Raman BiTiO

8/11/2019 Raman BiTiO

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Materials Science and Engineering B 128 (2006) 156160

Lattice vibration of bismuth titanate nanocrystalsprepared by metalorganic decompositionZ.C. Ling a,b,, H.R. Xia a,b, W.L. Liu a,b,c, H. Han d,X.Q. Wang b, S.Q. Sun a,b, D.G. Ran a,b, L.L. Yu a,b

a School of Physics and Microelectronics, Shandong University, Jinan 250100, PR Chinab State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China

c School of Material Science and Engineering, Shandong Institute of Light Industry, Jinan 250100, PR Chinad Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487-0324, United States

Received 14 July 2005; received in revised form 14 November 2005; accepted 28 November 2005

Abstract

Bismuth titanate (Bi 4Ti3O12 (BTO)) nanocrystals have been prepared by metalorganic decomposition. X-ray diffraction revealed that the BTOnanocrystals belong to the orthorhombic system with average size of

60 nm. Raman scattering and Fourier-Transform infrared measurements of

BTO nanocrystals showed a large decrease of the number of peaks comparing to the space group theoretical analysis. It could be interpreted bythe fact that the characteristic vibration modes of BTO nanocrystals arise mainly from the internal modes of TiO 6 octahedra, which in return leadto modes degeneration. Raman shifts and infrared absorption bands of BTO nanocrystals were tentatively assigned. Large distortions in Ti(2)O 6octahedra lead originally Raman active modes to become IR active as well and are testied by our FTIR result. An inactive mode for has beenfound in both our Raman and FTIR data. That might be originated from oxygen vacancies and internal strain, which is commonly explained forthe mechanism of the fatigue in BTO. 2005 Elsevier B.V. All rights reserved.

Keywords: Bismuth titanate; Lattice vibration; Space group analysis; Nanocrystals

1. Instruction

Bismuth titanate (Bi 4Ti3O12 (BTO)) is a member of theAurivillius family of bismuth layer structure perovskites, whichconsist of three perovskite-like units, sandwiched between bis-muth oxide layers along the c-axis as depicted in Fig. 1, where A, B, and C denote the (Bi 2Ti3O10)2 perovskite-like block,units of hypothetical perovskite structure, and (Bi 2O2)2+ sheets,respectively [1]. As a promising ferroelectric material, BTO isattracting considerable attention due to its unique ferroelectric,piezoelectric, and electro-optic switching behaviours [2,3]. Ithas become a key candidate for memory storage capacitor, opti-cal display, and electro-optical devices [46].

One of the most puzzling problems now for the Bi layeredferroelectric random access memory (FeRAM) products is its

Corresponding author. Tel.: +86 53188376539; fax: +86 53188565167. E-mail addresses: [email protected], [email protected]

(Z.C. Ling).

fatigue, which means that the remnant polarization of BTOis prone to decrease with repeated switching of polarization.Many mechanisms were put forward to explain the origin of the fatigue. Its well-known that lanthanum substituted bismuthtitanate (Bi 4 x La x Ti3O12 (BLT)) which possess better fatigueresistance properties than BTO were proposed to suppress thevolatilizationof Biandincreasethestabilityof themetaloxygenoctahedral in the crystal structure [7]. Lee et al. reveals that theinternal strain, as well chemical stability of oxygen ions, con-tributes to the ferroelectric fatigue of the BLT lms [8].

Lattice vibration studies on Aurivillius family have been aneffective method to study the crystal structure because of itssensitivity to the minor changes of the structure. Raman studieshave been early used on the single crystals of Bi-layered per-ovskites [9], giving some meaningful results of the structure of them. But lattice vibrationstudies on thenanocrystalsusing bothRaman and FTIR are still few for us. In this paper, the vibra-tion modes in a BTO primitive cell at point were calculatedby space group theory. The experiments of Raman scatteringand Fourier-Transform infrared (FTIR) spectra were performed

0921-5107/$ see front matter 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.mseb.2005.11.033


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Z.C. Ling et al. / Materials Science and Engineering B 128 (2006) 156160 157

Fig. 1. Schematic of the cell structure of BTO.

to study lattice vibration modes, which may help us to get abetter understanding of the structure about this material. Basedon the discussion of lattice vibration, we further discussed themechanism of the fatigue in Bi-layered ferroelectrics.

2. Experimental procedure

Among the various techniques, such as solid-state-reaction,coprecipitation, solgel, hydrothermal and molten salt synthe-sis, metalorganic decomposition (MOD) employed in this studyoffers the advantage of simplifying process, better homogene-ity, stoichiometric composition control, and low cost. Detailedprocess is as follows.

The precursor for BTO was prepared by rst dissolvingrequired amounts of bismuth nitrate in glacial acetic acid by stir-ring at 60 C up to achieve complete dissolution. Stoichiometrictetrabutyl titanate was slowly dropped into the above solutionunder constant-rate stirring and then the solution was dilutedwith 2-methoxyethanol to adjust viscosity and surface tension.Theresultant solutionwas stirred at room temperature for1 h andltered thereafter to form the stock solution, which was yellow,clear, and transparent. The precursor solution thus obtained wasprecalcined in a mufe at 350 C. It combustedrapidly, resultingin the formation of a ne powder. After milling, the precalcinedpowder was heat-treated at 700 C.

The existing phase in the calcined samples were studiedby X-ray diffraction (XRD). The grain size of BTO powderswas estimated by X-ray line broadening using the Scherrerequation.

The Raman scattering measurement was performed in theback-scattering geometry using a Ventuno21 NRS-1000DTinstrument at room temperature. The Raman spectra were accu-

mulated twice from100 to1000 cm 1. Theinfraredspectrawere

collected by the Nicolet-Nexus 670 FTIR spectrometer from 50to 1000 cm1 .

3. Results and discussion

3.1. XRD analysis

Fig. 2 shows the XRD patterns of BTO powder prepared byMOD method after annealed at 700 C, which revealed goodcrystalline had been obtained. It is found that the BTO powderhas an orthorhombic phase structure consistent with the JCPDFcards PDF # 732181, in which space group Fmmm (D 232h ) is rec-ommended. The measured cell parameters a = 0.541, b = 0.544,and c = 3.282nm match well with previous report [10].

The average grain size t was estimated to be 60 nm fromthe half-width of the X-ray diffraction peaks using Scherrersequation:

t

=

k

cos where is the diffraction angle, is the average wavelength of X-ray, k is the shape factor, and is taken as half-maximum linebreadth.

3.2. Space group analysis of BTO

The idealized structure of bulk BTO crystal belongs to pointgroup D4h (4/ mmm) forming the space group I 4/ mmm. How-ever, an orthorhombic phase with space group Fmmm (D 232h )has been got in our experiment. Recently, based on the XRDpowder renement, BTO lm produced by solgel method was

assigned as orthorhombic phase with space group Fmmm (D232h )[11] . Hence, we will conduct the factor group analysis by space

group Fmmm .The reducible representations for the space group Fmmm at

thecenter point of the rst Brillouin zone are given in Table 1 ,in terms of the space group theory and the International Tables for Crystallography [12]. The atomic positions in the BTO crys-

Fig. 2. The XRD pattern of BTO nanocrystals after annealed at 700 C.


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Table 1The reducible representations for Fmmm at point

Symmetry species of point group D2h Seitz operator Character of Wyckoff position

16i 8i 8e 8f 4b

E {1[000] |T (000)} 48 24 24 24 12C z2 {2[001] |T (000)} 16 8 8 0 4C

y2 {2[010] |T (000)} 0 0 0 0 4C x2 {2[100] |T (000)} 0 0 0 0 4

I {1[000] |T (000) } 0 0 0 24 12 z {2[001] |T (000) } 0 0 8 8 4 y {2[010] |T (000) } 0 8 0 0 4 x {2[100] |T (000) } 0 8 0 0 4

tal are given in Table 2 , referring to the experimental results inRef. [11]. Based on the factor group theory, the reducible repre-sentations shown in Table 1 can be reduced as follows:

16i : 4Ag +4B1g +8B2g+8B3g +4Au +4B1u +8B2u +8B3u

8i : 4Ag +4B2g +4B3g +4B1u +4B2u +4B3u8e : 4Au +4B1u +8B2u +8B3u8f : 4B1g +4B2g +4B3g +4B1u +4B2u +4B3u4b : 4B1u +4B2u +4B3uAccording to the crystal structure and The International Tables

for Crystallography , the atomic Wyckoff position is 8i for 1Ti,2O, and 2Bi atoms. Then the irreducible representation of thelattice vibration of the BTO crystal in a unit cell can be obtainedas follows:

24Ag +8B1g +32B2g +32B3g +8Au+36B1u +44B2u +44B3u

Considering four formula of BTO exist in a unit cell, but aprimitive cell contains only one formula, theoriginal irreduciblerepresentations should be:

6Ag +

2B1g +

8B2g +

8B3g +

2Au +

9B1u +

11B2u +

11B3u

On the basis of the character table of the point groups D2h ,the Raman (R) and infrared (IR) active optical modes at zerowave vector are:

vib =6Ag(R) +2B1g(R) +8B2g(R) +8B3g(R)+8B1u(IR) +10B2u(IR) +10B3u(IR)

Table 2The Wyckoff position of atoms in BTO unit cell

Atom Ti(1) Ti(2) Bi(1) Bi(2) O(1) O(2) O(3) O(4) O(5)

Position 4b 8i 8i 8i 8e 8f 8i 8i 16i

It is clear that the theoretically observable Raman peaks andinfrared absorption bands are no more than 24 and 28 in number,respectively.

3.3. Raman spectra analysis

Fig. 3 shows the Raman spectrum on BTO nanocrystals pre-pared by MOD technique, and only 13 Raman modes could beobserved in the spectrum, much less than the calculated results.Experimentally, modes counting in the Raman spectra of BTOis never quite exact because of possibility of overlapping bands,bands formally allowed but too weak to observe, and bands thatmaybe multi-phonon features [13].

From the point of view of lattice dynamics, these strongerRaman peaks imply the strong interactions between the ions,whichmainlyarisefromthestretchingandbendingof theshorter

metaloxygen bonds within the anionic groups. The TiO 6 octa-hedra in the BTO crystal, accordingly, should play an importantrole in the lattice vibration spectra.

An octahedral molecule XY 6 with the symmetry O h has 15internal vibrational degrees of freedom or sixnormalvibrationalmodes i . They can be represented from group theoretical con-

Fig. 3. Raman spectrum of BTO nanocrystals post-annealed at 700 C.


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siderations as:

vib =A1g(R) +E g(R) +2F 1u(IR) +F 2g(R) +F 2u(inactive)

where the subscripts g and u represent symmetric and anti-symmetric vibrations, respectively. 1, 2 , and 3 are stretchingmodes and 4, 5, and 6 are bending ones. Thus there mightbe three characteristic Raman peaks and two strong characteris-tic infrared absorption bands belonging to the internal vibrationmodes of the TiO 6 octahedra. A primitive cell of theBTO crystalcontains three TiO 6 octahedra, consequently, the Raman peaksandinfraredabsorption bands observed in theexperiment shouldbe fewer in number than those calculated by the group theory.

As a matter of experience [14,15] , there are the relationships:1 > 3 > 2 for the stretch vibrations, 4 > 5 > 6 for thebendvibrations,and 2 > 4 and 5 26. Inaccordance withRaman data of BTO, KTiOPO 4, BaTiO 3, and PbTiO 3 [9,1618] ,a shorter bond length of TiO than that of BiO suggests that

the corresponding higher wave numbers. The modes at 849,originating mainly from the vibration of atoms inside the TiO 6octahedron, could be assigned as 1. The peak at 849 cm 1 (1)is attributed to the symmetric TiO stretching vibration. Themode at 540 (TO) and 565cm 1 (LO) should be assigned as2, which indicate the doubly degenerate symmetric OTiOstretching vibrational mode. They split into longitudinal (LO)and transverse (TO) components due to the long-range electro-static forces associated with lattice ionicity. The lager distortionof Ti(2)O 6 octahedra testied by XRDresults [19,20,21] , whichmaybe ascribed to the larger impact of the [Bi 2O2]2+ layers,induces the splitting of peak. When the Bi ions in the A-sitein pseudoperovskite are substituted by La, the bands between400 and 600 cm 1 tend to merge into one another [22], whichmight bea witness for our assignment. The peak269 cm 1 couldbe assigned as 5, indicating the triply degenerate symmetricOTiO bending mode ( F 2g). It seems wide and strong, and hassplit into two peaks, 269 and 224 cm 1. The anomaly broaden269 cm1 suggest a large distortion of the octahedra.

The mode at 192 cm 1 which can be determined to6 (269 cm1 2192cm 1) is Raman inactive according totheOh symmetryof TiO 6. It is observed because of thedistortionof octahedra. The distortion may result from the oxygen vacan-cies or strain, which are commonly accepted as the cause of fatigue behaviour in Bi layered perovskites. However, the exis-

tence of internal strain and the oxygen vacancies might break the Raman selection rule and let the mode 192 cm 1 observable.

Mode 325 cm 1 was from a combination of the stretchingand bending vibrations. The TiO 6 octahedra showed consider-able distortion at room temperature so that some phonon modes,e.g. at 325, 540, 617, and 849 cm 1, appeared wide and weak,which may induce ferroelectric and dielectric anomaly of BTOas previous reports [11,23,24] . The Raman modes of the corre-sponding lower wave numbers, such as the mode at 119 cm 1,are originated mainly from the vibrations between Bi and Oatoms, which can be conrmed by the shift to a higher wavenumber due to the modication of a lighter Sm atom at a Bi site

with increasing doping concentration [23,24] .

Fig. 4. FTIR spectrum of BTO nanocrystals after 700 C annealing.

3.4. Infrared spectra analysis

The infrared spectrum of BTO is shown in Fig.4 . Thenumberof the absorption peaks in experimental results is much smallerthan that of predicted, which could also be ascribed to the samereasons as the Ramans referred above.

Compared with the octahedral groups in crystals such asKTiOPO 4, K:LiNbO 3 and Li6WO6 [17,25,26] , the absorptionband at 637 cm 1, assigned as 3, suggest the triply degenerateTiO antisymmetric stretch mode and 365 cm 1 (4) the triplydegenerate antisymmetric in-plane OTiO bending one in theOh symmetric octahedron Ti(1)O 6. From the XRD result of Raeet al. [21], Ti atoms in the Ti(2)O 6 octahedra deviated fromthe a b plane greatly, while the Ti(1)O 6 show relatively muchsmaller distortion. According to the dynamics of the octahedra,when the central atom in the O h symmetric octahedral devi-ate greatly from the ab plane, it will certainly take part in theantisymmetric modes, i.e. those originally Raman active modesaccording to the selection rule of O h point group may becomeIR active as well. Therefore, the large distortion in Ti(2)O 6 octa-hedra might lead 1 and 2 to become IR active modes and shiftto 823 and 553 cm 1, respectively. However, whats differentfrom the 2 in Raman data is that it doesnt split. Raman active5, also appears and shifts to 282cm 1 in our FTIR data. 6(184 cm1) can be also seen in the spectrum, which suggests alarge distortion appearing in the octahedral. From an inactivemode to a Raman and IR active mode, the breaking of selectionrule is tremendous. That may result from the large distortionof the TiO 6 octahedra caused by oxygen vacancies and inter-nal strain, and furthermore might shed some new light on themechanism of the fatigue in BTO.

4. Conclusions

Bismuth titanate nanocrystals have been successfully pre-pared by the MOD method. XRD result revealed that the BTOnanocrystals belong to the orthorhombic phase with average

size of 60 nm. Space group theoretical analyses predict 24


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Raman active modes and 28 IR active modes, respectively.Raman scattering and infrared absorptionmeasurementsof BTOnanocrystals showed a large decrease of the number of peaks,which could be interpreted by the fact that the characteristicvibrational modes of BTO nanocrystals arise mainly from theinternal modes of TiO 6 octahedra. Raman shifts and infraredabsorptionbands in BTOnanocrystals were tentatively assignedbased on experimental results accordingto thevibrational modesof metaloxygen anion groups. The octahedral Ti(2)O 6 showlarger distortions than Ti(1)O 6 which lead to the splitting of some Raman peaks. Originally Raman active mode 1, 2 , and5 become IRactiveaswell due to the large distortion in Ti(2)O 6and are all observed in our FTIR data. An inactive mode foroctahedra has been found in both our Raman and IR data, whichcan be explained by a large distortion appearing in BTO. Thedistortion may result from the oxygen vacancies and strain inthe BTO, which are commonly accepted as the origin of thefatigue in BTO. We hope our research may shed some light onthe mechanism of fatigue in Bi layered perovskites.

Acknowledgement

This work is supported by the National Natural ScienceFoundation of China (no. 10274043), the Shandong ProvincialNatural Science Fund and the State Key Laboratory of CrystalMaterials of Shandong University.

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Raman BiTiO