Cleaved and polished surfaces of CaCu$_3$Ti$_4$O$_{12}$ ceramics have been
investigated by x-ray photoelectron spectroscopy (XPS) and energy dispersive
x-ray spectroscopy (EDX), respectively. While EDX technique shows the identical
CaCu$_3$Ti$_4$O$_{12}$ stoichiometry for the two surfaces, XPS indicates that
the cleaved surface with grain-boundary layers is remarkably Ti-rich and
Cu-poor. The core-level spectrum of Cu 2$p$ unambiguously shows the existence
of monovalent copper only for the cleaved surface. Possible grain-boundary
structure and its formation are discussed.Comment: 8 pages, 3 figure

Cao, G. H.

He, P. M.

Wang, C.

Zhang, H. J.

English

arXiv

Cleaved and polished surfaces of Ca Cu3 Ti4 O12 ceramics have been investigated by x-ray photoelectron spectroscopy (XPS) and energy dispersive x-ray spectroscopy (EDX), respectively. While EDX technique shows the identical Ca Cu3 Ti4 O12 stoichiometry for the two surfaces, XPS indicates that the cleaved surface with grain-boundary layers is remarkably Ti-rich and Cu-poor. The core-level spectrum of Cu 2p unambiguously shows the existence of monovalent copper only for the cleaved surface. Possible grain-boundary structure and its formation are discussed

Wang, C

Zhang, H J

He, P M

Cao, G H

Research Online

University of Wollongong Research Online Faculty of Engineering - Papers (Archive) Faculty of Engineering and Information Sciences 1-1-2007 Ti-rich and Cu-poor grain-boundary layers of CaCu3Ti4O12 detected by x-ray photoelectron spectroscopy C Wang Zhejiang University, caow@uow.edu.au H J. Zhang Zhejiang University P M. He Zhejiang University G H. Cao Zhejiang University Follow this and additional works at: https://ro.uow.edu.au/engpapers  Part of the Engineering Commons https://ro.uow.edu.au/engpapers/5146 Recommended Citation Wang, C; Zhang, H J.; He, P M.; and Cao, G H.: Ti-rich and Cu-poor grain-boundary layers of CaCu3Ti4O12 detected by x-ray photoelectron spectroscopy 2007. https://ro.uow.edu.au/engpapers/5146 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au Ti-rich and Cu-poor grain-boundary layers of CaCu3Ti4O12 detected by x-ray photoelectron spectroscopyC. Wang, H. J. Zhang, P. M. He, and G. H. Cao  Citation: Appl. Phys. Lett. 91, 052910 (2007); doi: 10.1063/1.2768006 View online: http://dx.doi.org/10.1063/1.2768006 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v91/i5 Published by the American Institute of Physics.  Related ArticlesSelf-assembled NaNbO3-Nb2O5 (ferroelectric-semiconductor) heterostructures grown on LaAlO3 substrates Appl. Phys. Lett. 101, 132902 (2012) Pressure and electric field effects on piezoelectric responses of KNbO3 J. Appl. Phys. 112, 064106 (2012) Enhanced piezoelectric properties of (Na0.5+y+zK0.5−y)(Nb1−xTax)O3 ceramics Appl. Phys. Lett. 101, 012902 (2012) Multiferroic response of nanocrystalline lithium niobate J. Appl. Phys. 111, 07D907 (2012) Non-radiative complete surface acoustic wave bandgap for finite-depth holey phononic crystal in lithium niobate Appl. Phys. Lett. 100, 061912 (2012)  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 31 Oct 2012 to 130.130.37.85. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsTi-rich and Cu-poor grain-boundary layers of CaCu3Ti4O12 detected byx-ray photoelectron spectroscopyC. Wang, H. J. Zhang, P. M. He, and G. H. CaoaDepartment of Physics, Zhejiang University, Hangzhou 310027, People’s Republic of ChinaReceived 27 June 2007; accepted 11 July 2007; published online 2 August 2007Cleaved and polished surfaces of CaCu3Ti4O12 ceramics have been investigated by x-rayphotoelectron spectroscopy XPS and energy dispersive x-ray spectroscopy EDX, respectively.While EDX technique shows the identical CaCu3Ti4O12 stoichiometry for the two surfaces, XPSindicates that the cleaved surface with grain-boundary layers is remarkably Ti-rich and Cu-poor. Thecore-level spectrum of Cu 2p unambiguously shows the existence of monovalent copper only for thecleaved surface. Possible grain-boundary structure and its formation are discussed. © 2007American Institute of Physics. DOI: 10.1063/1.2768006CaCu3Ti4O12 CCTO has recently attracted consider-able interest due to its extraordinarily high dielectric permit-tivity 10 000 at low frequencies over a wide temperaturerange 100–400 K.1,2 The giant dielectric phenomenon hasbeen primarily elucidated as an extrinsic effect in terms of aninternal-barrier-layer-capacitor mechanism.3 It wassuggested3 and then confirmed4 that grain boundary GBwas the internal barrier for CCTO ceramics. As for CCTOcrystals which show even larger dielectric permittivity,2 in-ternal barriers were still inferred from impedance spectros-copy measurement.5 In fact, CCTO ceramics contain domainboundaries as well as grain boundaries, revealed by scanningelectron microscopy and high-resolution transmission elec-tron microscopy.6 As a result, the detailed dielectric re-sponses of CCTO ceramics can be well interpreted by usinga double-barrier-layer-capacitor model.7So far, however, the structure of the internal barriersgrain boundaries and domain boundaries remains unclear.It was initially suggested that twin boundary was the possiblebarrier layer.1 A structural model of planar defects due to atwining parallel to 100 planes was recently proposed,8 yetit needs further experimental support. According to a detailedtransmission electron microscopy investigation,9 such twindomains could not be detected in single crystals or polycrys-tallines; instead, high density of dislocations and cation-disorder-induced planar defects were observed. Very re-cently, x-ray diffraction under extremely high hydrostaticand uniaxial compression suggested that CCTO ceramicswere composed of grains with stiffer shells and softercores.10 If so, the GB layers should be different from thegrain interiors in structure and composition. However, an-other recent report11 on Mn-doping effect proposed that thegrain and GB regions in CCTO ceramics might consist of thesame phase but with slightly different compositions.In this letter, x-ray photoelectron spectroscopy XPSand energy dispersive x-ray spectroscopy EDX were em-ployed to detect the possible differences between a polishedsurface PS and a cleaved surface CS of CCTO ceramicsamples. For a CS, GB layers remain on the surface becauseof the relatively weak linkage between grains. In the case ofa PS, on the other hand, grain interiors are exposed on thesurface. Because the detecting depth of XPS is 1 nm inmost cases,12 XPS actually reveals the information of ultra-thin surface layers. In comparison, the information depth ofEDX is commonly at the micron scale, thus EDX measuresthe bulk composition. By examining the CS and PS layerswith the two techniques, one may obtain the compositionaland structural information, especially for the GB layers.The CCTO ceramic samples were prepared by conven-tional solid-state reaction using the powdered chemicals ofTiO2 99.99%, CaCO3 99.99%, and CuO 99.99%. Thestarting materials were weighed according to the stoichio-metric ratio and mixed thoroughly in an agate mortar. Themixed powder was calcined at 1273 K for 12 h in air. Thisprocedure was repeated for three times to ensure that thesamples were in single CCTO phase. Then the calcined pow-der was pressed into a disk  122 mm2 and a rod 1215 mm2. The pressed specimens were finally sinteredin air at 1353 K for 24 h followed by furnace cooling toroom temperature. X-ray diffraction identified single phasefor the two specimens. Dielectric measurement with an Agi-lent 4284A precision LCR meter confirmed that the samplesshowed the property of giant dielectric permittivity as re-ported elsewhere.1,7Prior to the XPS and EDX measurements, the as-prepared CCTO samples were treated to make a PS and aCS, respectively. For the PS sample, the sintered disk waspolished by using CeO2 fine powder, followed by removingthe remaining CeO2 with a mixture of nitric acid and hydro-gen peroxide. Then it was in turn cleaned in distilled water,ethanol, and acetone with a ultrasonic cleaner. The CSsample was obtained simply by cleaving the rod. As soon asthe surface was made, the specimen mounted on a sampleholder was transferred into the ultrahigh vacuum systemequipped with an x-ray generator Mg K, 1253.6 eV andan Omicron EAC2000-125 analyzer. The shift of core-levelspectra due to the charging effect was calibrated using thecontaminated C 1s peak located at 284.6 eV.13 The XPS in-tensity was calculated based on the areas of the relatedpeaks. After the XPS experiment, the identical samples werealso examined by employing a field-emission scanning elec-tron microscope SEM sirion FEI, Netherlands equippedwith a Phoenix EDAX x-ray spectrometer. The sampleswere coated with very thin layer of gold before they wereplaced into the SEM chamber.aAuthor to whom correspondence should be addressed; electronic mail:ghcao@zju.edu.cnAPPLIED PHYSICS LETTERS 91, 052910 20070003-6951/2007/915/052910/3/$23.00 © 2007 American Institute of Physics91, 052910-1Downloaded 31 Oct 2012 to 130.130.37.85. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsFigure 1 shows the SEM images of the cleaved and pol-ished surfaces of CCTO ceramics. The CS image showsgrain-packed morphology with the grain size of 6 m.One can see that the cleaving takes place mostly at the grainboundaries. For the PS image, a flat surface is shown exceptfor some cavities. This indicates that grain interiors show updue to the polishing. It is also noted that both surfaces havevery similar EDX spectra, as shown in the insets of Fig. 1.Quantitative analysis indicates that the atomic ratiosCa:Cu:Ti for CS and PS are 1.0:3.1:4.0 and 1.0:3.0:3.8,respectively, consistent with the stoichiometric ratio ofCCTO within the experimental errors 5% .Figure 2 shows the core-level spectra of the cleaved andpolished surfaces of CCTO ceramics. Although there is noobvious shift for the positions of the core-level peaks, therelative intensities vary remarkably. As can be seen in panela, the intensities of Ca 2p peaks are similar for the twosurfaces. In comparison, the intensity of Cu Auger peak isremarkably weaker for the CS. Accordingly, the intensity ofCu 2p peaks in panel b is also proportionally weaker forthe CS. For the Ti 2p peaks in panel c, however, the inten-sity is substantially higher for the CS. In the case of O 1speak, it is difficult to make a comparison because of thedisturbance of surface contamination, especially for the PSsample.Here we make a quantitative comparing analysis so thatthe element-specific and instrumental parameters do not needto be concerned. The cation ratio of the PS is assumed to beNCaPS:NCuPS :NTiPS=1:3 :4, because the grain interiors were ex-posed adequately. Consequently, with the XPS data of the PSas a reference, the composition of the CS was easily deter-mined as NCaCS:NCuCS:NTiCS=1.0:2.3:4.7 the measurement er-ror, mainly coming from the determination of the peak area,is no more than 5%. Since the information depth is onlyabout 1 nm, the XPS result of the CS reveals the informationof the GB layers. Therefore, we conclude that the GB layersare Ti rich and Cu poor, compared with the CCTO stoichi-ometry.Figure 3 separately shows the Cu 2p core-level spectraof CCTO ceramics. As can be seen, there is a shoulder atlower energy side of the main peak of 2p3/2 only for the CSspecimen. By separating the peaks one obtained a small peakat 932.2 eV, suggesting the existence of CuI at GB similarpeak separating for the PS specimen was unsuccessful. An-other evidence for the existence of CuI comes from theintensities of the Cu 2p shake-up peaks. The CS sampleshows relatively weak shake-up peaks, because CuI has nosuch component.One notes that the amount of excessive Ti is almostequal to the amount of the missing Cu for the CS. This resultsuggests that some Ti may replace Cu in the GB layers. InFIG. 1. SEM images of cleaved surface a and polished surface b ofCaCu3Ti4O12 ceramics. The insets show the corresponding EDX spectra,respectively. Both of the surfaces were gold coated prior to the SEMobservation.FIG. 2. Core-level spectra of Ca 2p a, Cu 2p b, Ti 2p c, and O 1s dfor the cleaved and polished surfaces of CaCu3Ti4O12 ceramics.FIG. 3. Color online Analysis on Cu 2p spectra of the cleaved and pol-ished surfaces of CaCu3Ti4O12 ceramics. Note that the background wassubtracted by using Shirley-plus-linear method.052910-2 Wang et al. Appl. Phys. Lett. 91, 052910 2007Downloaded 31 Oct 2012 to 130.130.37.85. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsfact, evidence of Ti on the Cu site was given in nonstoichio-metric Sr0.946Cu2.946Ti0.054Ti4O12 Ref. 14 andNaCu2.5Ti0.5Ti4O12 Ref. 15. In CCTO, however, the Ti onthe Cu site is far too small to be detected by refining siteoccupancies from neutron diffraction data.14 Nevertheless,there is a possibility for the GB in which significant amountof Ti occupies the Cu site. The Ti-for-Cu substitution resultsin monovalent copper due to the charge neutrality, in agree-ment with the existence of CuI only for the GB layers.With the clue of Ti on Cu site, Li et al.14 proposed aconvincing explanation of how CCTO develops conductingregions. At high temperature, CuII reduces to CuI accom-panying with a charge compensation via a slight substitutionof TiIV on Cu site, forming CaCu1−3x2+ Cu2x+ Tix4+3Ti44+O12.Upon cooling, the CuI converts to CuII, liberating elec-trons into the Ti 3d conduction band. Although this mecha-nism explains the conductivity of CCTO grains, one couldnot understand the formation of internal barriers. We noticethat the above mechanism ignores the cation migrations dur-ing the cooling process. It is possible that the Ti at Cu sitemigrates to GB layers and domain-boundary layers whencooling down, which forms Ti-rich and Cu-poor barrierlayers.It is also noted that the structure model proposed by Wuet al.9 is consistent with the result of Ti-for-Cu substitution inGB layers. The planar defect model involves a lattice shiftR= 14 110.16 Such a lattice shift naturally results in cationdisorder in Ca/Cu site, accommodating the Ti-for-Cu substi-tution. The planar defect may give rise to remarkable strainat the boundary, which explains the thermal etching effectwithin the grains.6 Furthermore, such a domain-boundarymay serves as a stiffer layer as suggested by the high-pressure x-ray diffraction result.10 Finally, due to the latticediscontinuity, ion-disorder and/or ion displacement, theinterface may become a barrier layer against the electronconduction.In summary, we have revealed the subtle differences incomposition of the bulk and grain boundary regions inCCTO ceramics by comparing the core-level spectra ofcleaved and polished surfaces. The grain boundary containsremarkably more Ti and less Cu than the grain interior does.Moreover, only the grain boundary layer contains monova-lent copper. These results provide crucial insight for the ori-gin of the special giant dielectric phenomenon as well as thegrain-boundary structure in CCTO system.This work was supported by National Science Founda-tion of China Grant No. 10274070.1M. A. Subramanian, D. Li, N. Duan, B. A. Reisner, and A. W. Sleight, J.Solid State Chem. 151, 323 2000.2C. C. Homes, T. Vogt, S. M. Shapiro, S. Wakimoto, and A. P. Ramirez,Science 293, 673 2001.3Derek C. Sinclair, Timothy B. Adams, Finlay D. Morrison, and AnthonyR. West, Appl. Phys. Lett. 80, 2153 2002.4S. Y. Chung, I. D. Kim, and S. J. L. Kang, Nat. Mater. 3, 774 2004.5J. Li, A. W. Sleight, and M. A. Subramanian, Solid State Commun. 135,260 2005.6T. T. Fang and C. P. Liu, Chem. Mater. 17, 5167 2005.7Guanghan Cao, Lixin Feng, and Cao Wang, J. Phys. D 40, 2899 2007.8M.-H. Whangbo and M. A. Subramanian, Chem. Mater. 18, 3257 2006.9L. Wu, Y. Zhu, S. Park, S. Shapiro, G. Shirane, and J. Tafto, Phys. Rev. B71, 014118 2005.10Yanzhang Ma, Jianjun Liu, Chunxiao Gao, W. N. Mei, Allen D. White,and Jahan Rasty, Appl. Phys. Lett. 88, 191903 2006.11Ming Li, Antonio Feteira, Derek C. Sinclair, and Anthony R. West, Appl.Phys. Lett. 88, 232903 2006.12S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal. 17, 9111991.13D. Dobler, S. Oswald, and K. Wetzig, Anal. Bioanal. Chem. 374, 6462002.14J. Li, M. A. Subramanian, H. D. Rosenfeld, C. Y. Jones, B. H. Toby, andA. W. Sleight, Chem. Mater. 16, 5223 2004.15M. Avdeev and V. B. Nalbandyan, Inorg. Chem. 45, 2217 2006.16The original model contains an extra oxygen atom at the interface, what ismore, two oxygen atoms are much too close. A more reasonable structurefor the planar defect can be simply viewed as 14 110 shear planes, involv-ing edge-sharing TiO6 octahedra.052910-3 Wang et al. Appl. Phys. Lett. 91, 052910 2007Downloaded 31 Oct 2012 to 130.130.37.85. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Ti-rich and Cu-poor grain-boundary layers of CaCu3Ti4O12 detected by x-ray photoelectron spectroscopy

C. Wang

H. J. Zhang

P. M. He

G. H. Cao

Crossref

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=8077&context=engpapers

Ti-rich and Cu-poor grain-boundary layers of CaCu$_3$Ti$_4$O$_{12}$
  detected by x-ray photoelectron spectroscopy

Ti-rich and Cu-poor grain-boundary layers of CaCu $_3$ Ti $_4$ O $_{12}$ detected by x-ray photoelectron spectroscopy

Abstract

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Ti-rich and Cu-poor grain-boundary layers of CaCu3_33​Ti4_44​O12_{12}12​ detected by x-ray photoelectron spectroscopy

Abstract

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Full text

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Ti-rich and Cu-poor grain-boundary layers of CaCu $_3$ Ti $_4$ O $_{12}$ detected by x-ray photoelectron spectroscopy