DOI: 10.1002/adma.200602153

High-Temperature Resistance Anomaly at a Strontium TitanateGrain Boundary and Its Correlation with the Grain-BoundaryFaceting–Defaceting Transition**

By Sung Bo Lee,* Jong-Heun Lee, Pyeong-Seok Cho, Doh-Yeon Kim, Wilfried Sigle, and Fritz Phillipp

With the development of nanotechnology, more attentionand care should be paid to grain-boundary structure and itsstructural transition in order to understand the behavior ofpolycrystalline materials with grain sizes down to nanometerlevels. Here, we report direct evidence suggesting a correla-tion between a grain-boundary structural transition and achange in the electrical property, using a strontium titanate(SrTiO3) bicrystalline grain boundary as a model system. Theelectrical properties of grain boundaries in SrTiO3 play a criti-cal role in barrier-layer devices, such as capacitors and varis-tors.[1] It is believed that the electrical behavior depends onthe double Schottky barrier established by the interfacecharge and the associated space charge across the grainboundary.[2–4] The interface charge arises from the segregationof point defects, which form because the energies to move an-ions and cations to the grain boundary are different.[5] Theconsequent electrostatic potential obstructs the transport ofcharge carriers across the grain boundary. For pure, undopedSrTiO3, Kim et al.,[6] using electron energy loss spectrometry(EELS), showed that the ratio of Ti to O concentration in var-ious boundaries is higher than in the bulk, indicating that thegrain boundaries are enriched in Ti or deficient in O. Brown-ing et al.[7] reported that a ∑5 grain boundary (∑ denotes thereciprocal of the fraction of common lattice points of the ad-joining grains) is segregated by oxygen vacancies. Klie and

Browning[8] reported the segregation of oxygen vacancies at agrain boundary (nominally undoped with a 58°/[001] tilt) inSrTiO3, which was compensated by a decrease in the Ti va-lence. The oxygen vacancies segregated at grain boundariesinduced a positive grain-boundary charge along with a nega-tive space-charge region, which formed a double Schottkyelectrical barrier.[9] In relation to barrier-layer formation, un-derstanding the relation between electrical and structural be-havior of individual grain boundaries is important. However,studies about the influence of high-angle grain-boundarystructure and its structural transition on electrical propertiesare rare.

Cahn[10] suggested that most grain-boundary structural tran-sitions belong to faceting–defaceting transitions where an ini-tially flat grain boundary is transformed into a faceted struc-ture with a hill-and-valley shape and vice versa, based on theassumption that the thermodynamics for the surface faceting–defaceting transition[11–13] also hold for the grain-boundarycounterpart. The defaceting transition of faceted grain bound-aries has been observed with temperature increase[14–16] orcomposition change.[15,17] Lee et al.[18] showed that a SrTiO3

grain boundary with a ∑5 [001] misorientation relationshipalso underwent a faceting–defaceting transition. Grain bound-aries annealed in air below 1600 °C showed faceting, whereasthey became defaceted after annealing at 1600 °C. In this re-port, we show the grain-boundary faceting–defaceting transi-tion in the SrTiO3 ∑5 bicrystalline grain boundary causes asudden change in the charge-carrier transport across the grainboundary.

Details of the experimental procedure are given in the Ex-perimental section. The dimensions of the bicrystal examinedand the position and size of electrodes for impedance mea-surements are illustrated in Figure 1. Figure 2 shows theimpedance spectra of the bicrystals examined. As shown inFigures 2a–d, only one semicircle, corresponding to theimpedance response of the bulk, appeared in the as-receivedbicrystal and in those annealed below 1600 °C: the grain-boundary impedance was negligible. Only in the bicrystalannealed at 1600 °C (Fig. 2e) did a semicircle for the imped-ance response of the grain boundary appear in the lower fre-quency range. The ratio of grain-boundary and bulk resis-tances, RGB/RB, was 0.281 (Fig. 2e). To check if the change ingrain-boundary impedance was reversible, additionally, abicrystal was step-annealed and its impedance was measured:The bicrystal was annealed at 1600 °C for 1 day, then cooled

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–[*] Prof. S. B. Lee[+]

Department of Ceramic Engineering, Hanyang UniversitySeoul 133-791 (South Korea)E-mail: bolee@snu.ac.krProf. J.-H. Lee, P.-S. ChoDepartment of Materials Science and Engineering, Korea UniversitySeoul 136-713 (South Korea)Prof. D.-Y. KimSchool of Materials Science and EngineeringSeoul National UniversitySeoul 151-744 (South Korea)Dr. W. Sigle, Dr. F. PhillippMax-Planck-Institut für MetallforschungHeisenbergstrasse 3, 70569 Stuttgart (Germany)

[+] Present Address: School of Materials Science and Engineering,Seoul National University, Seoul 151-744 (South Korea)

[**] The Alexander von Humboldt Foundation is acknowledged for thefinancial support.

directly to 1100 °C at a rate of 17 K min–1, and was maintainedat this temperature for 3 h. Then it was cooled in the furnaceto room temperature. Like the other bicrystals annealed

below 1600 °C, the step-annealed bicrystal exhibited no grain-boundary impedance, as shown in Figure 2f. (Hereafter, eachbicrystal examined in this study will be designated by itsannealing temperature. For example, the bicrystal annealed at1100 °C is designated as 1100.)

The as-received bicrystal grain boundary and the annealedones were imaged by using high-resolution transmission elec-tron microscopy (HRTEM) in the [001] projection. Figure 3ashows that the as-received bicrystal grain boundary was facet-ed into the symmetric (310) and short asymmetric (100)//(430)plane components. The grain boundary remained faceted ifannealed below 1600 °C. Figure 3b shows the faceting of the1500 bicrystal. (Images from the 1100 and 1300 bicrystals arenot shown here). At 1600 °C, the grain boundary became de-faceted and appeared wavy (Fig. 3c). The step-annealing pro-duced faceting into the symmetric (310) and asymmetric(100)//(430) planes (Fig. 3d), as in the other grain boundariesannealed below 1600 °C. No second phase was observed inany of the grain boundaries investigated.

Edges accessible for EELS analysis in the present studywere Ti L2,3 and O K, where the symbols, such as K, L, M,and so forth, are customarily used to denote the groups ofelectrons with principal quantum numbers n = 1, 2, 3, and soforth. To know how the grain-boundary defaceting transitionaffects the grain boundary’s chemical and electronic structure,the Ti:O intensity ratio, Ti-L3:L2 intensity ratio, crystal-fieldsplitting, and Ti-L edge-onset shift were measured from thegrain boundary and from the bulk in 1100, 1500, and 1600 bi-crystals. Whereas the 1100 and 1500 bicrystals did not showany discernible increase in Ti:O intensity ratio at the grainboundary with respect to the bulk, in the 1600 bicrystal theTi:O ratio at the grain boundary was increased by about 9 %with respect to the bulk (Fig. 4a). The Ti-L3: L2 intensityratios at the grain boundary were higher than in the bulk atall temperatures measured and, with increasing temperature,the Ti-L3:L2 ratio increased both in the bulk and at the grainboundary (Fig. 4b). Both the Ti-L3 and -L2 edges are split intotwo peaks, designated as eg and t2g, by the crystal-field ef-fect.[19] Whereas the separation between the eg and t2g peaksin the Ti-L edge did not show any change at the grain bound-ary with respect to the bulk in the 1100 bicrystal, the 1500 and1600 bicrystals showed lower crystal-field splitting at the grainboundary with respect to the bulk by (0.2 ± 0.1) eV and0.1 eV, respectively. The t2g:eg ratio increased with increasingannealing temperature. Only in the 1600 bicrystal was there adrop of the edge onset (both for Ti and O) by about 0.15 eVfor the grain boundary, whereas the 1100 and 1500 bicrystalsdid not clearly show any edge-onset shift.

Typically there are trivalent cation impurities, such as Fe3+

and Al3+, in undoped SrTiO3, although these are not reportedin the impurity data from the company, and so the nominallyundoped bicrystal used in the present study is assumed to beacceptor-doped.[20] As the bicrystal was pure and the anneal-ing was carried out at high temperatures in air (high oxygenpartial pressure), the oxygen-vacancy concentration was low,pinned at a constant level by background acceptors, and the

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Figure 1. A schematic diagram showing the dimensions of the bicrystaland the position of the electrodes. The thickness of the bicrystal is exag-gerated, compared with its width and length. Note that the two elec-trodes were placed in a diagonal direction with the grain boundary inbetween to prevent surface conduction and to force conduction to bethrough the grain boundary.

Figure 2. Impedance spectra measured at 450 °C. a) The spectrum fromthe as-received bicrystal. b–e) Spectra from the bicrystals annealed at1100 °C for 3 days, at 1300 °C for 3 days, at 1500 °C for 1 day, and at1600 °C for 1 day, respectively. f) The spectrum from the bicrystal under-going the step-annealing.

major charge carrier under the oxidizing or mildly reducingatmosphere is believed to be holes.[20,21] To support the sug-gestion, we briefly measured the electrical conductivity withincreasing oxygen partial pressure (PO2

) from 10–2 to 100 at600 °C. In the log conductivity versus log PO2

plot, the electri-cal conductivity increased with increasing PO2

, the slope beingca. 0.24. This value is close to the theoretical value of 1/4,[21]

which indicates that the bicrystal is p-type.The as-received bicrystal and those annealed in the temper-

ature range from 1100 to 1500 °C underwent faceting into thesymmetric (310) and asymmetric (100)//(430) components, asshown in Figure 3a and 3b, and defaceting of the initially fac-eted grain boundary occurred at 1600 °C (Fig. 3c), showingthat the defaceting transition occurred between 1500 and1600 °C. Such tendencies are also found in the previous studyof Lee et al.[18] where, as in the present study, a nominallyundoped ∑5 bicrystal was used. The observed grain-boundaryfaceting denotes that the average grain-boundary-plane orien-tation of the faceted grain boundaries deviates from the exactsymmetric (310) plane, a deviation that originates from the in-trinsic surface miscut of the mating single crystals. These twokinds of facets are regarded as singular planes correspondingto cusp orientations in the polar plot of the grain-boundaryenergy versus the grain-boundary-plane normal. The initiallyfaceted boundary shown in the as-received bicrystal (Fig. 3a)became defaceted at 1600 °C (Fig. 3c) and, as shown in Fig-ure 3d, a faceted boundary was recovered at 1100 °C by thestep-annealing, indicating that the faceting–defaceting transi-tion is reversible, as observed by Hsieh and Balluffi.[14]

As grain-boundary impedance distinctly appeared abovethe defaceting transition temperature (Fig. 2e) and disap-

peared again below that temperature because ofthe step-annealing (Fig. 2f), the change in grain-boundary impedance was shown to be reversible,further suggesting that the change in grain-bound-ary impedance was determined by the grain-boundary faceting–defaceting transition.

The higher Ti:O intensity ratio at the grainboundary at 1600 °C (Fig. 4a) evidences the pres-ence of oxygen vacancies at the grain boundary.The increase in the Ti-L3:L2 intensity ratio both atthe grain boundary and in the bulk with tempera-ture is caused by the reduction of the system at ele-vated temperatures.[8,19] The higher Ti-L3:L2 ratioat the grain boundary at all the observed tempera-tures (Fig. 4b) indicates an additional reduction ofthe Ti valence state at the grain boundary to com-pensate for oxygen-vacancy segregation at thegrain-boundary core indicated in Figure 4a.[8,19]

The observed loss of oxygen at the grain boundaryis consistent with the small reduction of the crys-tal-field splitting: missing oxygen atoms lead to adistortion in the symmetry of the Ti–O environ-ment and a reduction of the electrostatic fieldimposed by the oxygen on the Ti atom. For the1600 bicrystal, as mentioned above, the shifts in

the Ti-L and O-K edges at the grain boundary were very simi-lar in magnitude, resulting in no significant change in theseparation between the Ti-L and O-K edges. The separationwas always close to 72.5 eV, which is, according to Sankarara-man and Perry,[22] typical for TiO2. Those for Ti2O3 and TiOare reported to be 74.5 and 75.5 eV, respectively.[22] Althoughthe unchanged separation seems to indicate that there is nochange in the Ti valence at the grain boundary, the down-wards shift of the Ti-L edge, the increase in the Ti-L3:L2 inten-sity ratio, and the reduced crystal field are enough evidencefor a reduction of the Ti valence state at the grain bound-ary.[8,19]

Klie and Browning[8] compared Ti:O and Ti-L3:L2 intensityratios at room temperature and 724 K. The Ti:O ratio at thegrain boundary is higher than in the bulk, both values beingincreased at higher temperature. The Ti-L3:L2 intensity ratioexhibits similar tendencies. The loss of oxygen is driven by en-tropy and thus increases with temperature.[8] However, thesudden increase in oxygen-vacancy concentration at the grainboundary between 1500 and 1600 °C, as shown in the presentstudy (Fig. 4a), cannot be explained simply by the tempera-ture increase. We attribute this effect to the defaceting of thegrain boundary structure at 1600 °C (Fig. 3c). The faceting–defaceting transition is a first-order phase transition,[10–13] andthus explains the discontinuous change in the oxygen-vacancyconcentration at the grain boundary. For the Ti-L3:L2 ratio atthe grain boundary no indication of an abrupt change between1500 and 1600 °C was found (Fig. 4b). However, because boththe increase in the Ti-L3:L2 ratio and in the Ti:O ratio at thegrain boundary were evidence for a reduction or a loss ofoxygen at the grain boundary,[8,19] it appears that the Ti-L3:L2

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Figure 3. An HRTEM image of the grain boundary in a) the as-received bicrystal.b,c) Grain-boundary shapes of the bicrystals annealed at 1500 °C for 1 day and1600 °C for 1 day, respectively, and d) the step-annealed sample. The faceted structureis composed of symmetric (310) and asymmetric (100)//(430) plane components,regardless of the types of heat treatments.

intensity ratio at the grain boundary exceeded a critical levelat 1600 °C because of the defaceting transition.

The disordered grain-boundary structure observed at1600 °C can probably accommodate more oxygen vacanciesthan the ordered structure of the singular grain-boundaryfacet planes. The presence of oxygen vacancies at the grainboundary means that the grain boundary is positively charged,which induces the formation of a double Schottky barrier.The electrical barrier obstructs the charge-carrier transportacross the grain boundary. The high oxygen-vacancy concen-tration causes a high blocking effect, which is reflected in thestrong grain-boundary impedance at 1600 °C (Fig. 2e). Thepossibility of the grain-boundary contamination duringhigh-temperature heat-treatments can be excluded by the

negligible grain-boundary impedance of the step-annealedspecimen.

A combination of the impedance spectroscopy, HRTEM,and EELS of a 36.8° [001] tilt (∑5) grain boundary demon-strated that the change in grain-boundary impedance, grain-boundary electronic structure, and the grain-boundary facet-ing–defaceting transition were closely related to each other.The rough grain boundary observed at 1600 °C containedmore oxygen vacancies than the faceted grain boundaries ob-served below 1600 °C. Disordering of grain boundary by thedefaceting transition appears to have caused the abrupt ap-pearance of grain-boundary impedance at 1600 °C. Grainboundaries can be singular or rough, and it is expected thatthe grain boundary faceting–defaceting transition also signifi-cantly influences other grain-boundary properties (e.g., me-chanical), thus controlling bulk properties.

Experimental

We used nominally undoped SrTiO3 bicrystals containing a 36.8°[001] tilt (∑5) grain boundary (Shinkosha Co., Ltd., Japan). Two singlecrystals were diffusion-bonded into a bicrystal with the (310) sym-metric grain-boundary plane. The size of the bicrystals was10 mm × 5 mm × 0.5 mm. The grain-boundary plane was parallel to the5 mm × 0.5 mm surface. The bicrystal is reported to contain a smallamount of Ba and Ca, in total less than 50 ppm. The bicrystals wereannealed in air at 1100 °C for 3 days, at 1300 °C for 3 days, at 1500 °Cfor 1 day, and at 1600 °C for 1 day. They were heated at a rate of10 K min–1, and after annealing, were cooled in the furnace to roomtemperature. To fabricate the electrodes for the complex impedancemeasurements, Au paste (Model 5742, Dupont Chemical Co., Ltd.,USA) was screen-printed on part of both the 10 mm × 5 mm surfacesof the bicrystal (Fig. 1), and was heat-treated at 700 °C for 1 h. Theelectrodes were of 5 mm × 3 mm. The measurements were made at450 °C in air in the frequency range of 0.1–107 Hz by a dielectric/impedance analyzer (Model Alpha-N, Novocontrol, Germany). TheHRTEM work was carried out at 1.25 MeV using the StuttgartJEOL JEM-ARM1250 with 0.12 nm point-to-point resolution. Thelocal electronic structure of the grain boundary was obtained by usingEELS, which was done with the Gatan UHV ENFINA 766 systemattached to a VG HB501 UX dedicated scanning transmission elec-tron microscope (Vacuum Generators, Inc., UK). The electron probesize at the VG microscope was typically around 0.5–1 nm and theenergy resolution was about 0.8 eV. The convergence and collectionangles were both 6.5 mrad. Electron energy-loss spectra from thebulk and the grain boundary were measured at a dispersion of0.1 eV/channel.

Received: September 21, 2006Revised: October 29, 2006

Published online: January 11, 2007

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Figure 4. Ti:O and Ti-L3:L2 intensity ratios in the bicrystals annealed at1100, 1500, and 1600 °C. a) The Ti:O ratio at the grain boundary washigher than in the bulk at all examined temperatures. Note that the Ti:Ointensity ratio at the grain boundary shows an abrupt increase with re-spect to the bulk at 1600 °C. b) The Ti-L3:L2 intensity ratio, both at thegrain boundary and in the bulk, increased with temperature with the val-ues at the grain boundary higher than those in the bulk at all examinedtemperatures. (Here, the error bar shown is the standard error.)

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