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2005, 109, 1337-1339 Published on Web 01/11/2005
Near-edge X-ray Absorption Fine Structure Study of Disordering in Gd2(Ti1-yZry)2O7 Pyrochlores Ponnusamy Nachimuthu,*,†,‡ Suntharampillai Thevuthasan,§ Evan M. Adams,§ William J. Weber,§ Bruce D. Begg,| Bongjin S. Mun,‡ David K. Shuh,‡ Dennis W. Lindle,† Eric M. Gullikson,‡ and Rupert C. C. Perera‡ Department of Chemistry, UniVersity of NeVada, Las Vegas, NeVada 89154, Lawrence Berkeley National Laboratory, Berkeley, California 94720, Pacific Northwest National Laboratory, Richland, Washington 99352, and Australian Nuclear Science and Technology Organization, Menai, New South Wales 2234, Australia ReceiVed: NoVember 15, 2004; In Final Form: December 17, 2004
Disorder in Gd2(Ti1-yZry)2O7 pyrochlores, for y ) 0.0-1.0, is investigated by Ti 2p and O 1s near-edge X-ray absorption fine structure spectroscopy. Ti4+ ions are found to occupy octahedral sites in Gd2Ti2O7 with a tetragonal distortion induced by vacant oxygen sites. As Zr substitutes for Ti, the tetragonal distortion decreases, and Zr coordination increases from 6 to 8. The migration of oxygen ions from 48f or 8b sites to vacant 8a sites compensate for the increased Zr coordination, thereby reducing the number of vacant 8a sites, which further reduces the tetragonal distortion and introduces more disorder around Ti. This is evidence for simultaneous cation disorder with anion migration.
The use of pyrochlore-structured oxides in solid oxide fuel cells and as host matrices for actinide-rich wastes is receiving increasing attention because of recent discoveries showing that the isovalent substitution of Zr for Ti in Gd2Ti2O7 results in an increase of 4 to 5 orders of magnitude in the oxygen ion conductivity at 875 K and structural resistance to damage by energetic particle irradiation.1-8 Several experimental investigations using neutron diffraction, Raman and IR spectroscopies, extended X-ray absorption fine structure, and theoretical studies based on molecular dynamics and atomistic simulations have been carried out to understand the mechanisms responsible for the changes in oxygen ion conductivity and radiation tolerance in these materials.1-15 These studies show that the increase in the ionic conductivity in pyrochlore is most likely due to the increased oxygen vacancies at the 48f site as a result of cation and anion disordering.6-8 The increased radiation tolerance is attributed to the ease of rearrangement and relaxation of Gd, Zr, and O ions/defects within the crystal structure, which inhibits amorphization by causing the irradiation-induced defects to relax and form cation antisite defects and anion Frenkel defects.2-5 There is limited direct experimental evidence for the presence of cation antisite disorder in a highly ordered pyrochlore structure.1-15 Recently, Chen et al.16 reported on the disorder in Gd2(Ti1-yZry)2O7 pyrochlores measured by X-ray photoelectron spectroscopy (XPS). In the O 1s XPS spectra for Gd2Ti2O7, a broad feature with two components at binding energies (BEs) of ∼526 and 531 eV was observed, and these two distinct peaks * Corresponding author. E-mail address:
[email protected]. † University of Nevada. ‡ Lawrence Berkeley National Laboratory. § Pacific Northwest National Laboratory. | Australian Nuclear Science and Technology Organization.
10.1021/jp0447789 CCC: $30.25
merged into a single broad peak when 25% Ti was replaced by Zr, suggesting that the environments of oxygen ions have become similar. Recent work17 on a Gd2Ti2O7 single crystal using site-selective near-edge X-ray absorption fine structure (NEXAFS) and XPS showed no anisotropic distribution of Ti and O sites, and additional collaborative studies17 resolved that the discrepancies in the previous reports16 result from charging effects during XPS and surface contamination. Further study is needed to understand disorder in the Gd2(Ti1-yZry)2O7 pyrochlores and its role in conductivity and radiation tolerance. Gd2(Ti1-yZry)2O7 (y ) 0-1) pyrochlores were prepared by a sol-gel route using titanium isopropoxide, tetrabutyl zirconate, and gadolinium nitrate. The solutions were stir-dried and calcined at 975 K for 1 h in air. The calcined material was wetball milled, dried, pressed into pellets, and sintered in air at 1475 K for 12 h. The pellets were powdered and underwent the same processing steps before sintering in air at 1775 K for 30 h. The pellets were coated with boron nitride, encapsulated in glass, and hot-isostatically pressed under Ar at 1775 K for 2 h at 200 MPa to produce fully dense materials. The materials were annealed in air at 1625 K for 48 h to recover oxygen stoichiometry. Glancing-incidence X-ray diffraction revealed single-phase polycrystalline materials with the ordered pyrochlore structure.11,12 These ordered pyrochlores in general exhibit A2B2O6O′ (Fd3m) stoichiometry.8,9 The pyrochlore structures are derived from the fluorite structure, but with two cations and one-eighth fewer anions. Each unit cell of the pyrochlore structure contains eight formula units (Z ) 8) and four nonequivalent sites. By fixing the origin at the B cation as in Figure 1, the atoms A, B, O, and O′ occupy 16d, 16c, 48f, and 8b sites, respectively. In Gd2Ti2O7, Gd3+ occupies the eight-coordinate A (16d) sites with six 48f and two 8b oxygen anions forming a distorted cube, © 2005 American Chemical Society
1338 J. Phys. Chem. B, Vol. 109, No. 4, 2005
Letters
Figure 1. Partial unit cell structure of Gd2Ti2O7 pyrochlore.
whereas Ti4+(Zr4+) occupies six-coordinate B (16c) sites, which lie adjacent to the vacant 8a anion sites, forming a distorted octahedron with oxygen anions from 48f sites. Each oxygen anion in the 48f and 8b sites is tetrahedrally coordinated with two each of Gd3+ and Ti4+ and with four Gd3+ cations, respectively. The remaining unoccupied 8a oxygen site is surrounded by four Ti4+(Zr4+) cations. Furthermore, the 48f anions are shifted toward the smaller Ti4+(Zr4+) cations by an amount defined by the x positional parameter.1-15 Ti 2p and O 1s NEXAFS spectra of Gd2(Ti1-yZry)2O7 (y ) 0.0, 0.25, 0.50, 0.75, 1.0) were recorded at beamlines 6.3.2 and 9.3.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory.18 NEXAFS is a powerful tool for understanding the electronic and structural properties of materials.19 X-ray absorption originates because of the excitation of atomic core electrons to unoccupied electronic states, which can make NEXAFS atom-specific.19 Among the various experimental techniques employed in determining the local electronic structure and the symmetry of an atom of solids, NEXAFS plays a crucial role due to its simplicity and near-universal applicability.19 Single crystals of TiO2(110) (rutile), SrTiO3(100), yttriastabilized zirconia (Y-ZrO2), and Gd2O3 powder served as reference materials. Bulk-sensitive total fluorescence yield (TFY) signal was collected with a photodiode. Spectra were corrected for the photon flux and normalized to the edge jumps. The photon energy was calibrated to the Ti 2p3/2(t2g) peak at 457.9 eV and the O 1s pre-edge transition from SrTiO3(100) at 530.8 eV.19 Energy resolution was 125 meV at 530 eV.18 The Ti 2p and O 1s NEXAFS spectra of Gd2(Ti1-yZry)2O7 (y ) 0.0-1.0) and the references are shown in Figures 2-3. Ti 2p and O 1s NEXAFS features are assigned on a symmetrydetermined molecular orbital model obtained using the linear combination of atomic orbital method for the octahedral (TiO6)8- ion cluster in TiO2.17,19 The Ti 2p and O 1s NEXAFS, in general, probe the unoccupied 2t2g (Ti 3d + O 2pπ), 3eg (Ti 3d + O 2pσ), 3a1g (Ti 4s + O 2pσ), and 4t1u (Ti 4p + O 2pπ) states. The ground states of TiO2, Gd2(Ti1-yZry)2O7, and SrTiO3 consist mainly of Ti4+: (2p3/2,1/2)3d0-O2-: 2p6. The spin-orbit interaction splits the Ti 2p state into 2p3/2(L3) and 2p1/2(L2) states ∼5.4 eV apart. Thus, the L3 and L2 transitions in TiO2, Gd2(Ti1-yZry)2O7, and SrTiO3 predominantly result from transitions to the final states, Ti4+: (2p3/2,1/2)-13d1-O2-: 2p6, where (2p3/2,1/2)-1 denotes a hole in the 2p3/2 or 2p1/2 state. The t2g and eg states stem from transitions to the final states, Ti4+: (2p3/2)-13d(2t2g)1-O2-: 2p6 and Ti4+: (2p3/2)-13d(3eg)1-O2-: 2p6, respectively. The separation between t2g and eg states is related to the crystal field strength. The eg states, which consist of dz2 and dx2-y2 orbitals, are directed toward ligand anions and are therefore more sensitive to deviations from Ti octahedral symmetry. Consequently, the splitting of eg states into dz2 and dx2-y2 indicates the degree of distortion from octahedral symmetry. Previous Gd2Ti2O7(100) NEXAFS and XPS showed no
Figure 2. Titanium 2p NEXAFS of TiO2 (rutile), Gd2(Ti1-yZry)2O7 (y ) 0.0, 0.25, 0.5, 0.75), and SrTiO3.
Figure 3. Oxygen 1s NEXAFS of TiO2 (rutile), SrTiO3, Gd2(Ti1-yZry)2O7 (y ) 0.0, 0.25, 0.5, 0.75, 1.0), yttria-stabilized zirconia (Y-ZrO2), and Gd2O3.
anisotropic distribution of Ti and O sites; however, Ti4+ ions in Gd2Ti2O7 occupy octahedral sites with a tetragonal distortion, induced by vacant 8a oxygen sites in the ab plane adjacent to the TiO6 octahedron.17 The Ti 2p NEXAFS show the same features for single-crystal and polycrystalline Gd2Ti2O7;17 however, isovalent substitutions of Zr for Ti in Gd2Ti2O7 yield differing spectra. Figure 2 shows the splitting of the eg states into dz2 and dx2-y2, which indicates the degree of distortion from octahedral site symmetry, decreasing with increasing Zr substitution in Gd2(Ti1-yZry)2O7. Fitting transitions to the eg states using two Gaussians with constant bandwidths provides the separation between the dz2 and dx2-y2 orbitals, which are 1.0, 0.8, 0.6, and 0.4 eV for Gd2(Ti1-yZry)2O7 for y ) 0.0, 0.25, 0.5, and 0.75, respectively. This energy separation is 1.2 eV for TiO2. Ti in SrTiO3 has perfect octahedral site symmetry, and the transitions to the eg states are not split, although the profiles are slightly asymmetric. Fitting the
Letters transitions to the SrTiO3 eg states yields a separation of ∼0.5 eV, which is close to that of Gd2(Ti1-yZry)2O7 with y ) 0.75 and suggests that at this composition there is no distortion in the TiO6 octahedron. Intensities of the transitions to the eg states relative to those to the t2g states are enhanced by the increasing overlap of the transitions to dz2 and dx2-y2 states with increasing Zr substitution in Gd2(Ti1-yZry)2O7, although the ratio of the area under the bands and the energy separation between the t2g and eg states are constant within experimental error. The increasing substitution of Zr for Ti in Gd2(Ti1-yZry)2O7 gradually introduces disorder around Ti by decreasing the tetragonal distortion in the TiO6 octahedron, although Ti retains its octahedral site symmetry in all compositions. The t2g and eg transitions in the O 1s NEXAFS for TiO2, SrTiO3, and Gd2(Ti1-yZry)2O7 are to the final states, Ti4+: 3d(2t2g)1-O2-: (1s)-12p6 and Ti4+: 3d(3eg)1-O2-: (1s)-12p6, respectively, where (1s)-1 denotes a hole in the O 1s shell (Figure 3).17,19 The energy separation between t2g and eg states is 2.8, 2.7, and 2.5 eV for TiO2, Gd2Ti2O7, and SrTiO3, respectively. This is a direct measure of the crystal field strength from the TiO6 octahedron. Comparison of the O 1s NEXAFS of Gd2Ti2O7 with TiO2, SrTiO3, and Gd2O3 suggests that the oxygen ions coordinated to Gd3+ contribute to the intensity in the vicinity of eg states (533.3 eV) derived from the TiO6 octahedron for Gd2Ti2O7. Intensities of the transitions to t2g and eg states of TiO6 octahedron decrease, and the oxygen ions coordinated to Zr increasingly contribute to the intensity in the vicinity of the transitions to eg states in addition to those originating from Ti-O and Gd-O. This transition shifts systematically to lower energy with increasing Zr substitution for Ti. The systematic shift toward lower energy of the transition at 533.3 eV for y ) 0 to 532.5 eV for y ) 1 shows an increase of Zr coordination from 6 to nearly 8 as Zr substitutes for Ti. This is also substantiated by a transition observed at 532.2 eV for Y-ZrO2 in which Zr is in cubic symmetry with 8 oxygen ions.20 Both Zr-O and Gd-O contribute to the intensity of the transition at ∼532.5 eV for Gd2Zr2O7; thus, the Gd-O character shifts the transition ∼0.3 eV toward higher energy compared to the transition at ∼532.2 eV for Y-ZrO2. On the basis of the crystal structure information,1-15 Ti4+(Zr4+) occupies the 6-coordinate B (16c) sites adjacent to the vacant 8a oxygen sites, forming an octahedron with oxygen ions from 48f sites. The O 1s NEXAFS show an increase in coordination of Zr from 6 to nearly 8 with increasing substitution of Zr for Ti in Gd2(Ti1-yZry)2O7. Further, to compensate for the increased coordination of Zr, the vacant 8a oxygen sites adjacent to the Ti4+(Zr4+) coordination sphere need to be filled systematically. The oxygen ions, either from 48f or 8b, sites migrate to the vacant 8a oxygen sites, and thus provides evidence for the anion disorder. Furthermore, the vacant 8a oxygen sites are known to induce the tetragonal distortion in the TiO6 octahedron in Gd2Ti2O7.17 The migration of oxygen ions either from 48f or 8b sites to the vacant 8a oxygen sites reduces the number of the vacant 8a oxygen sites, which in turn should decrease tetragonal distortion in the TiO6 octahedron with increasing substitution of Zr for Ti in Gd2(Ti1-yZry)2O7. The decrease of tetragonal distortion in the TiO6 octahedron is indeed found in the Ti 2p NEXAFS with increasing Zr substitution for Ti in Gd2(Ti1-yZry)2O7. The Ti 2p and O 1s NEXAFS spectra show that Ti4+ ions in Gd2Ti2O7 occupy octahedral sites with a tetragonal distortion, which is induced by the vacant 8a oxygen sites located in ab plane adjacent to TiO6 octahedron. With increasing substitution of Zr for Ti in Gd2(Ti1-yZry)2O7, the tetragonal distortion in
J. Phys. Chem. B, Vol. 109, No. 4, 2005 1339 the TiO6 octahedron decreases, and coordination of Zr increases from 6 to nearly 8. Migration of oxygen ions either from 48f or 8b sites to vacant 8a oxygen sites to compensate the increased coordination of Zr reduces the number of vacant 8a oxygen sites, which decreases tetragonal distortion in the TiO6 octahedron with increasing substitution of Zr for Ti in Gd2(Ti1-yZry)2O7. A decrease of tetragonal distortion in the TiO6 octahedron gradually introduces more disorder around Ti with increasing substitution of Zr in Gd2(Ti1-yZry)2O7. This confirms that the disorder around the cation occurs simultaneously with anion migration. Acknowledgment. This work was supported by Nevada DOE EPSCoR State-National Laboratory Partnership under grant no. DE-FG02-01ER45898; the DOE BES Division of Materials Sciences and Engineering and the DOE BER EMSL User Facility at Pacific Northwest National Laboratory under contract no. DE-AC06-76RLO-1830; and the DOE BES Divisions of Materials Sciences and Engineering at the Advanced Light Source and Chemical Sciences, Geosciences, and Biosciences under contract no. DE-AC03-76SF00098 at Lawrence Berkeley National Laboratory. References and Notes (1) (a) Goodenough, J. B. Nature 2000, 404, 821. (b) Lacorre, P.; Goutenoire, F.; Bohnke, O.; Retoux, R.; Lallgant, Y. Nature 2000, 404, 856. (2) Ewing, R. C.; Weber, W. J.; Lian, J. J. Appl. Phys. 2004, 95, 5949. (3) Lian, J.; Wang, L. M.; Wang, S. X.; Chen, J.; Boatner, L. A.; Ewing, R. C. Phys. ReV. Lett. 2001, 87, 145901. (4) Purton, J. A.; Allan, N. L. J. Mater. Chem. 2002, 12, 2923. (5) (a) Sickafus, K. E.; Minervini, L.; Grimes, R. W.; Valdez, J. A.; Ishimaru, M.; Li, F.; McClellan, K. J.; Hartmann, T. Science 2000, 289, 748. (b) Weber, W. J.; Ewing, R. C. Science 2000, 289, 2051. (6) Wuensch, B. J.; Eberman, K. W. JOM 2000, 52, 19. (7) (a) Tuller, H. L. J. Electroceram. 1997, 1, 211. (b) Tuller, H. L. Solid State Ionics 1992, 52, 135. (c) Moon, P. K.; Tuller, H. L. Solid State Ionics 1988, 28-30, 470. (8) (a) Subramanian, M. A.; Aravamudan, G.; Subba Rao, G. V. Prog. Solid State Chem. 1983, 15, 55. (b) Tabira, Y.; Withers, R. L. Philos. Mag. A 1999, 79, 1335. (9) Minervini, L.; Grimes, R. W.; Sickafus, K. E. J. Am. Ceram. Soc. 2000, 83, 1873. (10) Heremans, C.; Wuensch, B. J.; Stalick, J. K.; Prince, E. J. Solid State Chem. 1995, 117, 108. (11) (a) Begg, B. D.; Hess, N. J.; Weber, W. J.; Devanathan, R.; Icenhower, J. P.; Thevuthasan, S.; McGrail, B. P. J. Nucl. Mater. 2001, 288, 208. (b) Begg, B. D.; Hess, N. J.; McCready, D. E.; Thevuthasan, S.; Weber, W. J. J. Nucl. Mater. 2001, 289, 188. (12) Hess, N. J.; Begg, B. D.; Conradson, S. D.; McCready, D. E.; Gassman, P. L.; Weber, W. J. J. Phys. Chem. B 2002, 106, 4663 and references therein. (13) (a) Willford, R. E.; Weber, W. J. J. Am. Ceram. Soc. 1999, 82, 3266. (b) Willford, R. E.; Weber, W. J. J. Nucl. Mater. 2001, 299, 140. (14) Pirzada, M.; Grimes, R. W.; Minervini, L.; Maguire, J. F.; Sickafus, K. E. Solid State Ionics 2001, 140, 201. (15) Lian, J.; Zu, X. T.; Kutty, K. V. G.; Chen, J.; Wang, L. M.; Ewing, R. C. Phys. ReV. B 2002, 66, 054108. (16) (a) Chen, J.; Lian, J.; Wang, L. M.; Ewing, R. C.; Wang, R. G.; Pan, W. Phys. ReV. Lett. 2002, 88, 105901. (b) Chen, J.; Lian, J.; Wang, L. M.; Ewing, R. C.; Boatner, L. A. Appl. Phys. Lett. 2001, 79, 1989. (17) Nachimuthu, P.; Thevuthasan, S.; Engelhard, M. H.; Weber, W. J.; Shuh, D. K.; Hamdan, N. M.; Mun, B. S.; Adams, E. M.; McCready, D. E.; Shutthanandan, V.; Lindle, D. W.; Balakrishnan, G.; Paul, D. M.; Gullikson, E. M.; Perera, R. C. C.; Lian, J.; Wang, L. M.; Ewing, R. C. Phys. ReV. B 2004, 70, 100101. (18) (a) Underwood, J. H.; Gullikson, E. M. J. Electron Spectrosc. Relat. Phenom. 1998, 92, 265. (b) Hussain, Z.; Huff, W. R. A.; Kellar, S. A.; Moler, E. J.; Heimann, P. A.; McKinney, W.; Padmore, H. A.; Fadley, C. S.; Shirley, D. A. J. Electron Spectrosc. Relat. Phenom. 1996, 80, 401. (19) (a) Chen, J. G. Surf. Sci. Rep. 1997, 30, 1. (b) de Groot, F. M. F.; Figueiredo, M. O.; Basto, M. J.; Abbate, M.; Petersen, H.; Fuggle, J. C. Phys. Chem. Miner. 1992, 19, 140. (c) Fischer, D. W. J. Phys. Chem. Solids 1971, 32, 2455. (20) Ostanin, S.; Craven, A. J.; McComb, D. W.; Valchos, D.; Alavi, A.; Paxton, A. T.; Finnis, M. W. Phys. ReV. B 2002, 65, 224109.
