The X-ray absorption measurements on the Bi L- and Ti K-edges were carried out with synchrotron radiation at the beam line 10B of the Photon Factory (KEK-PF) in Tsukuba. The samples were ground to a fine powder in a mortar with boron nitride as a diluent and pelleted to obtain an optimum absorption jump (∆µt ≈ 1).6 The data were recorded with a spacing of ∼0.3 eV for the XANES region and ∼1.5 eV for the EXAFS region in the transmission mode at room temperature. An Si(311) monochromator channelcut monochromator was used. The data analysis was performed according to the previously published procedures.7 Magnetic Measurements. The magnetic susceptibilities for the Li-intercalate were measured in the regime of T e 300 K on a Quantum Design vibrating sample SQUID magnetometer at 2000 G. Result and Discussion X-ray Diffraction Analyses. As shown in Figure 2, powder X-ray diffraction pattern for the Li-intercalate is almost the same as that for the pristine, Bi4Ti3O12. After lithiation, no significant
change in Bragg positions and peak intensities can be observed. According to the previous study on the electrochemically lithiated BIMEVOX phase,8 anion-deficient phase exhibits significant variation of Bragg peaks depending on discharge states. However, no additional peak due to such anion-deficient oxide can be observed in the present XRD pattern (Figure 2). Furthermore, an additional peak of 27.3° due to Bi metal cannot be noticed, which shows that BTO was not decomposed upon the lithiation. Bi L- and Ti K-edge XANES/EXAFS Spectra. When the n-BuLi is added to BTO bulk powder, its color changes from yellowish white to black due to a partial change of oxidation state in bismuth and titanium. As shown in Bi LIII-edge XANES spectra (Figure 3a), the position of the absorption edge jump for the Li-intercalate is shifted to the lower energy side by ca. 0.5 eV compared to the pristine compound, indicating the reduction of bismuth. Figure 3b represents the normalized Ti K-edge XANES spectra and their second derivatives of BTO and its Li-intercalate. Both spectra exhibit pre-edge peaks A around 4970 eV, which can be assigned as the transition from
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Figure 2. X-ray diffraction patterns for Bi4Ti3O12 and Li2Bi4Ti3O12.
TABLE 1: Bond Distances and Debye-Waller Factors for Ti-O Bond Obtained from EXAFS and Neutron Powder Diffraction Analyses.
compounds
scattering paths
Debye-Waller factor (×10-3 Å2)
Bi4Ti3O12 6 × (Ti-O) Li2Bi4Ti3O12 2 × (Ti-O) 4 × (Ti-O)
7.98 3.31 2.63
average (Ti-O) bond distance (Å) EXAFS neutron 1.962 1.947 9 1.927 2.041 average 2.003 2.029
TABLE 2: Atomic Position and Isotropic Temperature Factors for Li2Bi4Ti3O12 atom
site
x
y
z
B (Å2)
Li(1) Bi(1) Bi(2) Ti(1) Ti(2) O(1) O(2) O(3) O(4) O(5) O(6)
8b 8b 8b 4a 8b 8b 8b 8b 8b 8b 8b
0.227(6) 0.0a 0.006(2) 0.064(4) -0.044(3) 0.238(3) 0.248(3) 0.076(2) 0.047(3) 0.280(3) 0.180(2)
0.231(6) 0.974(1) 0.008(2) 0.0 -0.054(4) 0.204(2) 0.277(2) 0.048(2) 0.959(2) 0.290(2) 0.197(2)
0.3004(7) 0.0663(2) 0.2134(2) 0.5 0.3680(4) 0.0089(2) 0.2461(3) 0.4365(2) 0.3179(2) 0.1150(2) 0.8661(3)
0.63(9) 0.70(9) 0.70(9) 0.1(3) 0.1(3) 0.56(9) 0.56(9) 0.56(9) 0.56(9) 0.56(9) 0.56(9)
a
Coordinate fixed to define origin of polar axis.
2s core level to unoccupied 3d states. Although the overall XANES features do not show any changes in shape and transition energy, the curve-fitting analyses of EXAFS spectra clearly indicate that the local structure around titanium is significantly changed upon the Li-intercalation. Figure 4, parts a and b, represent the k3-weighted Ti K-edge EXAFS spectra and their Fourier transforms (FTs) of BTO and its Li-intercalate. The first peak in the FT corresponds to the nearest neighbors of titanium ion, i.e., (Ti-O) shells. In the fitting procedure, only the distance (R) and the Debye-Waller factor (σ2) were allowed to be refined. However, the coordination number (CN) was fixed to the crystallographic values to
TABLE 3: Selected Bond Distances for Li2Bi4Ti3O12 (Å) bond
bond length (Å)
bond
bond length (Å)
Ti(1)-O(1) Ti(1)-O(3)
1.86(5), 2.16(6) 2.10(2)
Bi(2)-O(2)
Ti(2)-O(3)
2.40(4)
Bi(2)-O(4)
Ti(2)-O(4) Ti(2)-O(5) Ti(2)-O(6) Bi(1)-O(1)
1.72(5) 1.69(6), 2.03(7) 2.03(7), 2.28(6) 2.95(4), 2.93(4), 2.60(4), 3.20(3) 2.35(4), 3.13(4), 3.14(3), 2.31(3) 2.79(3), 2.37(3) 2.59(3), 3.19(3)
Bi(2)-O(6) Li-O(2) Li-O(4) Li-O(6)
2.50(5), 2.26(4), 2.22(4), 2.11(4) 2.71(4), 3.11(4), 3.17(4), 2.63(4), 3.45(2) 2.99(3) 1.80(8) 1.86(10) 2.20(7)
Bi(1)-O(3) Bi(1)-O(5) Bi(1)-O(6)
Bi(2)-Li
2.06(10), 1.99(10) 3.32(7)
be free from the inaccuracies in refining the CN values, since they are strongly correlated with σ2. The results of the best fit to the first coordination shell are compared with the experimental spectra in Figure 4b and the fitted structural parameters are listed in Table 1. As shown in Table 1, the average Ti-O bond length increases upon the Li-intercalation due to a partial reduction of titanium. The existence of Ti3+ (3d1) ions makes the local symmetry more distorted, as confirmed by neutron diffraction analysis in the next section. In contrast to the pristine compound, the Ti-O first shell in the Ti K-edge EXAFS spectra for the Li-intercalate could not be fitted successfully when one Ti-O single scattering contribution is considered. Therefore, we performed a curve fitting analysis including two single scattering paths, 2 × Ti-O (short) and 4 × Ti-O (long). The average Ti-O bond length (2.00 Å) over two-shell fitting agrees fairly well with the neutron diffraction analysis result (2.029 Å). Crystal Structures of BTO and the Lithiated BTO. As shown in Figure 1, the crystal structure of BTO can be described as an intergrowth one composed of alternating layers of perovskite (Bi2Ti3O10)2- and intervening fluorite-like (Bi2O2)2+.
Analyses for Lithiated Aurivillius-Type Li2Bi4Ti3O12
J. Phys. Chem. B, Vol. 105, No. 33, 2001 7911
Figure 4. (a) k3-weighted Ti K-edge EXAFS spectra, (b) their Fourier transforms (open circles) and best fit (solid line) in the first coordination sphere of the titanium of the BTO and its Li-intercalate.
Figure 3. (a) Bi LIII-XANES spectra and their second derivatives for Bi4Ti3O12 and Li-intercalated Bi4Ti3O12. (b) Ti K-edge XANES spectra and their second derivatives.
At first, we have performed Rietveld refinement for the neutron diffraction pattern based on a structural model, where the Li atoms are omitted. The starting parameters used were those of Bi4Ti3O12 in the orthorhombic unit cell with the space group B2cb (Z ) 4) according to the previous neutron diffraction study.9 After the Rietveld refinement based on this structural model, an RBragg factor of 13.1 was reached. The small discrepancies between observed and calculated intensities allowed us to get information on the remaining scattering density, which was not considered in the starting model. From a Fourier synthesis, performed on the difference between observed and calculated structural factors,10 we were able to determined the location of lithium atoms at 8b (x,y,z) positions. As shown in Figure 5b, the peaks of negative density could be seen as located at (0.206, 0.324, 0.313) and equivalent positions, which is wholly consistent with the elemental analysis result of Li. After
introducing a lithium atom into the structural model, the RBragg factor was dropped to 8.11. The refined crystallographic data and selected bond lengths are listed in Tables 2 and 3. As previously reported, all the structural distortions in Bi4Ti3O12 are related to bonding requirements at the cation sites, especially the Bi ones.9 In other words, the predominant driving force for the structural distortion lies with the unsatisfactory coordination of the Bi cations which should be severely underbonded in the symmetrical environment. In the tetragonal phase at high temperature (800 °C), bond valence sum (BVS) values of Bi(1) and Bi(2) cations are calculated to be 2.22 and 2.70, respectively.11 Therefore, it is expected that a partial reduction of the bismuth cations reduces the structural distortion because the tendency to optimize the bonding requirement of Bi cations decreases. In the present lithiated BTO, the minimum and maximum Bi(1)-O and Bi(2)-O bond lengths are determined to be 2.32-3.21 Å and 2.11-3.17 Å, respectively, whereas those of the pristine are reported to be 2.29-3.30 Å and 2.14-3.25 Å for Bi(1) and Bi(2), respectively.9 Therefore, the range of the Bi-O bond lengths for the Li-intercalate becomes smaller than that for the pristine, which implies that the local distortion of the Bi sites is rather reduced upon the lithiation. In this work, the average Bi(1)-O bond length (2.796 Å), belonging to (Bi2Ti3O10)2- perovskite block, increases upon the lithiation (∆R ) 0.047 Å), but the average Bi(2)-O one (2.59 Å) in the (Bi2O2)2+ layer slightly decreases (∆R ) -0.017 Å). This result is well consistent with the previous theoretical calculation result on the electronic structure of BTO at room
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Figure 6. Magnetic susceptibility for Li2Bi4Ti3O12.
exhibits temperature independent behavior, implying Pauli paramagnetism. This metallic magnetism is totally different from the diamagnetism of the pristine BTO. Such metallic behavior of Li-intercalate is well consistent with the X-ray absorption spectroscopic results indicating the partial reduction of Bi3+ and Ti4+. Further studies on the magnetic and transport properties are in progress. Acknowledgment. This work was financially supported by the Ministry of education (BK 21 program) and the Ministry of Science and Technology (NRL project). References and Notes
Figure 5. (a) Neutron diffraction profiles for Li2Bi4Ti3O12. Calculated (solid line), observed (cross), difference (solid line underneath), and Bragg position (vertical mark underneath) are presented. (b) Difference Fourier map for z ) 0.313 showing the location of Li atoms at (0.206, 0.324, 0.313) and equivalent positions. The scattering density at Li positions is negative (blue line).
temperature.12 According to this result, the 6p state of Bi(1) cation is relatively lower than that of Bi(2) by ca. 1.5 eV. Therefore, it is expected that the extra charge due to the lithiation is mainly accumulated at the Bi(1) site, which is well consistent with the increase of Bi(1)-O bond length. On the other hand, it is thought that the local distortion around titanium is mainly due to an electronic effect; i.e., the existence of Ti3+ ions with d1 electronic configuration. As for the Ti-O bonds, the average Ti(1)-O and Ti(2)-O bond lengths for the Li-intercalate are much larger than the average bond length for the pristine (∆R ) 0.082 and 0.083 Å for Ti(1) and Ti(2), respectively). As expected from EXAFS analysis result, the local distortion around titanium becomes stronger due to the Liintercalation and, therefore, the ranges of Ti(1)-O and Ti(2)-O bond lengths become broader upon Li-intercalation. The minimum and maximum Ti(1)-O and Ti(2)-O bond lengths for the Li-intercalate are 1.856-2.098 Å and 1.719-2.402 Å, respectively, whereas the ranges of Ti(1)-O and Ti(2)-O bond lengths for the pristine are determined to be 1.916-1.988 Å and 1.753-2.302 Å, respectively. Magnetic Susceptibility Measurement. As shown in Figure 6, the measured susceptibilty for the Li-intercalated Bi4Ti3O12
(1) Scott, J. F.; Paz de Araujo, C. A. Science 1989, 246, 1400; Paz de Araujo, C. A.; Cuchiaro, J. D.; McMillan, L. D.; Scott, M. C.; Scott, J. F. Nature 1995, 374, 627. (2) Kanatzidis, M. G.; Marks, T. J. Inorg. Chem. 1987, 26, 783. (3) Divigalpitiya, W. M. R.; Frindt, R. F.; Morrison, S. R. Science 1989, 246, 369; Tsai, H. L.; Heising, J.; Schindler, J. L.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1997, 9, 879; Murphy, D. W.; Hull, G. W., Jr. J. Chem. Phys. 1975, 62, 973. (4) (a)Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618; Evans, S. D.; Ulman, A.; Goppert-Beraducci, K. E.; Gerenser, L. J.; J. Am. Chem. Soc. 1991, 113, 5866; Ollivier, P. J.; Kovtyukhova, N. I.; Keller, S. W.; Mallouk, T. E. Chem. Commun. 1998, 1563. (b) Blodgett, K. B.; J. Am. Chem. Soc. 1935, 57, 1007; Ulman, A.; An Introduction to Ultrathin Organic Films. In Langmuir Blodgett to SelfAssembley; Academic Press: Boston 1991. (c) Choy, J. H.; Kwon, S. J.; Park, G. S. Science 1998, 280, 1589. (5) Rodriguez-Carvajal, J. Physica (Amsterdam) 1993, 192B, 55. (6) Lytle, F. W.; van der Laan, G.; Greegor, R. B.; Larson, E. M.; Violet, C. E.; Wong, J. Phys. ReV. B 1990, 41, 8955; Stern, E. A.; Kim, K. Phys. ReV. B 1981, 23, 3781. (7) Choy, J. H.; Kim, D. K.; Demazeau, G.; Jung, D. Y. J. Phys. Chem. 1994, 98, 6258.; Choy, J. H.; Kim, D. K.; Hwang, S. H.; Demazeau, G.; Jung, D. Y. J. Am. Chem. Soc. 1995, 117, 8557. Choy, J. H.; Kim, J. Y.; Kim. S. J.; Demazeau, G. Int. J. Inorg. Mater. 2000, 2, 61. (8) Patoux, S.; Vanier, R.; Mairesse, G.; Nowogrocki, G.; Tarascon, J. Chem. Mater. 2001, 13, 500. (9) Hervoches, C. H.; Lightfoot, P. Chem. Mater. 1999, 11, 3369. (10) Alonzo, J. A.; Sanz, J.; Santamarı´a, J.; Leo´n, C.; Va´lez, A.; Ferna´ndez-Diaz, M. T. Angew. Chem., Int. Ed. 2000, 39, 619. (11) (a)Bresse, N. E.; O’Keepe, M. Acta Cryst. B 1991, 47, 192. (b) Unfortunately, the Ro values for the reduced Bi such as Bi2+ is not available and, therefore, we could not obtain the BVS values for Bi. However, assuming Ro ) 2.094 (Bi3+), the BVS values for Bi(1) and Bi(2) are calculated to be 2.56 and 3.14, respectively, which implies that Bi(1) is mainly reduced. As for Ti, the BVS values are calculated for the Ro values of Ti4+ and Ti3+, respectively. Using the Ro value of Ti4+, the BVS for Ti(1) and Ti(2) are 3.52 and 4.30, respectively. For Ro of Ti3+, the BVS for Ti(1) and Ti(2) are 3.29 and 4.03, respectively. The present BVS calculations show that significant amounts of Bi and Ti are reduced upon the lithiation, which is well consistent with the X-ray absorption spectroscopic analysis results. (12) Postnikov, A. V.; Bartkowski, St.; Merch, F.; Neumann, M.; Kurmaev, E. Z.; Cherkashenko, V. M.; Nemnonov, S. N.; Galakhov, V. R. Phys. ReV. B 1995, 52, 11805.
