Six-fold coordinated silicon at grain boundaries in sintered α- Al 2 O 3Kenji Kaneko, Isao Tanaka, and Masato Yoshiya Citation: Applied Physics Letters 72, 191 (1998); doi: 10.1063/1.120681 View online: http://dx.doi.org/10.1063/1.120681 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/72/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Role of the interface on radiation damage in the SrTiO3/LaAlO3 heterostructure under Ne2+ ion irradiation J. Appl. Phys. 115, 124315 (2014); 10.1063/1.4870052 Effects of Ti O x physical vapor deposition parameters on the preferred orientation and adhesion of Pt films on γ -Al 2 O 3 J. Vac. Sci. Technol. A 24, 1540 (2006); 10.1116/1.2194925 Grain boundary dependency of nonlinear current–voltage characteristics in Pr and Co Doped ZnO Bicrystals J. Appl. Phys. 95, 1258 (2004); 10.1063/1.1636816 Direct measurements of grain boundary sliding in yttrium-doped alumina bicrystals Appl. Phys. Lett. 82, 1179 (2003); 10.1063/1.1555690 Evaluation of lattice distortion near c- ZrO 2 /α- Al 2 O 3 interface by plasmon loss J. Appl. Phys. 88, 5206 (2000); 10.1063/1.1314321

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Six-fold coordinated silicon at grain boundaries in sintered a-Al2O3

Kenji Kanekoa)

ICORP JST, c/o JFCC, Atsuta, Nagoya 456, Japan

Isao Tanaka and Masato YoshiyaDepartment of Materials Science Engineering, Kyoto University, Sakyo, Kyoto 606-01, Japan

~Received 8 July 1997; accepted for publication 11 November 1997!

High-resolution transmission electron microscopy~HRTEM! and analytical electron microscopy~AEM! have been carried out on Si-doped sintereda-Al2O3. HRTEM shows that there is noamorphous phase at grain boundaries. The Si-segregated boundary is found to be much moresensitive to irradiation damage than undoped Al2O3 grain boundaries. AEM with energy dispersivex-ray spectroscopy~EDS! shows the significant segregation of Si at grain boundaries, and AEMwith electron energy-loss spectroscopy~EELS! reveals the existence of six-fold coordinated Si atthe grain boundaries. The theoretical calculations obtained by the molecular orbital method supportthe data obtained by EELS. ©1998 American Institute of Physics.@S0003-6951~98!03602-X#

Silicon oxides usually exhibit a 4:2 coordination exceptfor the high-pressure form, known as stishovite, which ex-hibits a 6:3 coordination. Amorphous forms of silicon oxide,such as silicate melts, exhibit four-fold coordinations whenthey are synthesized at ordinary pressures. However, underhigh pressures, as in the lower mantle of the Earth, a coor-dination transition from four-fold to five- and/or six-foldtakes place.In situ x-ray measurements of static structurefactors have shown that the average coordination number ofthe silicon completely shifts from 4 to 6 by around 42 GPa.1

The coordination of silicon may also change to six-fold fromfour-fold when it substitutes for a cation of a six-fold com-pound, such asa-Al2O3.

Silicon is known to segregate to grain boundaries in sin-tereda-Al2O3. Lee and Kro¨ger used the direct current con-ductivity of Si-doped Al2O3 to measure the solubility limit ofsilicon in Al2O3 as 220 ppm at 1500 °C and as 300 ppm at1600 °C.2 Elemental profile analysis across grain boundariesby EDS has been used extensively to study sintereda-Al2O

3 since the introduction of analytical transmission electronmicroscopes with sub-nanometer probes.3 When the dopantlevel was 200 ppm or less, no amorphous film was detectedat the grain boundaries though silicon was still confirmed tosegregate to the grain boundaries.2 Recent work on Zr-dopedhigh-purity a-Al2O3 ceramics using a combination ofHRTEM, EDS, and EELS has revealed that no glassy filmswere present at the grain boundaries even when zirconiumwas found to segregate to those grain boundaries.4

In the present study, we combine HRTEM, EDS andEELS to characterize the grain boundaries of 100 ppm Si-doped Al2O3 ceramics. By combining HRTEM and EDS, weare able to show that the silicon is localized near the grainboundaries of Al2O3 without forming any secondary phases.This implies that silicon is dissolved into the Al2O3 crystalpredominantly near the boundary. Electron energy loss nearedge structure~ELNES! analysis of the O-K edge acquiredfrom the boundary region shows that the spectrum exhibits apre-edge structure specific to the boundary. First principlesmolecular orbital calculations using a model cluster of sub-

stitutional Si41 in Al2O3 reproduces the pre-edge structure. Itis therefore concluded that this signal originates from six-fold coordinated silicon located near the boundaries.

The silicon dopeda-Al2O3 specimens were produced bysintering a fine, high-puritya-Al2O3 powder at 1400 °C. Im-aging and EDS studies were carried out with a JEOL 2010FTEM, equipped with a thermal Schottky field emission gunhaving a point resolution of 2.3 Å. Electron energy-lossspectra were acquired using a Gatan 766 parallel EELS on adedicated scanning TEM~STEM! ~Thermo, Vacuum-Generators HB 601 UX! with a cold field emission gun. Fig-ure 1 shows a lattice fringe image of a grain boundary. Noamorphous or crystalline intergranular phase is observed. Anorientation mismatch is detected between the two grains andthe atomic arrangement near the boundary is distorted. Thewidth of the distorted region is observed to be less than 0.5nm.

Figure 2 shows a line profile analysis of the SiKa EDSintensity across a typical grain boundary. The segregation ofSi was detected within a 10 nm width of the boundary. As-

a!Electronic mail: kaneko@ngo.jst.go.jpFIG. 1. High resolution TEM image~acquired at an accelerating voltage of200 kV! taken from an area containing an edge-on grain boundary.

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suming that the sample thickness at the grain boundary is200 nm, the maximum count measured on the boundary cor-responds to 0.02 at. % of Si. Thickness changes cause varia-tions in the at. % from one detection point to another. There-fore, instead of the at. %, the intensity itself was used tomonitor the segregation.

Grain boundaries exhibiting silicon segregation werefound to be extremely sensitive to irradiation damage. Al-though no amorphous alumina was present during the analy-sis, resistance to the irradiation damage at the Si-segregatedboundary was as low as that of amorphous Al2O3. It wasnoted that the undoped boundary is much more tolerant toirradiation. This implies that the Si segregation significantlyweakens the grain boundary bonding. In order to minimizethe effects of irradiation damage, ELNES edges on theboundary were obtained using a beam diameter of almost 5Å. The Si L2,3 ELNES edges were impossible to quantifybecause of the complexity introduced by the superposition ofthe intense Al L1 ELNES edge.

Comparison between the O-K ELNES edge obtainedfrom the grain interior ofa-Al2O3, shown in Fig. 3~a!, andfrom the grain boundaries, shown in Fig. 3~b!, showed subtlechanges as a pre-peak edge near the threshold of the O-KELNES edge appeared at approximately 537 eV~marked X!.It is possible to emphasize the peak X by subtraction of thetwo spectra as shown in Fig. 3~c!. This is known as thespatial difference~SD!-ELNES.5 The presence of the peak Xindicates a modification of the oxygen environment in theboundary region. Since the segregation of silicon at theboundary is detected by EDS, it is natural to suspect that thepeak X originates from oxygen atoms that are bound withsilicon instead of aluminum in the boundary region. In orderto examine this hypothesis theoretically, first principles mo-lecular orbital~MO! calculations have been carried out bythe discrete-variational~DV!-Xa method.

The DV-Xa method has been used successfully for thereproduction of ELNES edges of various compounds byTanaka and Adachi.6,7 Molecular orbital calculations weremade fora-Al2O3 anda-Al2O3 with a substitutional Si41 ionusing model clusters of~Al8O33)42- and ~SiAl7O33)41-, re-spectively. These clusters were modeled to be centered on acation, and were embedded in the Madelung potential gener-

ated by point charges outside the clusters. In the case of the~SiAl7O33)41- cluster, a Si41 ion was put at the center of thecluster. For comparison, two more clusters,~Si5O16)12- and~Si11O44)44-, were employed in order to modela-quartz~SiO2 with four-fold coordination! and stishovite~SiO2 withsix-fold coordination!. The basis sets applied were 1s-2p foroxygen and 1s-3d for aluminum and silicon. The electric-dipole selection rule allowed a transition from the O-1s or-bital to the unoccupied 2p orbital in the case of the O-KELNES edge. Therefore, the unoccupied partial density ofstates~PDOS! of O-2p corresponds to the O-K ELNES edge.Since the presence of a core hole was found to change theO-2p PDOS significantly ina-quartz,6 all calculations weremade including a half-filled core hole, i.e., at Slater’s transi-tion state.8 The absolute transition energy was evaluated bytaking account of temporary spin polarization during thetransition. More details of the computational procedures aregiven elsewhere.5

Figure 4 shows the unoccupied O-2p PDOS obtained for

FIG. 3. O-K ELNES edges acquired~a! from the grain interior and~b! fromthe grain boundary of Si-dopeda-Al2O3 . A strong pre-peak~marked X! isobserved at the grain boundary compared to the bulk ofa-Al2O3 . Thespatial difference spectrum obtained by subtracting~a! from ~b! is shown in~c!.

FIG. 2. EDS spectrum recorded with the beam located on the grain bound-ary as seen in Fig. 1. The presence of silicon is confirmed at grain bound-aries in dopeda-Al2O3 while no Si is detected within the grain interior or atthe grain boundaries ofa-Al2O3 .

192 Appl. Phys. Lett., Vol. 72, No. 2, 12 January 1998 Kaneko, Tanaka, and Yoshiya This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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two polytypes of SiO2, a-quartz and stishovite respectively@Figs. 4~a! and 4~b!#, in comparison with experimental O-KELNES edges@Figs. 4~c! and 4~d!# reported by Sharpet al.9

The framework structure of both ELNES edges was wellreproduced by the calculation. The absolute transition energyof the first peak top ofa-quartz, given as 539 eV by theory,agreed well with the experimental transition energy, 535 eV,with an accuracy of better than 1%.

Figure 5 shows a comparison of the unoccupied O-2pPDOS obtained for oxygen neighboring substitutional Si41

in a-Al2O3, Fig. 5~a!, with that in undopeda-Al2O3, Fig.5~b!. An additional peak appears at 3.5 eV lower than thefirst peak of purea-Al2O3. This extra peak, marked Z inFig. 5~a!, was validated by population analysis composed of69% Si 3s and only 0.2% Al 3s atomic orbitals. Therefore

the origin of peak Z can be ascribed to silicon oxide with asix-fold coordination.

The energy of peak Z was found to be 2 eV smaller thanthat of stishovite. This is probably due to the difference inthe chemical environment of the Si41 ion in the two struc-tures. Peak X in the experimental spectrum which matchespeak Z in the theoretical calculation is therefore explained bythe presence of six-fold coordinated Si in ana-Al2O3 matrixnear the grain boundary. This provides evidence that the seg-regated silicon ions dissolve into thea-Al2O3 in the vicinityof the grain boundary with a six-fold coordination.

In order to understand the chemical bonding around theSi1 ion at the grain boundary, additional work is required.First, a theoretical estimate of the lattice relaxation aroundthe substitutional silicon atom near the grain boundary isneeded. Second, an experimental technique capable of ob-taining spectra with good quality and free from irradiationdamage must be developed.

One author~K.K.! would like to acknowledge Dr. M.Saunders~U.S. Naval Postgraduate School!, and Dr. S.Tsurekawa~University of Tohoku! for valuable discussions.

1W. Jin, R. K. Kalia, P. Vashishta, and J. P. Rino, Phys. Rev. B50, 118~1994!.

2C. H. Lee and F. A. Kro¨ger, J. Am. Ceram. Soc.68, 92 ~1985!.3C. W. Li and W. D. Kingery, inStructure and Properties of MgO andAl2O3 Ceramics, edited by W. D. Kingery~The American Ceramics So-ciety, Columbus, OH, Advances in Ceramics Vol. 10, 1984!, p. 368.

4K. Kaneko, T. Gemming, I. Tanaka, and H. Mu¨llejans, Philos. Mag. A~inpress!.

5J. Bruley, Microsc. Microanal. Microstruct.4, 23 ~1993!.6I. Tanaka and H. Adachi, Phys. Rev. B54, 4604~1996!.7I. Tanaka and H. Adachi, J. Phys. D29, 1725~1996!.8J. C. Slater,Quantum Theory of Molecules and Solids~McGraw–Hill,New York, 1974!.

9R. Sharp, Z. Wu, F. Seifert, B. Poe, M. Doerr, and E. Paris, Phys. Chem.Miner. 23, 17 ~1996!.

FIG. 4. MO calculations of the unoccupied O 2p PDOS obtained for two polytypes of SiO2 , ~a! a-quartz and~b! stishovite, in comparison with experimentalO-K ELNES edges reported by Sharpet al. ~Ref. 9!, and~c! a-quartz and~d! stishovite.

FIG. 5. MO calculations of the unoccupied O 2p PDOS obtained for oxygenneighboring substitutional Si41 in ~a! a-Al2O3 , with that ~b! in undopeda-Al2O3 .

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