Effect of an Internal Electric Field on the Redox Energies of ALnTiO4

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Effect of an Internal Electric Field on the Redox Energies of ALnTiO4 (A = Na or Li, Ln = Y or rare-earth) Sang-Hoon Song, Kyunghan Ahn, Mercouri G. Kanatzidis, Jose Antonio Alonso, jinguang Cheng, and John B. Goodenough Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm401814z • Publication Date (Web): 28 Aug 2013 Downloaded from http://pubs.acs.org on September 11, 2013

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Chemistry of Materials

Effect of an Internal Electric Field on the Redox Energies of ALnTiO4 (A = Na or Li, Ln = Y or rare-earth). Sang-Hoon Song1, Kyunghan Ahn2, Mercouri G. Kanatzidis2, José Antonio Alonso3, Jin-Guang Cheng1 and John B. Goodenough1* 1

Materials Science and Engineering and The Texas Materials Institute, University of Texas at Austin, Austin, Texas 78712, USA 2 Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA 3 Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spain Key words: layered titanates; ion-exchange; Li-insertion potentials; cation order. ABSTRACT: The layered perovskite compounds NaLnTiO4 (Ln = Y, La to Tb) have Ln2O2 rock-salt layers alternating with Na2O2 rock-salt layers either side of a TiO2 sheet. The cation ordering introduces an internal electric field perpendicular to the layers. Ion exchange of Li+ for Na+ changes the rock-salt Na2O2 layers into OLi2O antifluorite layers, but not the charge of the layers or, significantly, the internal electric field. The internal fields induce a ferroic displacement of the Ti(IV) towards the alkali-ion layers. Reversible Li insertion is observed in both NaLixLnTiO4 and Li1+xLnTiO4 with 0≤x≤1; the voltage profiles show Ti(IV)/Ti(III) redox plateaus at 0.5 V and 0.1 V versus Li+/Li0 and a 4f6/4f7 Eu3+/Eu2+ plateau at 0.8 V. The shift of 1 eV of the Ti(IV)/Ti(III) redox energy relative to that found in the spinel Li4Ti5O12 could be attributed to a capacitance energy associated with the formal +2e and -2e charges, respectively, on the Ln2O2 layers and the alkali-ion layers. Optical measurements showed that the energy gap between the Ti: 3d0 and O: 2p6 band edges is not significantly different in the layered and spinel titanates.

energy of an internal electric field perpendicular to the layers and the relative energies of the Ti(IV)/Ti(III) and Eu3+/Eu2+ couples.

INTRODUCTION Lithium insertion into a transition-metal-oxide host framework is accompanied by a change in the oxidation state of the framework ions; the host cations accept a single electron for each inserted Li+ ion for charge neutrality. The energy of the host state that accepts the electrons is the 'redox energy' of the host1,2. Reversible lithium insertion/extraction into/from a host oxide is the fundamental science of a reversible electrode in a Li-ion battery. A wide range of transition-metal oxide frameworks has been developed and tested as electrode materials3-13. One of the most successful anode hosts, Li4Ti5O12, contains the Ti(IV)/Ti(III) redox couple in a [Li1/3Ti5/3]O4 spinel framework; it has a redox energy about 1.5 eV below the Fermi energy of metallic lithium, which is well matched to the window of the organic liquid-carbonate electrolyte used in a Li-ion battery10, 12, 14-18. In this paper, we report the use of electrochemistry to investigate the relative energies of the Ti(IV)/Ti(III) and Ln3+/Ln2+ redox couples in a series of isostructural layered titanates LnATiO4, (A= Na or Li). Of interest is the effect on the Ti(IV)/Ti(III) redox

EXPERIMENTAL DETAILS Structural Characterizations. Polycrystalline samples of NaLnTiO4 (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, and Tb) were synthesized by conventional solid-state reaction with Na2C2O4 (Alfa Aesar, 99.5%), TiO2 (SigmaAldrich, 99.9%) and Y2O3, La2O3, Pr6O11, Nd2O3, Sm2O3, Eu2O3, Gd2O3, and Tb4O7 (Alfa Aesar, 99.999%) as starting materials. Prior to mixing the powders with stoichiometric ratios in an agate mortar, the rare-earth oxides were heated at 900 ◦C for 12 hours to remove adsorbed water and carbonate impurities. In addition, 30 mol% of excess Na2C2O4 was added to compensate for the loss of Na during the heat treatment. The mixed powders for the NaLnTiO4 were heat-treated at 900 ◦C for 12 hours followed by another subsequent 12 hours with a grinding in between. To synthesize LiLnTiO4, the sodium ions in the parent NaLnTiO4 compounds were ion-exchanged with lithium ions in

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equipped with an integrating sphere detector and controlled by a computer. BaSO4 was used as a 100% reflectance standard. The samples were prepared by grinding the polycrystalline samples to a powder and spreading the powder on a compacted surface of the powdered standard material, preloaded into a sample holder. The reflectance versus wavelength data generated were used to estimate the band gap of the materials by converting reflectance to absorption data with the Kubelka-Munk equation20; F(R)=(1-R)2/2R, (R= reflectance).

molten LiNO3. The products were washed with distilled water and air-dried in an oven. To increase the cyclability of the charge/discharge coin cell tests, the synthesized samples were then coated with carbon from glucose by heating at 600 ◦C under Ar atmosphere. For the preparation of the samples with additionally inserted lithium ions for neutron diffraction and magnetic measurements, the synthesized NaLnTiO4 and LiLnTiO4 powder samples were cold-pressed and Li+ ions were electrochemically inserted into the pellets by fully discharging down to 0.01 V versus Li+/Li0 with lithium metal as the reference electrode. Powder X-ray diffraction (XRD) patterns were collected with a Philips X-pert powder X-ray diffractometer and monochromatized Cu Kα radiation in a step-scanning mode with counts for 10 seconds at 0.02◦ intervals over the 2θ range from 5◦ to 100◦. Neutron powder diffraction (NPD) patterns for LiLnTiO4 and Li1+xYTiO4 samples were collected at the HRPT diffractometer of the SINQ spallation source at ETH-Zurich. The patterns were collected at room temperature with a wavelength of 1.494 Å. The highflux mode was used; the collection time was 3 h. A 6mm-dia. vanadium can was used as sample holder. The patterns were refined by the Rietveld method19 with the FULLPROF program. A pseudo-Voigt function was chosen to generate the line shape of the diffraction peaks. The following parameters were refined in the final runs: scale factor, background coefficients, zeropoint shift, pseudo-Voigt function corrected for asymmetry parameters, positional coordinates and isotropic thermal factors. The coherent scattering lengths for Li, Y, Ti and O atoms are -1.900, 7.750, 3.438 and 5.803 fm, respectively. Electrochemical Characterizations. The electrochemical tests were performed with CR2032 coin cells and a battery-testing system Arbin BT2000. The oxide electrodes were fabricated with mixtures of active materials, acetylene black, and polytetrafluorethylene (PTFE) in 60:30:10 weight percentages. A typical electrode is a 7-mm diameter circular disk of 5-10 mg. 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (50:50 by volume) was used as an electrolyte and lithium-metal foil (14-mm-diameter circular disk) was used for both the counter and reference electrodes. Assembling electrodes, separator, and electrolyte as well as sealing of the cells were carried out inside an Arfilled glove box. The typical charge and discharge rate was 0.2 C. Magnetic Characterizations. The dc magnetic susceptibility was measured under an applied magnetic field of 0.1 T in the temperature range 5-300 K with a commercial superconducting quantum interference device (SQUID) magnetometer (Quantum Design). Optical Characterizations. Optical diffuse reflectance measurements were performed at room temperature with a Shimadzu UV-3101PC double-beam, doublemonochromator spectrophotometer. The instrument is

RESULTS Orthorhombic La2CuO4 contains two LaO rock-salt layers alternating with CuO2 sheets; the resulting cornershared CuO6 octahedra undergo a cooperative rotation about the tetragonal [010] axis, which lowers the symmetry to orthorhombic. The layered LnNaTiO4 compounds are obtained by substituting one Na+ for one Ln3+ and Ti(IV) for Cu2+; the Ln3+ and Na+ ions order into alternate rock-salt layers as is illustrated in figure 1(a). This order introduces internal electric fields perpendicular to the layers as is illustrated in figure 1(b). The powder X-ray diffraction patterns for NaLnTiO4 and LiLnTiO4 (Ln = Y, La, Pr, Nd, Sm, Eu, Gd and Tb) are shown in figure 2; their crystal structure refinements were carried out by the Rietveld method with the FULLPROF program based on the crystallographic data in the previously reported paper on NaYTiO421. Although there are no literature data for the crystal structure of LiYTiO4, it can be refined well in the orthorhombic model with space group Pbcm; the results of the refinement from NPD data are included in Table I. With respect to Li1+xYTiO4, containing interstitial Li, a first refinement was also carried out in the same space group based upon the LiYTiO4 structural model. The small discrepancies between observed and calculated intensities indeed contain information on the remaining scattering density from the interstitial Li atoms that was not considered in the starting model. A Fourier synthesis performed over the difference between observed and calculated structure factors yields information on the location of the “missing” atoms. A difference Fourier map at this step allowed us the unambiguous location of Li atoms at 8e (x,y,z) positions, x∼0.40, y∼0.45, z∼0.20. After introduction of Li in the structural model, the refined occupancy factor of this site was around 1.0 per formula unit. Figure 4c illustrates the good agreement between the observed and calculated NPD patterns at room temperature for the new intercalation compound. Although they all share the same space group, the lattice parameters along the long axis for the Na compounds are longer than those for Li compounds, whereas the other two axes in the perpendicular planes are shorter for the Na compounds than for the Li compounds. The variation of the lattice parameters along the long axis a,


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exchanged samples of LiLnTiO4 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb).

and short axis c, with the size of substituted rare earth ions are shown in figure 3.

Figure 1. (a) Representation of the crystal structure of NaYTiO4 and (b) the internal electric field formed along the x-axis (the arrows and numbers indicate the internal electric field and the charges for each bilayer of (NaO)22and (YO)22+, respectively.)

Figure 3. Variations of the lattice parameters of NaLnTiO4 and LiLnTiO4 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb) as a function of the radii of lanthanide ions.

Figure 4. The observed (red circles), calculated (black line), and the difference (blue line) X-ray and neutron-diffraction profiles: (a) NaYTiO4; (b) LiYTiO4; (c) Li1+xYTiO4 (x≈1). The green vertical lines are the expected peak positions.

Figure 2. XRD patterns of the prepared samples (space group Pbcm): (a) parent samples of NaLnTiO4; (b) ion-



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As the radius of the rare-earth ion increases from Y to La, the lattice parameter a increases monotonically; however, the lattice parameter c shows a minimum at Ln = Nd. To locate the position of Li, neutron diffraction analysis has been performed for the ion-exchanged sample LiYTiO4 and the intercalated sample Li1+xYTiO4 (x ≈ 1), which was fully discharged down to 0.01 V versus Li+/Li0. The resultant refined parameters of X-ray and neutron diffraction patterns for the parent Na compound (NaYTiO4), the ion-exchanged compound (LiYTiO4), and the Li-inserted compound (Li1+xYTiO4, (x ≈ 1)) are shown in figure 4 and Table I; the crystal structures of LiYTiO4 and Li1+xYTiO4, based on the refined data, are shown in figure 5. According to the refinements, the Na+ ions are coordinated by 9 oxygen atoms (figure 1) whereas the smaller ion-exchanged Li+ ions are located in tetrahedral oxygen sites to form a single antifluorite layer, which enlarges the cell parameters b and c and shortens a. The Ln3+ ions remain in (LnO)2 rock-salt layers coordinated with 9 oxygen atoms. In both the Na and Li compounds, the positions of the Ti atoms in the TiO6/2 octahedra are displaced from the center of their site toward the (NaO)22- or (Li2O2)2- layer and away from the (LnO)22+ layer in response to the internal electric field between a negatively charged (NaO)22- or (Li2O2)2- layer and a positively charged (LnO)22+ layer (figure 1b). The magnitude of the Ti(IV) displacements can be assumed to be an indicator of the strength of the internal electric field perpendicular to the bilayers. Therefore, the ratio of the long to short Ti-O bond lengths on the displacement axis is also given in Table I. After Li intercalation to x ≈ 1 in Li1+xYTiO4, the displacement ratio Ti-O(3)/Ti-O(4) drops from 1.27 for x = 0 to 0.94 for x ≈ 1, consistent with reduction of the a-axis internal electric field and reduction of Ti(IV) to Ti(III); ordering of the Ti(III) electron into a yz orbital allows xy, zx bonding to provide some Ti(III) displacement. Figure 6 shows the voltage profiles of the Li/LiYTiO4 cell test under a constant current (0.2 C) over the voltage range 0.01≤V≤2.5 V versus Li+/Li0. A solidelectrolyte interface (SEI) layer is formed in the 1st discharge below 1.0 V; subsequent charge/discharge profiles showed reversible plateaus below 0.5 V. At least two plateaus can be seen near 0.1 and 0.3 V; the one near 0.1 V is flatter than the one at higher voltage. Very similar behaviors were observed with other Ln compounds. Figure 7 shows the variation of the 5th charge curves of Na and Li compounds have similar features; however, the cyclability of the Li compounds is better than that of the Na compounds, as is shown in figure 8. Except for the Eu compounds, the plateaus are interpreted to correspond to the Ti(IV)/Ti(III) redox couple, not the Ln3+/Ln2+ couple, based on the fact that the shape and position of the plateaus are almost the same regardless of the rare-earth ion. The Eu compounds show a plateau about 0.8 V versus Li+/Li0

Figure 5. Representation of the crystal structure; (a) LiYTiO4 and (b) Li1+xYTiO4 (x≈1).

Figure 6. Galvanostatic charge-discharge voltage profiles of LiYTiO4 between 0.5 and 0.01 V at a rate of 0.2 C for 5 cycles; inset is the enlarged view of the profiles after 1st discharge.

showing access to the Eu3+/Eu2+ instead of the Ti(IV)/Ti(III) couple (figure 9.) This deduction is also reflected in the magnetic properties. Figure 10 shows the temperature dependent inverse magnetic susceptibilities of Li1+xYTiO4 and Li1+xEuTiO4 (x ≈ 1), respectively. The effective magnetic moment for each formula unit supports reduction of Ti(IV) to Ti(III) for the Y compound and of Eu3+ to Eu2+ for the Eu compounds, viz 1.89 µB/Ti(III) and 7.47 µB/Eu(II). More details on the Eu compounds will be reported in a separate paper. The positions of the reversible plateaus observed in these compounds are at least 1 V lower than those of the Ti(IV)/Ti(III) redox couples observed in other titanium oxide compounds reported in the literature; their operative Ti(IV)/Ti(III) redox couples are 1.3-2.5 V versus Li+/Li0 depending on the crystal structure and counter ion24-32. In order to test whether a change in the Madelung energy is responsible for raising the energy of the Ti(IV)/Ti(III) couple, we measured the band gap of LiYTiO4 with optical diffuse reflectance


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Figure 7. Galvanostatic voltage profiles versus lithium content after 4 cycles between 0.5 and 0.01 V at a rate of 0.2 C for C-coated samples; (a) NaLnTiO4; (b) LiLnTiO4 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb).

Figure 10. Temperature dependence of the inverse magnetic susceptibility of Li1+xYTiO4 and Li1+xEuTiO4 (x≈1); effective magnetic moment (µeff) for each formula unit was obtained from the Curie constant of Curie-Weiss model by fitting the high temperature region over 100 K.

Figure 8. Cycling performance at a rate of 0.2 C for Ccoated samples; (a) NaLnTiO4; (b) LiLnTiO4 (Ln= Y, La, Pr, Nd, Sm, Eu, Gd, Tb).

Figure 11. (a) Diffuse reflectance spectra of TiO2 (rutile) TiO2 (anatase), Li4Ti5O12 and LiYTiO4 and (b) variation of band gap energies of NaLnTiO4 and LiLnTiO4; The band gap was determined from the Absorbance versus Photon energy graph by linear extrapolation from the region of maximum slope down to the Photon energy axis (inset).

and compared it with those of the rutile and anatase TiO2, and the spinel Li4Ti5O12; the optical band gaps obtained from the absorption edge for each sample are 3.06, 3.37, 3.81 and 3.95 eV for TiO2 (rutile), TiO2 (anatase), Li4Ti5O12 and LiYTiO4, respectively. As for the Li4Ti5O12, very similar behavior was reported in the previously reported paper33; in which the low-energy shoulder band near 3 eV was attributed to the splitting of the t2g band due to the tetragonal distortion of the oxygen octahedra. The variation of the band gap with

Figure 9. Comparison of the charge-discharge voltage profiles after 4 cycles at a rate of 0.2 C between LiYTiO4 and LiEuTiO4.



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the Ln3+ ion is shown in figure 11(b) for the NaLnTiO4 and LiLnTiO4 compounds. The more basic character of the Na+ ion increases the band gap a little (< 0.3 eV). The smaller band gaps of Ln = Pr and Tb are caused by 4fn configurations above the top of the O-2p bands to give the respective charge transfers 4f2→5d1 and 4f8→ 5d1 for Pr and Tb34, 35. These results show that the global Madelung energy cannot account for the low Ti(IV)/Ti(III) voltage observed in the half-cell tests of the Li1+xLnTiO4 compounds with Ln ≠ Eu.

Finally, the ability to shift relative energies in a layered oxide by an internal electric field, as has been pointed out in the magnetoplumbites in 198736, is a strategy that has not been exploited in the design of technical materials. CONCLUSION The layered perovskite compounds NaLnTiO4 (Ln = Y and La to Tb) contain Ln2O2 and Na2O2 rock-salt layers that alternate with one another either side of TiO2 sheets. This cation arrangement creates an internal electric field perpendicular to the layers that raises the energies of the O-2p6 and Ti-3d0 bands equally by approximately 1 eV. Ion exchange of Li+ for Na+ changes the structure, but not the charge of the alkaliion layers, from rock-salt to antifluorite. Reversible Li insertion into the alkali-ion layers provides a measure of the shift of the Ti(IV)/Ti(III) redox energy, and optical diffuse reflectance shows the energy gap between the bottom of the Ti:3d0 and the top of the O:2p6 bands changes little between Li4Ti5O12 and LiYTiO4. Modeling the energy shift as a capacitance energy of a parallel-plate condenser can account for the shift in band energies. The 5d0 band edge of the Ln3+ ions remains above the Ti(IV)/Ti(III) redox energies, but the 4f6/4f7 Eu3+/Eu2+ redox energy is about 0.3 eV below the bottom of the Ti:3d0 band. Tuning of energy levels with internal electric fields in layered compounds is a strategy waiting to be exploited.

DISCUSSION The two interesting observations are (1) an increase of both the O-2p and Ti(IV)-3d0 energies by about 1 eV relative to their positions in other titanium oxides and (2) a 4f6/4f7 Eu3+/Eu2+ redox energy about 0.3 eV below the Ti-3d0 band edge, but a Ti(IV)/Ti(III) redox couple below the Ln3+-5d0 band edge. The first observation comes from the combined results of the electrochemical cell tests and the band-gap measurements, which show that the splitting of the bottom of the Ti-3d0 band and the top of the O-2p6 band is not changed significantly between the spinel and layered titanates; both are raised by about 1 eV in the layered oxides. The essential difference between the two structures is the existence of an internal electric field perpendicular to the layers in the layered titanates. Our structural analysis indicates that this electric field is responsible for a ferroic displacement of the Ti(IV) ions toward the negatively charged alkali-ion layers and that the magnitude of the electric field differs little with the position of the Na+ versus Li+ in the alkali-ion layers. From figure 1, we can model the expected energy shift from the capacitance of a parallel-plate condenser charged positively in the Ln2O2 layers and negatively in the alkali-ion layers. From Table I we obtain for NaYTiO4

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interest.

2 2

4 77 ≅ V 2 2

ACKNOWLEDGMENT The authors would like to thank Jian-Shi Zhou for his help with the magnetic measurements. This work was supported by the Robert A. Welch Foundation (Grant No. F-1066). JAA thanks the financial support of the Spanish Ministry of Education to the project MAT2010-16404.

where A = bc, d = a/2, Q ≈ 2e. An energy shift of about 1 eV would correspond to an r ≈ 77. The second observation is consistent with the observation that the Eu3+/Eu2+: 4f6/4f7 redox energy is about 1 eV below the Eu: 5d0 band edge in EuO. Moreover, a magnetic moment of 7 µB on the Eu2+ ion signals the electrons introduced into Li1+xEuTiO4 by the inserted Li are localized, creating a Eu: 4f7 6 1 configuration and not a 4f 5d configuration.

REFERENCES 1. Padhi, A. K. Ph.D. thesis, The University of Texas at Austin, Austin, TX, 1997. 2. Goodenough, J. B. Mol. Crysr. Liq. Cryst. 1998, 311, 409. 3. Dampier, F. W. J. Electrochem. Soc. 1974, 121, 656. 4. Dickens, P. G.; French, S. J.; Hight, A. T.; Pye, M. F. Mater. Res. bull. 1979, 14, 1295. 5. Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. bull. 1980, 15, 783.


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6. Thackeray, M. M.; Johnson, P. J.; de Picciotto, L. A.; Bruce, P. G.; Goodenough, J. B. Mater. Res. bull. 1984, 19, 179. 7. Manthiram, A.; Goodenough, J. B. J Solid State Chem. 1987, 71, 349. 8. Dahn, J. R.; Sacken, U.v.; Michal, C. A. Solid State Ionics 1990, 44, 87 9. Barboux, P.; Tarascon, J. M.; Shokoohi, F. K. J. Solid State Chem. 1991, 94, 185. 10. Ferg, E.; Gummow, R. J.; de Kock, A.; Thackeray, M. M. J. Electrochem. Soc. 1994, 141, L147. 11. Masquelier, C.; Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Solid State Chem. 1998, 135, 228. 12. Robertson, A. D.; Trevino, L.; Tukamoto, H.; Irvine, J. T. S. J. Power Sources 1999, 81-82, 352. 13. Whittingham, M. S. Chem. Rev. 2004, 104, 4271. 14. Colbow, K. M.; Dahn, J. R.; Haering, R. R. J. Power Sources 1989, 26, 397. 15. Ohzuku, T.; Ueda, A.; Yamamoto, N. J. Electrochem. Soc. 1995, 142, 1431. 16. Kavan, L.; Gratzel, M. Electrochem Solid-State Lett. 2002, 5, A39. 17. Nakahara, K.; Nakajima, R.; Matsushima, T.; Majima, H. J. Power Sources 2003, 117, 131. 18. Wang, G. J.; Gao, J.; Fu, L. J.; Zhao, N. H.; Wu, Y. P.; Takamura, T. J. Power Sources 2007, 174, 1109. 19. Rietveld, H.M. J. Appl. Crystallogr. 1969, 2, 65. 20. Pankove, J. I. In Optical Processes in Semiconductors; Dover Publications: New York, 1975. 21. Toda, K.; Kameo, Y.; Kurita, S.; Sato, M. J. Alloys Comp. 1996, 234, 19.

22. Toda, K.; Kurita, S.; Sato, M. J. Ceram. Soc. Jpn. 1996, 104, 140. 23. Ozawa, T.C.; Ikoshi, A.; Taniguchi, T.; Mizusaki, S.; Nagata, Y.; Noro, Y.; Samata, H. J. Alloys Comp. 2008 448, 38. 24. Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 2581. 25. Garnier, S.; Bohnke, C.; Bohnke, O.; Fourquet, J. L. Solid State Ionics 1996, 83, 323. 26. Mukai, K.; Ariyoshi, K.; Ohzuku, T.; J Power Sources 2005, 146, 213. 27. Huang, S.; Wen, Z.; Zhu, X.; Lin, Z. J Power Sources 2007, 165, 408. 28. Wang, B. L.; Chen, Q.; Hu, J.; Li, H.; Hu, Y. F.; Peng, L. Chemical Physics Letters 2005, 406, 95. 29. Yang, J. J. Mater. Sci. 2005, 40, 3765. 30. Dominko, R.; Baudrin, E.; Umek, P.; Arčon, D.; Gaberšček, M.; Jamnik, J. Electrochem Commun. 2006, 8, 673. 31. Dambournet, D.; Belharouak, I.; Amine, K. Inorg. Chem. 2010, 49, 2822. 32. Kataoka, K.; Awaka, J.; Kijima, N.; Hayakawa, H.; Ohshima, K.; Akimoto, J. Chem. Mater. 2011, 23, 2344. 33. Kellerman D. G.;Munkhina N. A.; Zhuravlev, N. A.; Valoca, M. S.; Gorshkov, V. S. Phys. Solid State 2010, 52, 459. 34. White, W. B. Applied Spectroscopy 1967, 21, 167. 35. Lal, H. B.; Gaur, K. J. Mater. Sci. 1988, 23, 919. 36. Fontcuberta, J.; Obradors, X.; Goodenough, J. B. J. Phys. C.: Solid State Phys. 1987, 20, 441.



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Table I. Structural Parameters of NaYTiO4 and Li1+xYTiO4 (0≤x≤1). BISO (Å2)

Ti-O(3)

Ti-O(4)

! "3 ! "4

1.31(1)

2.336(9) Å

1.721(9) Å

1.36

z

B (Å2)

Ti-O(3)

Ti-O(4)

! "3 ! "4

0.2500

0.0000

0.60(2)

0.8812(3)

0.0354(8)

0.2500

0.22(5)

0.2780(5)

0.0101(17)

0.2500

0.04(9) 2.314(8) Å

1.825(7) Å

1.27

BISO (Å2)

Ti-O(3)

Ti-O(4)

! "3 ! "4

2.06(6)

2.02(4)Å

2.18(5) Å

0.94

X-ray diffraction

Atom

Site

x

y

z

NaYTiO4

Na

4d

0.5904(4)

0.0286(11)

0.2500

Pbcm (No. 57)

Y

4d

0.8922(2)

0.0279(7)

0.2500

a = 12.2136 (6) Å

Ti

4d

0.2611(4)

0.0092(14)

0.2500

b = 5.3521(4) Å

O(1)

4c

0.2067(6)

0.2500

0.0000

c = 5.3488 (4) Å

O(2)

4c

0.7561(6)

0.2500

0.0000

Rwp = 21.9 %

O(3)

4d

0.0747(6)

-0.0890(16)

0.2500

Rp = 15.6 %

O(4)

4d

0.3991(6)

0.0743(16)

0.2500

Neutron diffraction

Atom

Site

x

y

LiYTiO4

Li

4d

0.5094(1)

Pbcm (No. 57)

Y

4d

a = 11.0134 (3) Å

Ti

4d

b = 5.3900 (2) Å

O(1)

4c

0.2301(6)

0.2500

0.0000

1.25(9)

c = 5.3891 (2) Å

O(2)

4c

0.7345(5)

0.2500

0.0000

0.98(9)

Rwp = 8.39 %

O(3)

4d

0.0711(5)

-0.0647(14)

0.2500

1.4(1)

Rp = 6.00 %

O(4)

4d

0.4428(4)

0.0455(14)

0.2500

0.95(7)

Neutron diffraction

Atom

Site

x

y

z

Li1+xYTiO4 (x≈1)

Li1

4c

0.509(1)

0.250

0.000

Pbcm (No. 57)

Li2

8e

0.410(2)

0.459(4)

0.154(6)*

a = 10.777 (7) Å

Y

4d

0.880(2)

0.034(5)

0.2500

b = 5.524 (3) Å

Ti

4d

0.257(4)

-0.033(16)

0.2500

c = 5.518 (2) Å

O(1)

4c

0.2322(2)

0.2500

0.0000

Rwp = 5.24 %

O(2)

4c

0.7274(2)

0.2500

0.0000

Rp = 4.10 %

O(3)

4d

0.0724(2)

-0.0840(6)

0.2500

RB = 6.47 %

O(4)

4d

0.4491(3)

0.0963(7)

0.2500

RB = 7.45 %

RB = 7.35 %

* The occupancy factor for Li2 is 0.26(1) (i.e. 1.04(4) per formula unit)


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Effect of an Internal Electric Field on the Redox Energies of ALnTiO4

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