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Journal of Crystal Growth 310 (2008) 434–441

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Hydrothermal synthesis of nanocrystalline BaTiO3 particles andstructural characterization by high-resolution transmission

electron microscopy

Xinhua Zhu�, Jianmin Zhu, Shunhua Zhou, Zhiguo Liu, Naiben Ming

National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, PR China

Received 23 September 2007; received in revised form 16 October 2007; accepted 30 October 2007

Communicated by T.F. Kuech

Available online 6 November 2007

Abstract

Nanocrystalline BaTiO3 (BT) particles were synthesized via hydrothermal method, and their microstructure was characterized by

X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron

diffraction (SAED), and high-resolution TEM (HRTEM). The results show that the BT nanoparticles remain a metastable cubic

structure at room temperature, as revealed by XRD and SAED. Such an abnormal crystallographic phenomenon was caused by the

lattice defects such as OH� defects and barium vacancies. The BT nanoparticles exhibit a spherical morphology with an average grain

size of 70 nm, and narrow particle size distribution. HRTEM images of individual particles indicate that the BT nanoparticles have a

good crystallinity and smooth surfaces. However, dark-field TEM images revealed high strains in BT nanoparticles, which were probably

resulted from the surface defects. Anti-phase boundaries were also observed in some BT nanoparticles, which were formed by the

intersection of two crystalline parts with a relative displacement from each other by 12d100 or

12d111, as revealed by HRTEM images. Some

local dark TEM contrast observed, near the pure edge dislocations with Burgers vector value of d110, was due to the existence of local

strains around the pure edge dislocations.

r 2007 Elsevier B.V. All rights reserved.

PACS: 81.07�b; 61.46+w; 68.37.Lp

Keywords: A1. Characterization; A1. Crystal morphology; A1. Crystal structure; A2. Hydrothermal crystal growth; B1. Barium compounds

1. Introduction

Barium titanate (BaTiO3: BT) is a key member of thefamily of technologically potential materials mainly be-cause of its ferroelectric response and high dielectricconstant that are regarded to be quite useful for thefurther development of the electronics industry. Inparticular, BT can be a suitable material for multilayerceramic capacitors (MLCCs) because of its high dielectricpermittivity and low losses [1,2]. Recent advances inmicroelectronics and communications have led to theminiaturization of MLCCs. Higher capacitance in smaller

e front matter r 2007 Elsevier B.V. All rights reserved.

rysgro.2007.10.076

ing author. Tel.: +8625 83592772; fax: +86 25 83595535.

ess: xhzhu1967@yahoo.com.cn (X. Zhu).

size requires a reduction of the thickness of the ceramiclayers below 2–3 mm and the increase of the number oflayers over 200. To achieve this goal, particles withimproved quality and uniform size of the order of 100 nmare highly required. However, conventional BT particlesobtained by solid-state reaction between BaCO3 and TiO2

at high temperatures above 900 1C are generally rathercoarse with uncontrolled and irregular morphologies,which are not suitable to realize very thin dielectric layers.Recent emphasis has been focused on wet chemical

methods to synthesize high quality BT particles [3,4].Among the various chemical methods developed so far, thehydrothermal method has been found to be a verypromising method for synthesis of ultra-fine (o100 nm)BT particles from the viewpoint of stochiometry control,

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reproducibility, purity, and particle size. Furthermore, thehydrothermal route has additional advantages to formcrystalline particles directly at rather low temperatures.The low-temperature hydrothermal synthesis of BT nano-particles is normally carried out by suspending titaniananoparticles or titania gel in an aqueous Ba(OH)2solution, and then autoclaving at 150–300 1C. Up to date,the hydrothermal preparation of BT nanoparticles hasbeen investigated extensively [5–15]. The effects of thesynthesis temperature, the pH value of the reactivemedium, and the Ba/Ti ratio on the particle size andmorphology have also been investigated [9–12], butdifferent results were reported by various researchers.Clearly, much remains to be done in order to establish thefactors which influence and control the particle size andmorphology of sub-micron BT particles synthesized byhydrothermal method.

To control the growth of BT particles with the desiredsize and particle morphology, the reactive kinetics andcrystallization mechanism of BT particles during thehydrothermal processing are also investigated. Hertl [16]investigated the formation of BT nuclei on the surface ofTiO2 particles suspended in an aqueous Ba(OH)2 solution,and proposed an in situ transformation of TiO2 into BT asBa diffuses into the structure. Ba reacts at the surface of theTiO2 particles to form an inwardly growing shell of BT.The BT particles obtained through this mechanism shouldmaintain the size and morphology of the precursor TiO2

particles. On the other side, a dissolution–precipitationmodel is also proposed by several authors [17–19]. In thismodel, a low concentration of TiO2 dissolves in the form ofsoluble Ti(OH)x

4�x, and then reacts with Ba2+ ions in asolution to precipitate BT. The BT particles obtained fromthis route are usually different from the precursor TiO2

particles with regard to their size and shape. Eckert et al. [6]reviewed these models in detail, and performed a kineticstudy of hydrothermal reactions involving barium hydro-xide octahydrate and anatase precursors. They found thatthe mechanism evolved from a dissolution–precipitationprocess (at the early stages of the reaction) to an in situ

mechanism for longer reactive times. Later Walton et al.[20] in situ studied the hydrothermal crystallization of BTby time-resolved powder neutron diffraction methods usingthe newly developed Oxford/ISIS hydrothermal cell. Theydirectly observed that the rapid dissolution of the bariumsource was followed by dissolution of the titanium sourcebefore the onset of crystallization of BT. These qualitativeobservations strongly suggest that a homogeneous dis-solution–precipitation mechanism dominates in the hydro-thermal crystallization of BT. The contradictive experi-mental observations reported previously are probablyresulted from the different hydrothermal conditions.However, the mechanism for the crystallization of BTmay be strongly dependent on the interactions amongvarious Ba and Ti complexes generated from the startingmaterials in the reactive medium and on their reactionsunder hydrothermal conditions.

As is known, BT has a perovskite structure and undergoesa cubic-to-tetragonal phase transition around the Curiepoint 130 1C [21]. The tetragonal phase is the onlythermodynamically stable phase for a bulk BT material atroom temperature. The TiO6 structure has to be completelydistorted in the tetragonal BT with a displacement of theTi ion in its oxygen coordination octahedron for 0.12 Aand an oxygen displacement about 0.003 nm [21]. However,in the hydrothermal BT nanoparticles, particularly thosesynthesized at lower temperatures, the results based on theX-ray diffractometry (XRD) and differential scanningcalorimetry (DSC) reveal an apparent cubic structurethat is normally observed only at temperatures higherthan the ferroelectric Curie temperature (�130 1C) [22].This indicates that the distortion of TiO6 structure leadingto a cubic-to-tetragonal phase transition by cooling thesample through the Curie point is impossible. A variety ofreasons for the room-temperature stabilization of thecubic structure and non-ferroelectric properties in the fineBT nanoparticles have been proposed, which include thepresence of lattice defects like hydroxyl ions associatedwith particles formed by wet chemical methods [23],small deviations in Ba/Ti in the stoichiometry [24],and excess surface energy associated with the ultrafineparticles [25,26]. Clearly, much effort is required to be donefor establishing the relationship between the abnormalcrystallographic feature (the retention of cubic phase atroom temperature) and the defect structures of hydrother-mal BT nanoparticles.It is generally believed that the physical properties of BT

nanoparticles are dependent upon their microstructureconsidering, e.g., grain boundaries, point and extendeddefects, as well as the surface morphology. It is consideredimportant to investigate the microstructure of BT nano-particles to facilitate a better understanding of the impactof size effects onto the physical properties. However, only afew detailed microstructural analyses, at the atomic level,of BT nanoparticles synthesized by the hydrothermalmethod, have been reported so far [27–30]. In the presentwork, the BT nanoparticles were synthesized by a modifiedhydrothermal technique using the as-prepared titaniumhydroxide and barium hydroxide as starting materials.Their microstructures (i.e., crystal structure, particle sizeand morphology, surface morphology, and so on) arecharacterized at atomic scale, especially by high-resolutiontransmission electron microscopy (HRTEM), to achievenanoparticles with a controlled microstructural character-istics.

2. Experimental procedure

The BT nanoparticles are synthesized by a modifiedhydrothermal technique using the as-prepared titaniumhydroxide and barium hydroxide as starting materials.Commercial reactants (Ba(OH)2 � 8H2O, reagent grade,Sigma-Aldrich; TiCl4, reagent grade, Acros) were em-ployed to prepare for these starting materials. Equimolar

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Fig. 1. (a) XRD pattern of the produced BT nanoparticles. Inset is the

local fine XRD pattern scanned at around 2yE451. (b) SEM image of the

produced BT nanoparticles.

X. Zhu et al. / Journal of Crystal Growth 310 (2008) 434–441436

quantities of the as-prepared Ba(OH)2 � 8H2O and Ti(OH)4were mixed in a 50mL autoclave together with 45mL ofdeionized water. The reaction mixture was stirred for fewminutes and then transferred into the autoclave. Thehydrothermal reaction was carried out in an autoclave at100 1C for 5 h. After the reaction, the product was washedwith organic acids and deionized water for several times,and finally dried in an oven for 24 h at 85 1C.

The phase structure of the BT nanoparticles wasexamined by X-ray diffraction (XRD, Philips X’PertMRD four-circle diffractometer) using Cu Ka radiationat 30 kV and 20mA. A typical scan rate was 0.011 persecond, and the 2y range was from 201 to 801. Morphologyand grain size of the BT particles were determined via SEMand TEM images. The TEM specimens were prepared bydispersing a small amount of the BT particles in purealcohol, mixing it in an ultrasonic generator, and putting adroplet of this dispersion on a copper grid with a supportedcarbon thin film. The particles on the supported carbonfilms were then examined using a field emission TEM(model, FEI Tecnai F20 TEM, operated at 200 kV) toinvestigate their morphology and crystalline structure; andthe microstructure was examined at atomic-scale by aHRTEM (model JEOL 4000 EX, operated at 400 kV). Allthe TEM, HRTEM images, and SAED patterns wererecorded by Gatan multiscan charge-coupled device (CCD)camera system (Gatan, Model 794). The fast Fouriertransform (FFT) pattern and the corresponding Fourierfilted HRTEM images are obtained by using the Gatandigital micrography (DM) software (revised version 2.0,Gatan Inc.).

3. Results and discussion

The XRD pattern of the produced BT nanoparticles isshown in Fig. 1, and the inset is the local fine XRD patternscanned at around 2yE451. As shown in Fig. 1(a), theXRD pattern matches well to the peak positions of thestandard cubic BT phase (JCPDS no. 31-174). Further-more, only a single diffraction peak at 2yE451 can beobserved in the inset, in other words, no split of the {2 0 0}peaks around 2yE451 can be seen. This implies that theparticles are stabilized in the cubic form at roomtemperature. This fact is also confirmed by the followingselected area electron diffraction (SAED) patterns of thesample. It is also noticed in Fig. 1(a) that a relatively strongdiffraction peak appears at 2yE251 in the sample, whichwas found out to be barium carbonate in the witherite form(JCPDS no. 45-1471). The very small diffraction peakaround 2y ¼ 281 was also contributed from BaCO3 in thewitherite form, in which the diffraction peak (2 0 0) ispositioned at 2y ¼ 27.71. The sample contained someBaCO3 impurities, probably formed by the dissolution ofatmospheric CO2 in the alkaline solution, followed by theprecipitation of insoluble BaCO3. BaCO3 interfering withthe complete conversion of BT was also reported by Eckertet al. [6] The production of BaCO3 impurities is normal in

the hydrothermal synthesis of BaTiO3 unless great care istaken to ensure that the precursors and the reactionenvironment are free of CO2 [31].In the present work, the BT particles (with an average

size of 70 nm) remain a metastable cubic structure at roomtemperature, which implies that the distortion of the TiO6

structure resulting in a cubic-to-tetragonal phase transitionas cooled the sample through the Curie temperature hasobviously not taken place. A plausible reason is the smallsize of the BT nanocrystals, which are so small that thestructural defects in the particles prevent the completion ofthe structural transition, leading to high strains within thecrystals. The high strains inside the nanoparticles (con-firmed by the dark-field TEM image of nanoparticle, seebelow) introduced by structural defects (e.g., latticedefects), would make the unit cell distortion (c/a ratio)much smaller than that in the standard BT. That is thereason why no peak splitting phenomenon is observed inthe XRD patterns of hydrothermal BT powders eventhough they are in tetragonal phase. The structural defectsof BT nanocrystals synthesized by hydrothermal method

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are primarily in the form of lattice OH� ions, which arecompensated by barium vacancies (V 00Ba) created on thesurfaces of individual particles to maintain electro-neu-trality [7,32–34]. Vivekanandan and Kutty [23] suggestedthat the strains in the crystallites are related to the latticepoint defects, and the presence of the metastable cubicphase at room temperature is resulted from the compensa-tion of the residual hydroxyl ions in the oxygen sublatticeby cation vacancies. Hennings and Schreinemacher [35]reported the observation of lattice hydroxyls and the effectof their release on the crystallographic recovery inhydrothermally prepared BT particles. Noma et al. [36]also reported that in the as-prepared BT nanopaticles withaverage size of 66 nm, there was a high concentration of thehydroxyl group and barium vacancy, and its crystalstructure was assigned to cubic with an expanded latticeby using the Rietveld. Shi et al. reported that thestabilization of the cubic phase of BT synthesized by thehydrothermal method was caused by surface defectsincluding OH� defects and barium vacancies [37].

The particle size and morphology of the produced BTparticles are revealed by both SEM and TEM images.Fig. 1(b) shows a SEM image of the produced BT particles,which have a fairly narrow size distribution and a sphericalgrain morphology with a high dispersibility. The averageparticle size (determined via SEM image) was about 70 nm.To demonstrate the particle size and morphology of the BTparticles in a fine-scale, TEM image of the produced BTparticles is demonstrated in Fig. 2(a), and Fig. 2(b) is thecorresponding SAED pattern. As shown in Fig. 2(a), theBT nanoparticles exhibit a spherical morphology and gooddispersibility, and the typical particle size distribution wasin the range of 60–100 nm. It is interesting to notice thatsome population of intragranular pores with diameters of2–3.5 nm exists at the cores of some BT nanoparticles, asmarked by arrows. It is known that the crystallineperovskite phase of BT can be directly synthesized underhydrothermal conditions; however, the obtained BTpowders are generally highly defective in the crystal-lographic structure [23,28,35,37]. For example, the hydro-xyl groups (OH�) exist in the lattice as point defects in the

Fig. 2. (a) TEM image of the produced BT particles, and (b) the

corresponding selected area electron diffraction pattern. The first seven

diffraction rings are indexed as (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0), (2 1 1)

and (2 2 0), respectively.

hydrothermal BT powders, occupying the regular oxygensites (O2�) of the perovskite structure. To maintain theelectro-neutrality condition, acceptor-type metal vacanciessuch as barium vacancies (V 00Ba) are introduced into thepowders [23,28,35]. Thus, there is large quantity of latticedefects in the hydrothermal BT powders. The coexistenceof large amounts of barium, titanium, and oxygenvacancies makes a rather unstable situation for theperovskite BT lattice. It is believed that these point defectsoccupying different lattice sites in a perovskite structurewould combine and annihilate each other. As a result, thevanishing vacancies formed upon dehydration are believedto be responsible for the formation of intragranular pores,as shown in Fig. 2(a). The SAED pattern shown inFig. 2(b) exhibits polycrystalline diffraction rings consistingof discrete diffraction spots. The first seven diffraction ringsare indicated in Fig. 2(b). The diameters (Di, i ¼ 1–7) of thefirst seven diffraction rings have been measured and theD2

i =D2i ratios (i ¼ 1–7) calculated. It was found that the D2

i

ratios equal to 1:2:3:4:5:6:8. The diffraction rings corre-spond well to the cubic perovskite structure, indicating theparticles being cubic BaTiO3. The first seven diffractionrings can be indexed as (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0),(2 1 1), and (2 2 0), respectively. That is well in agreementwith the XRD results. However, no rings corresponding tothe BaCO3 impurities are observed in the SAED pattern.To reveal the high strains in the produced BT

nanoparticles, both bright- and dark-field TEM imageswere recorded from the produced BT nanoparticles.Fig. 3(a) is a bright-field TEM image recorded by using asmall objective aperture that selects only the (0 0 0) centraltransmitted beam, which shows a narrow-distributionspherical nanoparticles. The dark-field image, shown inFig. 3(b), was recorded by using a smaller objectiveaperture that selects the part of the {1 0 0} and {1 1 0}reflections, as indicated by a circle in Fig. 3(c). The dark-field image displayed in Fig. 3(b) clearly shows high strainsin some BT nanoparticles. By using the bright- and dark-field TEM images, Lu et al. reported several types of TEM

Fig. 3. (a) Bright-field and (b) dark-field TEM images recorded from the

produced BaTiO3 nanoparticles. (c) A selected area electron diffraction

pattern from the BT particles showing a perovskite structure. The circle

indicates the size and position of the objective aperture used to record the

dark-field image displayed in (b).

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contrast variations in an individual BT nanocrystalsynthesized via hydrothermal method at higher tempera-ture (230 1C) by using polyoxyethylene sorbitan mono-oleate as a surface modifier [22]. It is believed that thedifferent types of variations of TEM contrast indicate theexistence of different strains in BT nanograins. Therefore,in a TEM image large strain is indicated by a contrastvariation across a particle. If a particle is single crystallineand has no strain, it should be uniform in contrast.However, for a single crystalline particle, if the TEM imageshows dark–bright variation in contrast, it is likely to havea high strain within the grain. Strain affects the diffractionbehavior of the electrons, resulting in dramatic contrastchange. The present hydrothermal BT nanoparticles

Fig. 4. (a) An overview TEM image of a single nanoparticle with size of 75 nm

and (c) a Fourier filtered image and the corresponding fast Fourier transfor

HRTEM image of another individual nanoparticle viewed from the [0 0 1] zon

filtered image.

exhibit a cubic structure (a high temperature phase) atroom temperature, such an abnormal crystallographicphenomenon is closely related to the existence of highstrains in these BT nanoparticles. The strains introduced bya high concentration of lattice defects, such as OH� ionsand barium vacancies, can make the unit cell distortion(c/a ratio) much smaller compared with that of thestandard BT. As a result, no peak splitting was detectedin the XRD patterns of the hydrothermal BT powders eventhough they belong to the tetragonal phase.The particles are also found to be single crystalline,

additionally proven by high-resolution lattice imaging ofindividual particles. Fig. 4(a) is an overview TEM image ofa single nanoparticle with size of 75 nm. The surface profile

, (b) a surface profile HRTEM image of the local part in this nanoparticle

m (FFT) pattern (see inset) of the single BT nanoparticle. (d) A profile

e axis and the insets are the FFT pattern and the corresponding Fourier

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HRTEM image of the local part (marked by a rectangularbox) in this nanoparticle is shown in Fig. 4(b). The well-developed lattice fringes are clearly observed, indicating agood crystallinity of the nanoparticle. The lattice fringes ofthe HRTEM image were examined to be 0.40 nm, close tothe (1 0 0) lattice spacing of the cubic BT. The Fourierfiltered image and the corresponding FFT pattern (seeinset) of the single BT nanoparticle are shown in Fig. 4(c),which reveal undistorted (1 0 0) lattice fringes. In addition,the surrounding edges of this particle are very smooth, nosurface steps are observed. Fig. 4(d) shows a profileHRTEM image of another individual nanoparticle viewedfrom the [0 0 1] axis (see the inset of FFT pattern).Obviously, the FFT pattern can be well indexed accordingto the cubic BT structure. The 2D lattice fringescorresponding to the two perpendicular sets of (1 0 0) and(0 1 0) planes are observed. To demonstrate this moreclearly, a Fourier filtered image of the single nanoparticlewas shown as an inset at the right-bottom corner, whichshows a certain degree of distortion of {1 0 0} latticefringes. That indicates the existence of some strains in theBT nanoparticle. It is also noticed that some surface stepslying on {1 0 0} planes are observed at the edge of thisparticle, as indicated by white arrows. It is also noticed thatthe {1 1 0} surface is rarely observed in the BT nanoparticlepossibly because of their higher surface energy. Thedominant morphology of the BT particles synthesized byhydrothermal method is usually spherical due to theequilibrium between crystallographic habit growth andpreferential dissolution of high-energy faceted edges.

Fig. 5. (a) and (b) The profile HRTEM images of two isolated nanoparticles (

(see insets).

Microstructural defaults like anti-phase boundaries(APBs) and edge dislocations are also observed in thenanoparticles from the BT powders. Fig. 5(a) and (b)shows the profile HRTEM images of two individualnanoparticles, respectively. The (1 0 0) and (1 1 1) latticefringes are clearly observed in Fig. 5(a) and (b), respec-tively. The corresponding Fourier filtered images and theFFT patterns (see insets) of the selected areas marked byboxes in the two HRTEM images are shown in Fig. 5(c)and (d), respectively. The Fourier filtered images clearlydemonstrates that how the APBs are formed during theparticle growth. There are two crystalline regions markedby I and II in the ellipses with fine white line in Fig. 5(c)and (d). The two crystalline regions in Fig. 5(c) aredeviated from each other by a relative displacement of 1

2d100

(d100 the inter-planar distance between two adjacent (1 0 0)planes), whereas in Fig. 5(d), the relative displacement is12d111 (d111 the inter-planar distance between two adjacent(1 1 1) planes). During the solid-phase crystallization, thecrystalline growth fronts of I and II parts intersect eachother. Then, the intersection of growth fronts accommo-dates the deviation as APBs. A similar condition wasobserved in a crystalline SrBi2Ta2O9 grain [38].Fig. 6(a) shows an overview image of a BT nanoparticle.

The HRTEM image of the local part (marked by a box inFig. 6a) is shown in Fig. 6(b), and the correspondingFourier filtered image and the FFT pattern (see inset) ofthe HRTEM image is shown in Fig. 6(c). Obviously, theFFT pattern can be well indexed according to the cubic BTstructure, which demonstrates that the nanopaticle is

c) and (d) the corresponding Fourier filtered images and the FFT patterns

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Fig. 6. (a) An overview image of a single BT nanoparticle, (b) the HRTEM image of the local part in (a), and (c) the corresponding Fourier filtered image

and the FFT pattern (see inset). (d) A profile HRTEM image of another individual nanoparticle, and the insets are the Fourier filtered image and the FFT

pattern of the selected area from the HRTEM image.

X. Zhu et al. / Journal of Crystal Growth 310 (2008) 434–441440

oriented along the [1 1 1] direction. The lattice distancesmeasured from the HRTEM image (Fig. 6b) are in perfectagreement with the cubic BT perovskite structure. Threepure edge dislocations (the value of Burgers vector is d101)are clearly observed in the Fourier filtered image (Fig. 6c),as marked in Fig. 6c. It is also noticed that the {1 1 0}lattice fringes shown in Fig. 6(c) have some distortion, andin some places the {1 1 0} lattice fringes are severelydisturbed. That implies the local high strains existing inthe nanoparticles. Fig. 6(d) shows a profile HRTEM imageof another single nanoparticle, in which the (1 1 0) latticefringes are clearly observed. The Fourier filtered image andthe FFT pattern of the selected area from the HRTEMimage are shown as insets in Fig. 6(d). The Fourier filteredimage clearly demonstrates the pure edge dislocations inthe presence of the nanoparticle. The value of their Burgervectors is determined to be d110. Similarly, around the edgedislocations some local dark TEM contrast was observeddue to the existence of local strains nearby the edgedislocations. The strains in the BT nanoparticles are

generally believed to be caused by the lattice defects likeOH� ions and their compensation by cation vacancies.

4. Conclusions

Nanocrystalline BaTiO3 particles were prepared viahydrothermal method by using the as-prepared titaniumhydroxide and barium hydroxide as starting materials.Their microstructure was characterized by XRD, SEM,TEM, SAED, and HRTEM. The XRD and SAED resultsdemonstrate that the produced BT nanoparticles remain ametastable cubic structure at room temperature, such anabnormal crystallographic phenomenon is caused by thestructural defects including OH� defects and bariumvacancies. Both SEM and TEM images show that the BTnanoparticles have a fairly narrow size distribution and aspherical grain morphology, with an average grain size of70 nm. Dark-field TEM images demonstrate the presenceof high strains in BT nanoparticles. Lattice fringes inHRTEM images reveal that the BT nanoparticles have a

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uniform and perfect crystal structure. Their surroundingedges are very smooth, and no surface steps are observed.However, in some nanoparticles, microstructural defectslike APBs and edge dislocations are also observed. TheHRTEM images reveal that the APBs can be formed by theintersection of two crystalline parts with a relativedisplacement from each other by 1

2d100 or 1

2d111. Around

the edge dislocations some local dark TEM contrast wasobserved due to the existence of local strains nearby theedge dislocations with Burgers vector value of d110.

Acknowledgments

This work is supported by Natural Science Foundationof Jiangsu Province (Project no. BK2007130), openingproject of National Laboratory of Solid State Microstruc-tures, and also by National Natural Science Foundation ofChina under Grant nos. 60576023 and 60636010.

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