Effect of Bond Covalency on the Lattice Stability and Fatigue Behavior

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J. Phys. Chem. C 2008, 112, 3434-3438

Effect of Bond Covalency on the Lattice Stability and Fatigue Behavior of Ferroelectric Bismuth Transition-Metal Oxides Tae Woo Kim, Su Gil Hur, Ah Reum Han, Seong-Ju Hwang,* and Jin-Ho Choy Center of Intelligent Nano-Bio Materials (CINBM), DiVision of Nano Sciences and Department of Chemistry, Ewha Womans UniVersity, Seoul 120-750, Korea ReceiVed: NoVember 29, 2007; In Final Form: December 13, 2007

The effects of bond covalency on the chemical bonding nature and lattice stability of ferroelectric bismuthbased transition-metal oxides have been investigated systematically through comparative X-ray absorption spectroscopic studies for several Aurivillius-structured materials and their chemically reduced derivatives. According to Ti K-edge X-ray absorption near-edge structure (XANES) analysis, Bi3TaTiO9 shows a weaker strength of (TiIV-O) bonds than Bi4Ti3O12, which could be interpreted as a result of the bond competition with highly covalent (TaV-O) bonds. Also, it was found that a lithiation process gives rise to a remarkable variation of Ti local structure commonly for Bi3TaTiO9 and Bi4Ti3O12. In contrast, there is only a slight spectral change in the Ta LIII-edge XANES spectra of Bi3TaTiO9 and SrBi2Ta2O9 upon the chemical reduction, underscoring the higher stability of more covalent (TaV-O) bonds than (TiIV-O) bonds. Alternatively, an energy difference between the peaks corresponding to Ta 2p3/2 f Ta 5dt2g and Ta 2p3/2 f Ta 5deg transitions appears to be greater for Bi3TaTiO9 than for SrBi2Ta2O9, indicating that the bond competition between collinear (Ta-O) and (Ti-O) bonds has a greater influence on the crystal field of TaO6 octahedra than an interaction between perpendicularly aligned (Ta-O) and (Bi/Sr-O) bonds. On the basis of the present experimental findings, we are able to conclude that the highly resistive nature of the (TaV-O) bonds against the increase of electronic charge is responsible for the excellent cycle characteristics of the SrBi2Ta2O9 phase. In this regard, the incorporation of highly covalent metal ions into the octahedral sites of the Aurivillius-structured lattice is supposed to be effective in improving the lattice stability and memory performance of this ferroelectric metal oxide.

Introduction Over the past decade, intense research activities have been devoted to Aurivillius-type bismuth-based transition-metal oxides because of their applicability for nonvolatile ferroelectric random access memory (FRAM).1 Among these materials, SrBi2Ta2O9 (hereafter this is denoted as SBT) has attracted special attention because it shows superior memory performance without a distinct fatigue phenomenon.1 Although the (Bi2O2)2+ layer in the Aurivillius-structure is believed to play an important role in preventing the reduction of the remnant polarization during repeated polarity switching,1 a serious fatigue phenomenon occurs for other Aurivillius-structured bismuth metal oxides like Bi4Ti3O12 (hereafter this is denoted as BTO) and Bi3TaTiO9 (hereafter this is denoted as BTT). This strongly suggests the presence of other factors affecting the memory performance of the Aurivillius phase.2,3 Park et al. reported that a fatigue phenomenon of BTO becomes effectively suppressed upon the partial substitution of La for Bi in dodecahedral sites of the perovskite block.3 Recently, we have demonstrated that the lattice stability of BTO becomes significantly enhanced by quenching an orbital interaction between octahedral Ti ions and dodecahedral Bi ions via bridging oxygen ligands.4,5 The incorporation of less-covalent La3+ ions into the Bi sites of the perovskite blocks can depress interactions between the (Bi-O) and (Ti-O) bonds in the BTO, which is responsible for the improvement of memory performance upon La substitution. * To whom all correspondences should be addressed. Tel: +82-23277-4370. Fax: +82-2-3277-3419. E-mail: [email protected].

Judging from an angle between neighboring metal-oxygen bonds, an interaction between collinear bonds of octahedral metal-oxygen pairs is expected to be stronger than that between perpendicularly aligned (Bi-O) and (Ti-O) bonds. To date, however, there has been no clear understanding about the relationship between the covalency of octahedral metal-oxygen bonds and the fatigue behavior of Aurivillius phases. In the present study, we have carried out systematic X-ray absorption spectroscopic (XAS) analyses for Aurivilliusstructured SBT, BTT, BTO, and their chemically reduced derivatives not only to probe the variations of their chemical bonding nature upon the increase of electronic charge but also to understand the effect of bond covalency on their lattice stability and fatigue behavior. Experimental Section Sample Preparation. The polycrystalline samples of BTT, BTO, and SBT were prepared by solid-state reactions with the stoichiometric mixture of Bi2O3, SrCO3, TiO2, and Ta2O5. Highpurity samples could be obtained by calcining the mixture at 800 °C and subsequently sintering at 1000 °C for 2 days with intermittent grindings.6,7 The chemical reduction of these pristine compounds was achieved in two ways of Ar-annealing and lithiation. The Ar-annealing was done at 900 °C for 24 h under Ar flow. The lithiated samples were prepared by the reaction with 5 equiv excess 1.6 M n-BuLi solution for 3 days because this condition could induce the reduction of Aurivilliusstructured bismuth transition-metal oxides through the incorporation of the lithium ion.7

10.1021/jp711283s CCC: $40.75 © 2008 American Chemical Society Published on Web 02/07/2008

Effect of Bond Covalency

J. Phys. Chem. C, Vol. 112, No. 9, 2008 3435

Figure 1. Crystal structures of SBT, BTT, and BTO.

Figure 2. Powder XRD patterns of (a) BTT, (b) Ar-annealed BTT, (c) lithiated BTT, (d-f) the corresponding data for SBT, and (g-i) the corresponding data for BTO.

Sample Characterization. The crystal structures of the pristine bismuth transition-metal oxides and their chemically reduced derivatives were studied by powder XRD measurement using Ni-filtered Cu KR radiation. The chemical bonding natures of the present materials were investigated using XAS analysis at Ti K-, Ta LIII-, and Bi LIII-edges. The XAS experiments were carried out at room temperature in a transmission mode in the beam line 7C at the Pohang Accelerator Laboratory (PAL) in Korea. All of the present spectra were calibrated carefully by measuring the reference spectrum of Ti, Ta, or Bi metal simultaneously. The data analysis for the experimental spectra was done with the WINXAS 2.0 program.8 Results and Discussion Crystal Structure and Powder XRD Analysis. All of the present compounds of SBT, BTT, and BTO crystallize with an Aurivillius-type structure composed of puckered Bi2O2 planes and perovskite blocks. As illustrated in Figure 1, there are two perovskite layers in the unit cells of SBT and BTT whereas the BTO possesses three perovskite layers in the unit cell. Also, there are significant differences in the type of metal ions in the octahedral sites of perovskite blocks, that is, Ta for SBT, Ta/Ti for BTT, and Ti for BTO. Besides, the dodecahedral sites of perovskite blocks are occupied by Bi3+ ions commonly for BTT and BTO, whereas Sr2+ ions exist in the same sites of SBT. According to previous neutron diffraction study on the Liintercalated Aurivillius phase,7 the intercalated lithium ions are located between the Bi2O2 layers and the perovskite blocks.

We have examined the effect of chemical reduction on the crystal structure of bismuth-based transition-metal oxides with powder XRD analysis. As shown in Figure 2, all of the present compounds display typical XRD features of the Aurivillius phase. In comparison with the BTT and SBT, the BTO compounds show a low-angle shift of Bragg reflections, indicating the expansion of unit cell volume. This is consistent with their larger unit cells containing three perovskite layers. Before and after the Ar-annealing and lithiation reactions, there are no significant changes in the powder XRD patterns of all of the pristine materials, strongly suggesting little influence of chemical reduction on their Aurivillius-type structures. According to least-squares fitting analysis, the unit cells of the bismuth transition-metal oxides are expanded slightly upon the chemical reduction process, suggestive of the reduction of component metal ions.9 Ti K-Edge XANES Analysis. We have probed the evolution of the chemical bonding nature of titanium ions upon the chemical reduction process using the Ti K-edge X-ray absorption near-edge structure (XANES) technique. The Ti K-edge spline and second-derivative XANES spectra for the pristine BTT and BTO compounds and their reduced derivatives are plotted in Figure 3, in comparison with those for the reference anatase TiO2 and Ti2O3. The edge energies of BTT and BTO are nearly the same as that of TiO2 but higher than that of Ti2O3, indicating the tetravalent oxidation state of titanium ions. All of the present compounds except Ti2O3 show three peaks (denoted P1, P2, P3) in the preedge region, which are assigned as the transitions from the core 1s level to unoccupied 3d states.10 In the main-edge region, there are three spectral features (denoted A, B, and C) corresponding to the dipole-allowed 1s f 4p transitions. Although the main-edge feature A would originate from the 1s f 4pz transition with the shakedown process, peaks B and C can be interpreted as the transitions to out-of-plane 4pz and inplane 4px,y orbitals, respectively.5 Because of the elongation of an axial metal-oxygen bond in the present ferroelectric materials,11,12 the peak B related to the 1s f 4pz transition appears at lower energy than the peak C corresponding to the 1s f 4px,y transitions. As shown in Figure 3, main-edge features B and C are somewhat weaker and broader for the BTT than for the BTO, reflecting more severe distortions in the local environment of titanium ions in the former. This stems from the coexistence of Ti and Ta ions in the same octahedral sites of the BTT, resulting in significant dispersions in (Ti-O) bond distances and the energies of final Ti 4p orbitals. Also, the BTT displays the lower energy of main-edge peak C than the BTO, reflecting the weakening and elongation of the (Ti-O) bonds in the former. This can be explained on the basis of competing bond model, where the more-electronegative

3436 J. Phys. Chem. C, Vol. 112, No. 9, 2008

Figure 3. Powder Ti K-edge XANES (a) spline and (b) secondderivative spectra for (i) BTT, (ii) Ar-annealed BTT, (iii) lithiated BTT, (iv) BTO, (v) Ar-annealed BTO, and (vi) lithiated BTO, in comparison with those for the reference (vii) anatase TiO2 and (viii) Ti2O3. Arrows put an emphasis on spectral variations after the chemical reduction.

Figure 4. Schematic model for competitions between metal-oxygen bonds of TiO6 (or TaO6) octahedra in BTT.

TaV cation deprives the electron density of adjacent (Ti-O) bonds, leading to the increased covalency of (Ta-O) bonds and the weakening of the (Ti-O) bonds; see Figure 4.13,14 Of special interest is that the lithiation process for the BTT and BTO compounds causes remarkable spectral changes such as peak broadening and peak splitting especially for peak C, implying the significant change of the chemical environment of titanium ions. The observed structural frustration upon the reduction process provides strong evidence on the low stability of TiO6 octahedra that would be responsible for a serious fatigue phenomenon in the Aurivillius-structured titanates.2,3 Ta LIII-Edge XANES Analysis. The local crystal structures around tantalum ions in the present Aurivillius-type compounds have been investigated with the Ta LIII-edge XANES technique. The Ta LIII-edge XANES spectra of the pristine BTT and SBT,

Kim et al.

Figure 5. Ta LIII-edge XANES (a) spline and (b) second-derivative spectra for (i) BTT, (ii) Ar-annealed BTT, (iii) lithiated BTT, (iv) SBT, (v) Ar-annealed SBT, (vi) lithiated SBT, and (vii) the reference Ta2O5.

their reduced derivatives, and the reference Ta2O5 are plotted in Figure 5, together with the corresponding second-derivative data. All of the spectra presented here exhibit two strong features, A and B. From their relative intensities and their positions, we are able to assign them as the transitions of 2p3/2 f 5dt2g and 2p3/2 f 5deg, respectively. Because the energy difference between the two peaks is surely proportional to the strength of crystal field around Ta ions,15 this value can provide direct information on the chemical environment around Ta ions. In Figure 5, the BTT shows a larger peak splitting than the SBT,16 indicating the stronger crystal field of TaO6 in the former. Again, this can be explained in terms of the competition between collinear metal-oxygen bonds in which the stronger (TaV-O) bonds in BTT become further reinforced through the competition with weaker collinear (TiIV-O) bonds, leading to the increase of crystal field around tantalum ions (Figure 4). Such bond reinforcement does not exist in the SBT because of the absence of titanium ions. It should be noted that, in addition to the type of octahedral metal ions, there is another significant structural difference between these compounds; that is, bismuth ions exist in the dodecahedral site of the perovskite blocks of the BTT, whereas strontium ions are present in the same site of SBT (see Figure 1). As reported previously,5 a significant interaction between Bi 6p and O 2p orbitals in the BTT can weaken adjacent (Ta-O) bonds and hence depresses the energy splitting between Ta 5dt2g and Ta 5deg orbitals. In this regard, the presence of dodecahedral Bi ions in the BTT should induce the weakening of the crystal field around Ta ions, which is contrasted with the competition effect originating from weak neighboring (TiO) bonds. Such an interaction between dodecahedral metal and oxygen is negligible in the SBT because of the less-interacting ionic character of strontium ions. If the influence of dodeca-

Effect of Bond Covalency

Figure 6. Bi LIII-edge XANES (a) spline and (b) first-derivative spectra of (i) BTT, (ii) Ar-annealed BTT, (iii) lithiated BTT, (iv) SBT, (v) Ar-annealed SBT, (vi) lithiated SBT, (vii) BTO, (viii) Ar-annealed BTO, (ix) lithiated BTO, and (x) Bi2O3. The solid, dashed, and dot-dashed lines represent the data of the pristine, Ar-annealed, and lithiated bismuth transition-metal oxides.

hedral Bi ions prevails over the bond competition effect between collinear (Ta/Ti-O) bonds, then the energy difference between peaks A and B should be smaller for BTT than for SBT. But this is not the case. In this context, we are able to conclude that the bond competition between collinear (Ta-O) and (Ti-O) bonds has a greater influence on the crystal field of TaO6 octahedra in the BTT than an interaction between perpendicularly aligned (Ta-O) and (Bi-O) bonds. As presented in Figure 5, a lithiation process for BTT and SBT gives rise to slight red-shifts of peaks A and B by ∼0.2-0.5 eV, reflecting the reduction of tantalum ion. Taking into account the fact that an energy change by ∼1 eV in Ta LIII-edge region corresponds to a change of ∼0.7 electronic charge on Ta,18 the observed red shift suggests only a slight decrease of the Ta oxidation state (i.e., ∼0.15-0.35) upon the lithiation. In addition to the variation of Ta oxidation state, the energies of peaks A and B are dependent on the strength of crystal field around the Ta ions. As shown in Figure 5, the lithiation process leads to a moreprominent low-energy shift for peak B related to the 2p f 5deg transition than for peak A related to the 2p f 5dt2g one.16 This observation could be understood by the fact that, in comparison with the Ta 5dt2g orbitals, the Ta 5deg orbitals interact more directly with O 2p orbitals and hence the change of (Ta-O) bond lengths has more prominent influences on the energy of Ta 5deg orbitals, resulting in the greater shift of peak B. Of special note is that, in contrast to the Ti K-edge data, the chemical reduction does not induce severe spectral changes in the Ta LIII-edge spectra such as peak broadening and peak splitting, underscoring the higher stability of more covalent (Ta-

J. Phys. Chem. C, Vol. 112, No. 9, 2008 3437 O) bonds. A closer inspection on the second-derivative data reveals that the peak splitting of SBT is decreased slightly by the lithiation while there is no significant spectral variation upon the lithiation for BTT.16 Negligible influence of lithiation on the Ta LIII-edge spectrum of the BTT can be attributed to the existence of weak (Ti-O) bonds playing a sacrificial role for accommodating an increase of electronic charge. Bi LIII-Edge XANES Analysis. The chemical bonding nature of bismuth ions in the present Aurivillius-type materials has been examined with Bi LIII-edge XANES spectroscopy. As plotted in Figure 6, all of the present materials exhibit two peaks A and B corresponding to 2p3/2 f 6dt2g and 2p3/2 f 6deg transitions, respectively.17 No preedge peak related to the 2p3/2 f 6s transition could be observed for all of the present bismuthbased metal oxides, indicating the trivalent Bi oxidation state with fully occupied 6s orbitals. The observed spectral feature is surely contrasted with the previously reported spectrum of pentavalent bismuth compound NaBiO3 with Bi electronic configuration of 5d106s0 showing a distinct preedge peak.17 From the first-derivative spectra, the edge energy was determined to be 13421.5 ( 0.1 eV for BTT, 13421.7 ( 0.1 eV for SBT, and 13421.3 ( 0.1 eV for BTO, respectively. Among the present Aurivillius-type compounds, the BTO shows the lowest edge energy, which would be related to the highest proportion (50%) of bismuth ions in the dodecahedral sites of the perovskite blocks with respect to the bismuth ions in the Bi2O2 layer. The coordination of dodecahedral Bi3+ ions by the larger number (i.e., twelve) of oxygen ligands seems to compensate their positive charge more efficiently through the formation of covalent bonds between bismuth and oxygen, resulting in the lower oxidation state of bismuth ions on average. This explanation is further supported by the fact that the order of the edge energy of BTT and SBT is in good agreement with their relative populations of the dodecahedral bismuth ions: BTT (33%) and SBT (0%). In addition, it was observed that the chemical reduction process results in a slight red shift of the main-edge jump commonly for the pristine SBT and BTO (0.2-0.4 eV). It was reported that a decrease of Bi valence from +5 to +3 gives rise to a low-energy shift of the Bi LIII-edge by ∼2.2 eV.19 In light of this, the observed decrease of edge energy indicates a slight depression of the Bi oxidation state by ∼0.2-0.4 upon the lithiation process. In particular, the BTT does not show any notable displacement of the Bi LIII-edge upon the lithiation process like the Ta LIII-edge case. This phenomenon would be attributed to the presence of bond competition between (TiO) and (Ta-O) accommodating the effect of the chemical reduction. Conclusions In the present work, we have carried out systematic XAS studies on several ferroelectric bismuth-based transition-metal oxides and their chemically reduced derivatives, clearly demonstrating that the covalency of octahedral metal-oxygen bonds plays an important role in the lattice stability of these ferroelectric materials through bond competition. In the case of the BTT, a competition with adjacent (TiIV-O) bonds has a greater influence on the bonding nature of (TaV-O) bonds compared to the interaction with (Bi-O) bonds. This phenomenon stems from the fact that the competition between collinear (Ti-O) and (Ta-O) bonds occurs through a direct interaction between collinear σ-bonds whereas perpendicularly aligned (Bi-O) and (Ta-O) bonds show a weak interaction between perpendicular σ- and π-bonds. Moreover, it was found that highly covalent (Ta-O) bonds experience negligible frustrations upon the

3438 J. Phys. Chem. C, Vol. 112, No. 9, 2008 lithiation, underscoring their highly resistive nature against the increase of electronic charge. Hence, such a resistive nature of the (Ta-O) bonds plays an important role in the excellent cycle characteristics and negligible fatigue phenomenon of the SBT during repeated polarity switching.2,3 This conclusion can be further supported by a recent report on Ta5+-substituted BTO phases revealing that the cycle characteristics of the BTO phase can be improved by the Ta substitution.20 In that paper,20 the suppression of the fatigue phenomenon upon Ta substitution was attributed to the decrease of oxygen vacancies, which would be related to the stronger strength of (Ta-O) bonds evidenced by the present XANES results. In light of this, we are able to suggest that, along with the incorporation of less-covalent metal ions in the dodecahedral sites,3,5 the stabilization of highly covalent transition-metal ions in the octahedral sites of the Aurivillius-structure can provide an effective way of synthesizing new efficient ferroelectric materials suitable for FRAM applications. Acknowledgment. This work was performed by the financial support by Ministry of Environment (Grant No. 022-061-023) and partly by the SRC/ERC program of the MOST/KOSEF (Grant R11-2005-008-03002-0). The experiments at PAL were supported in part by MOST and POSTECH. References and Notes (1) Paz de Arauzo, C. A.; Cuchiaro, J. D.; McMillan, L. D.; Scott, M. C.; Scott, J. F. Nature 1995, 374, 627. (2) Joshi, P. C.; Krupanidhi, S. B. Appl. Phys. Lett. 1993, 62, 1928. (3) Park, B. H.; Kang, B. S.; Bu, S. D.; Noh, T. W.; Lee, J.; Kim, H.-D.; Kim, T. H. J. Korean Phys. Soc. 1999, 35, S1306. (4) Park, B. H.; Kang, B. S.; Bu, S. D.; Noh, T. W.; Lee, J.; Jo, W. Nature 1999, 401, 682. (5) Hur, S. G.; Park, D. H.; Kim, T. W.; Hwang, S. J. Appl. Phys. Lett. 2004, 85, 4130. (6) Noguchi, Y.; Miyayama, M.; Kudo, T. J. Appl. Phys. 2000, 88, 2146.

Kim et al. (7) Choy, J. H.; Kim, J. Y.; Jung, I. J. Phys. Chem. B 2001, 105, 7908. (8) Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986. (9) Lattice parameters for BTT (a ) 5.421 Å, b ) 4.945 Å, c ) 23.974 Å), Ar-BTT (a ) 5.444 Å, b ) 4.978 Å, c ) 24.121 Å), Li-BTT (a ) 5.431 Å, b ) 4.971 Å, c ) 24.000 Å), SBT (a ) 3.897 Å, b ) 3.902 Å, c ) 24.986 Å), Ar-SBT (a ) 3.900 Å, b ) 3.904 Å, c ) 24.967 Å), LiSBT (a ) 3.912 Å, b ) 3.906 Å, c ) 25.005 Å), BTO (a ) 5.407 Å, b ) 5.437 Å, c ) 32.826 Å), Ar-BTO (a ) 5.411 Å, b ) 5.444 Å, c ) 32.849 Å), and Li-BTO (a ) 5.414 Å, b ) 5.449 Å, c ) 32.790 Å). (10) Hess, N. J.; Balmer, M. L.; Bunker, B. C.; Conradson, S. D. J. Solid State Chem. 1997, 129, 206. (11) Hervoches, C. H.; Lightfoot, P. Chem. Mater. 1999, 11, 3359. (12) Miura, K. Appl. Phys. Lett. 2002, 80, 2967. (13) Huheey, J. H.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and ReactiVity; HarperCollins: New York, 1993; p 171. (14) On the basis of the higher oxidation state of the TaV ion than the TiIV ion, the former is reasonably expected to be more electronegative than the latter because the electronegativity becomes higher with the increase of the oxidation state. This expectation is further supported by the fact that a neutral tantalum element has nearly the same electronegativity as the divalent titanium ion (χPauling(Ta0) ) 1.5, χPauling(TiII) ) 1.54; χPauling(O-II) ) 3.4) (ref 12). Using the Pauling’s formula for (a-b) bond ionicity (I) calculation, I ) 1 - exp[-1/4(χa - χb)2], the bond ionicity is expected to be smaller for (TaV-O) bonds than for (TiIV-O) bonds, implying the greater covalency for the former bonds. (15) Choy, J. H.; Hwang, S. J.; Park, N. G. J. Am. Chem. Soc. 1997, 119, 1624. (16) Peaks positions for BTT (A, 9881.2 eV; B, 9886.0 eV; ∆E ) 4.8 eV), Ar-BTT (A, 9881.0 eV; B, 9885.7 eV; ∆E ) 4.7 eV), Li-BTT (A, 9881.0 eV; B, 9885.7 eV; ∆E ) 4.7 eV), SBT (A, 9881.8 eV; B, 9886.3 eV; ∆E ) 4.5 eV), Ar-SBT (A, 9881.7 eV; B, 9886.2 eV; ∆E ) 4.5 eV), and Li-SBT (A, 9881.6 eV; B, 9885.8 eV; ∆E ) 4.2 eV). The standard deviation of the peak positions is (0.1 eV (17) Studer, F.; Bourgault, D.; Martin. C.; Retoux, R.; Michael, C.; Raveau, B.; Dartyge, E.; Fontaine, A. Physica C 1989, 159, 609. (18) Nemana, S.; Okamoto, N. L.; Browning, N. D.; Gates, B. C. Langmuir 2007, 23, 8845. (19) Retoux, R.; Studer, F.; Michael, C.; Raveau, B.; Fontaine, A.; Dartyge, E. Phys. ReV. B 1990, 41, 193. (20) Gu, J.; Wu, X.; Lu, X.; Zhu, J. Integr. Ferroelectr. 2006, 84, 227.