Homogeneity of Pb(Zr,Ti)O3 thin films by chemical solution deposition:Extended x-ray absorption fine structure spectroscopy study of zirconiumlocal environmentBarbara Malic, Iztok Arcon, Alojz Kodre, and Marija Kosec Citation: J. Appl. Phys. 100, 051612 (2006); doi: 10.1063/1.2337079 View online: http://dx.doi.org/10.1063/1.2337079 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v100/i5 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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JOURNAL OF APPLIED PHYSICS 100, 051612 �2006�

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Homogeneity of Pb„Zr,Ti…O3 thin films by chemical solution deposition:Extended x-ray absorption fine structure spectroscopy studyof zirconium local environment

Barbara Malica�

Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

Iztok ArconJozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia and University of Nova Gorica, Vipavska 13,5000 Nova Gorica, Slovenia

Alojz KodreJozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia and Faculty of Mathematics and Physics,University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia

Marija KosecJozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

�Received 18 July 2006; accepted 18 July 2006; published online 15 September 2006�

Sols for Pb�Zr0.53Ti0.47�O3 �PZT� thin films were prepared by 2-methoxyethanol route from leadacetate, titanium n-propoxide, and zirconium n-propoxide, the latter either unmodified or modifiedwith acetylacetone or acetic acid in a 2/1 molar ratio and deposited on sapphire �0001�. By ZrK-edge extended x-ray absorption fine structure �EXAFS� spectroscopy, the structural changes in theZr local environment, induced by the addition of the two modifiers, were followed from thesynthesis of the PZT sol to the transition to the amorphous film. In the unmodified PZT solsegregation of Zr species occurs from the original dimers present in the Zr propoxide solution in2-methoxyethanol. The immediate neighborhood of Zr atoms changes markedly at the transitionfrom the sol to the amorphous film: the local structure around Zr atoms is similar to the one foundin tetragonal zirconia particles. The modification of Zr propoxide with acetylacetone in2-methoxyethanol results in Zr monomers. In PZT sol, clustering of Zr species is observedcontinuing into the amorphous film. By modification with acetic acid the original dimeric structureof the Zr precursor is retained in the PZT sol and further in the amorphous film. Selectivemodification of Zr propoxide with acetic acid therefore results in a more homogeneous distributionof Zr atoms in the PZT sol and amorphous film than in both as-received and acetylacetone-modifiedZr propoxide. © 2006 American Institute of Physics. �DOI: 10.1063/1.2337079�

INTRODUCTION

The research on chemical solution deposition �CSD�methods for Pb�Zr,Ti�O3 �PZT� ferroelectric thin films isoriented towards higher reliability and lower processingtemperatures.1–5 The 2-methoxyethanol based route6 is prob-ably the most widely spread of CSD. It is based on reactionsof transition metal alkoxides TM�OR�4 �TM=Zr,Ti� withsoluble lead compounds, such as acetate, in 2-metho-xyethanol resulting in stable polymeric sols. From the earlydays of CSD the problems of local heterogeneities in thefilms, in the form, for example, of the nonferroelectric sur-face pyrochlore phase7–9 or zirconium-rich regions10 havebeen recognized. Heterogeneity on the atomic level has beendemonstrated in amorphous alkoxide precursors of PZT byextended x-ray absorption fine structure �EXAFS�spectroscopy.11–13

The TM alkoxides have been modified by substitution oraddition of ligands less reactive towards hydrolysis such asacetylacetone �CH3COCH2COCH3� or acetic acid�CH3COOH�.14 Examples include the use of acetylacetone

a�

Electronic mail: barbara.malic@ijs.si

0021-8979/2006/100�5�/051612/8/$23.00 100, 0516

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to stabilize Ti alkoxide in the synthesis of PbTiO3 thinfilms15 or both Zr and Ti alkoxides in the synthesis of PZTthin films.16–18 This modifier has been also used to increasethe pyrolysis temperature of the PZT precursor solution andconsequentially influence the crystallization of the respectivethin film; however, it has been added to the already synthe-sized PZT solution prior to the film deposition.19,20 Aceticacid has also been used in PZT thin film processing in therole of a solvent and modifier.9,21

We have used a selective modification of zirconiumalkoxide by acetic acid and by acetylacetone to decrease itsreactivity prior to the PZT sol synthesis. We have shown byEXAFS spectroscopy that a selective modification of the Zralkoxide by acetic acid results in a more homogeneous dis-tribution of constituent metal atoms in the PZT sol as com-pared to the sols prepared from either as-received oracetylacetone-modified Zr propoxide.22 Structural changesinduced by the Zr modifiers are also reflected in the localneighborhood of the Pb atoms.23 The differences between thethree sols in terms of functional group contents could addi-tionally be deduced from the differences in their thermal de-

composition pathways. The improved homogeneity of acetic

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acid modified PZT films is reflected in its functional re-sponse: optimum values of remanent polarization, coercivefield, and dielectric permittivity are obtained after annealingat 500 °C while the unmodified and acetylacetone-modifiedPZT films need to be annealed at 600 °C.24

In this work we exploit Zr K-edge EXAFS analysis tofollow structural changes in the Zr local environment in-duced by the addition of the two modifiers, from the Zr pre-cursor through the synthesis of the PZT sol to the transitionfrom the sol to the amorphous film.

EXPERIMENT

PZT sols corresponding to the target composition ofPbZr0.53Ti0.47O3 with 10 mol % PbO excess were preparedby diluting zirconium n-propoxide �Alfa, 72.49% in n-PrOH� and titanium n-propoxide �Alfa, 99.84%� in2-methoxyethanol �Aldrich, 99.3%� and by adding lead ac-etate in the required quantity. Water-free lead acetate wasobtained from lead acetate trihydrate �Alfa, ACS� by dryingin vacuum at 70 °C for 16 h and in air at 140 °C to constantmass. The metal contents of the starting compounds weredetermined gravimetrically.

Zirconium n-propoxide was either used as received ormodified prior synthesis. As modifiers, acetic acid�CH3COOH, Aldrich, 100%� or acetylacetone�CH3COCH2COCH3, Alfa, 99%� were used. Acetic acid wasadmixed to Zr propoxide in the molar ratio of 2 /1 and di-luted to 1M by 2-methoxyethanol. Acetylacetone, diluted by2-methoxyethanol, was added to Zr propoxide solution in2-methoxyethanol in 2/1 molar ratio, the concentration wasadjusted to 1M with respect to Zr. The as-prepared 1M so-lutions were also used for EXAFS measurements.

The reaction mixtures were heated to approximately60 °C to dissolve lead acetate. After 2 h of refluxing, distil-lation of the by-products, and after cooling to room tempera-ture, addition of formamide �4 vol %, Alfa� stable sols�0.5M, 100 ml batches� were obtained. All manipulationswere performed in dry nitrogen atmosphere. For EXAFSmeasurements the PZT sols were further concentrated to 1Msolutions in order to increase signal-to-noise ratio.

PZT thin films were prepared from the 0.5M PZT sols onsapphire �0001� by spin coating and pyrolyzed at 350 °C for1 min. The thickness of the films was approximately 200 nmas determined on a Rank Taylor Hobson profilometer.

The unmodified, acetic-acid and acetylacetone-modifiedZr propoxide sols are denoted as �Zr/O�, Zr/OAc, and Zr/Acac, respectively. For the PZT sols and films analogousnotation is used.

Zr K-edge EXAFS spectra of the Zr and PZT sols weremeasured at the X1 experimental station in HASYLAB atDESY �Hamburg, Germany�. A Si�311� double-crystalmonochromator was used with 3 eV resolution at 18 keV.Harmonics were effectively eliminated by detuning themonochromator crystal using a stabilization feedback con-trol. The sols were contained in a liquid absorption cell with

0.5 mm lucite windows. Sample thickness of about 2 mm

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provided total absorption thickness of about 2 above ZrK-edge. The empty absorption cell served for reference spec-trum measured in identical conditions.

Zr EXAFS spectra of the PZT films were measured withfluorescence detection technique using a four-channel Gefluorescence detector. The absorption spectra were obtainedas the ratio of the fluorescence detector signal and the signalof the incident photon beam from the ionization cell filledwith argon at ambient pressure. The same stepping progres-sion was adopted as in the case of the sol with an integrationtime of 4 s/step. To improve signal-to-noise ratio, four con-secutive runs were superimposed.

RESULTS

Zr sols

EXAFS spectra were analyzed by the University ofWashington analysis programs using a FEFF6 code for abinitio calculation of scattering paths.25,26 Fourier transformsof the k3 weighted spectra of the different Zr sols are shownin Fig. 1. Zr neighbors are discerned in two wide shells. Eachshell is fitted separately in the k interval from 4 to 14 Å−1.The R intervals of 1.3–2.1 Å and 2.7–3.6 Å are used forrespective shells. The best fit results are collected in Table I.

The immediate neighborhood of Zr atoms in the unmodi-fied Zr sol �Zr/O� is populated with six O atoms at twoslightly different distances, namely, 1.96 Å �approximatelytwo atoms� and 2.17 Å �approximately four atoms�. The sec-ond shell of neighbors consists of one Zr atom at 3.53 Å andof a large number �approximately ten� of C atoms at threedistances between 2.96 and 3.69 Å.

The modification of Zr alkoxide with acetylacetone �Zr/Acac� results in a significant change of coordination, namely,from approximately six to approximately eight O atoms dis-tributed nonuniformly at three distances: one at 2.01 Å, threeat 2.14 Å, and four at 2.23 Å. The second shell of neighbors

FIG. 1. The k3 weighted Fourier transforms of zirconium EXAFS spectra ofZr sols. Solid line—experiment and dotted line—model.

consists only of six C atoms at two distances. Models includ-

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ing Zr atoms in the second coordination sphere were alsotested, but no Zr–Zr correlations were found. A presence of asmaller amount of Zr neighbors �average coordination num-bers below detection limit of 0.3� cannot be excluded.

By modification of Zr alkoxide with acetic acid�Zr/OAc� the immediate neighborhood of Zr atoms is popu-lated by six to seven O atoms: one at 1.96 Å and the rest at2.16 Å. The wide second shell of neighbors consists of oneZr atom at 3.54 Å and a large number ��12� of C atomsbetween 2.97 and 3.69 Å.

Amorphous PZT films on sapphire

The PZT films pyrolyzed at 350 °C are amorphous asdetermined by x-ray diffraction �XRD�.22 The comparison ofthe Zr EXAFS spectra of the PZT sols, recorded in transmis-sion mode, and respective thin films is shown in Fig. 2. Inspite of the much longer detection time the signal-to-noiseratio of the spectra measured on the films is about one orderof magnitude lower. Nevertheless, the local environment ofZr atoms in the PZT films can be reliably deduced in thequantitative EXAFS analysis in the k interval from4 to 11 Å−1.

TABLE I. Parameters of the nearest coordination shells around Zr atoms in dfactor �2. Uncertainty of the last digit is given in parentheses.

Zr neighbor

Zr/O

N R �� �2 N

O 1.7�5� 1.96�1� 0.003�1� 1.0�3�O 4.5�5� 2.17�1� 0.007�1� 3.1�5�O ¯ ¯ ¯ 3.9�5�C 2.1�6� 2.96�2� 0.002�1� ¯

C 3.5�8� 3.09�3� 0.002�1� 0.8�5�Zr 0.9�2� 3.53�2� 0.004�1� ¯

C 4.2�9� 3.69�5� 0.002�1� 4.5�9�

FIG. 2. Zr K-edge EXAFS spectra of PZT sols �solid line� and amorphousthin films on �0001� sapphire �dots�. The spectra of PZT/O sol and film are

from Ref. 38.

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Figures 3–5 show the k3 weighted Fourier transforms ofthe Zr EXAFS spectra. Two shells of Zr neighbors are dis-cerned in all three spectra. The spectra of the respective PZTsols22 are added for comparison. The complete list of best fitstructural parameters is collected in Table II. Relatively highuncertainties in the number of neighbors in individual shellsare due to the high correlation of the shell parameters N and�2.

The local neighborhood of Zr atoms in PZT/O �Fig. 3� inthe R range of 1.3–3.8 Å consists of about four O atoms at2.14 Å and three O atoms at a much larger distance of2.67 Å. Three Zr and six O atoms are determined in thesecond coordination shell. In PZT/Acac film the local Zrneighborhood �Fig. 4� strongly differs from the one deter-mined in PZT/O. The first coordination shell consists ofabout seven O atoms: five at 2.14 Å and two at 2.29 Å. Thesecond shell is composed of about two Zr atoms at 3.39 Åand about seven C atoms at 3.71 Å. In the PZT/OAc film thelocal zirconium environment is again different �Fig. 5�. Thefirst shell of neighbors is comprised of about six O atoms:

nt Zr sols: atomic species, average number N, distance R, and Debye-Waller

Zr/Acac Zr/OAc

R �� �2 N R �� �2

2.01�1� 0.002�1� 1.2�4� 1.96�1� 0.002�1�2.14�1� 0.002�1� 5.7�4� 2.16�1� 0.006�1�2.23�1� 0.005�1� ¯ ¯ ¯

¯ ¯ 2.4�6� 2.97�2� 0.003�1�3.19�3� 0.002�1� 3.4�8� 3.11�3� 0.003�1�¯ ¯ 1.0�2� 3.54�2� 0.004�1�

3.67�1� 0.003�1� 6�2� 3.69�5� 0.002�1�

FIG. 3. The k3 weighted Fourier transforms of zirconium EXAFS spectra ofPZT/O sol and the respective amorphous thin film on �0001� sapphire, cal-culated in the k range of 4–11 Å−1: �Solid line—experiment and dashed

iffere

line—model�. The data are from Ref. 38.

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four at 2.14 Å and two at 2.32 Å. The second shell of neigh-bors consists of one Zr atom at 3.44 Å and about ten C atomsat three distances between 2.83 and 3.68 Å.

DISCUSSION

Zr sols

Zr n-propoxide reacts with the solvent 2-methoxyethanolby the transalcoholysis reaction, i.e., by exchange of alkox-ide groups,14 resulting in a less reactive mixed alkoxid �Eq.�1��.The extent of the exchange has not been determinedwithin this study; we nevertheless expect that x in Eq. �1� isclose to 4 in agreement with Ref. 27. In addition, the solventmay coordinatively bond to the alkoxide molecules via thehydroxy and the ether groups.28

Zr�OC3H7�4 + xCH3OCH2CH2OH

→ Zr�OC3H7�4−x�OCH2CH2OCH3�x + xC3H7OH.

�1�

EXAFS analysis of Zr/O sol reveals a sixfold coordina-tion of O atoms forming a distorted octahedron and one Zr

FIG. 4. The k3 weighted Fourier transforms of zirconium EXAFS spectra ofPZT/Acac sol �from Ref. 22� and the respective amorphous thin film on�0001� sapphire, calculated in the k range of 4–11 Å−1: �Solid line—experiment and dashed line—model�.

TABLE II. Parameters of the nearest coordination shells around zirconiumDebye-Waller factor �2. Uncertainty of the last digit is given in parentheses

Zr neighbor

PZT/O filma

N R �� �2 N

O 4.0�7� 2.14�4� 0.003�1� 5.0�8�O 2.8�7� 2.67�4� 0.002�1� 2.0�4�C ¯ ¯ ¯ ¯

C ¯ ¯ ¯ 3.3�5�Zr 3�1� 3.43�2� 0.006�2� 1.6�4�C ¯ ¯ ¯ 7.0�9�O 6�1� 4.19�5� 0.007�3� ¯

a

The data for PZT/O film are from Ref. 38.

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atom at 3.53 Å. A large number of carbon atoms in the sec-ond shell of neighbors stems from different organic groups inthe solution �Table I�.

Our results indicate oligomeric Zr units, most probablydimeric, in agreement with the model of Peter et al.29 whoexamined the structure of Zr n-propoxide in a n-propanolsolution and found six O atoms evenly distributed at threedistances, one Zr atom at 3.52 Å, and proposed dimeric unitswith two terminal, two bridging and two ROH-coordinatedZr–O bonds. We found a simpler distribution of bond dis-tances in our case: two shorter and four longer Zr–O bonds.Such distribution of Zr–O bond distances has been observedalso in a mixed Zr–Ti n-propoxide in n-propanol.30

The reaction of Zr alkoxide with acetylacetone in a 1/2molar ratio should result in the exchange of the functionalgroups14 �Eq. �2�� with an increase of coordination numberof zirconium as a consequence of chelate bonding of theacetylacetonate groups. Note that the equation does not in-clude any coordinative bonding of the solvent.

FIG. 5. The k3 weighted Fourier transforms of zirconium EXAFS spectra ofPZT/OAc sol �from Ref. 22� and the respective amorphous thin film on�0001� sapphire, calculated in the k range of 4–11 Å−1: �Solid line—experiment and dashed line—model�.

s in PZT thin films: atomic species, average number N, distance R, and

ZT/Acac film PZT/OAc film

R �� �2 N R �� �2

2.14�2� 0.004�2� 4.4�4� 2.14�1� 0.0010�5�2.29�2� 0.004�2� 2.0�4� 2.32�2� 0.007�5�¯ ¯ 1.5�7� 2.83�4� 0.001�1�

3.36�3� 0.003�1� 2.0�7� 3.32�4� 0.001�1�3.39�2� 0.004�2� 1.0�4� 3.44�4� 0.004�2�3.71�3� 0.003�1� 6.5�8� 3.68�4� 0.001�1�¯ ¯ ¯ ¯ ¯

atom.

P

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Zr�OR�4 + 2CH3COCH2COCH3

→ Zr�OR�2�CH3COCHCOCH3�2 + 2ROH. �2�

The absence of Zr–Zr correlations in the Zr/Acac sol,established by EXAFS �Table I�, indicates monomers. Theeightfold coordination of O atoms is strongly asymmetricwhich confirms that different groups—chelating acetylaceto-nate, 2-methoxyethoxide, and/or propoxide—are bonded toZr atoms, in agreement with stoichiometric considerations.We cannot exclude coordinative bonding of the solvent mol-ecules. This leads to the conclusion that the original Zrdimers disintegrate into monomers upon reaction of acetylac-etone with Zr alkoxide.

In contrast, Peter et al.31 observed that upon addition ofacetylacetone to Zr butoxide in a strongly diluted �0.03m�n-butanol solution, dimers were obtained with 2 mol andmonomers with 4 mol of acetylectone. The difference couldarise from the dilution, and the choice of the solvent,2-methoxyethanol in our case, and consequently its exchangereaction with the original Zr propoxide.

Zr alkoxide reacts with acetic acid in a 1/2 molar ratioto yield a mixed Zr alkoxide acetate �Eq. �3��.

Zr�OR�4 + 2CH3COOH → Zr�OR�2�CH3COO�2 + 2ROH.

�3�

In Zr/OAc sol the local neighborhood of Zr atoms ispopulated with six to seven O atoms and about one Zr atomat 3.53 Å �Table I�. Note that both coordination numbers 6and 7 are common for zirconium species. The result againindicates dimeric Zr units as in the case of the unmodified Zrprecursor. The Zr–O and Zr–Zr distances remain unchanged,however; the distribution and possibly the number of Oneighbors in the nearest coordination shell are altered incomparison to unmodified Zr sol. We ascribe this change tothe reaction of acetic acid with the Zr alkoxide. In agreementwith stoichiometric considerations each Zr atom is coordi-nated by two acetate and at most four alkoxide groups, de-pending on the type of reaction.

This result is in agreement with similar findings for Zrn-propoxide or n-butoxide modified with 2 mol of aceticacid in a 0.03m parent alcohol solution, where Fourier-transform infrared �FTIR� and EXAFS spectroscopy demon-strated that acetate groups substitute terminal alkoxidegroups and that the dimeric Zr–Zr units are preserved.31

PZT sols

Upon the synthesis of PZT sol the heterometallic precur-sor should be formed by a reaction between individual tran-sition metal �TM=Zr,Ti� alkoxide and lead acetate.6,28 Thereaction can proceed by ester elimination and/or additionleading to the formation of oxo, viz., acetate bridges betweenthe two metal atoms �Eqs. �4� and �5��. �The schematic reac-tions below are for one functional group per reactant.� Inreality both reactions occur, leading to a product containingoxo, alkoxo, and acetate groups �Eq. �6��.32–34

–Pb – OAc + OR – TM � Þ – Pb – O – TM � + ROAc,

�4�

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–Pb – OAc + OR – TM � Þ – Pb – OAc → TM�OR� � ,

�5�

Pb�OAc�2 + TM�OR�4 Þ PbTMOx�OAc�y�OR�z. �6�

The three Zr sols have been used in the synthesis of thePZT sols and the local neighborhoods of Zr in the PZT solshave been examined.22,24 These results are briefly summa-rized here. In all cases the local environment of Zr atomsconsists of two broad shells of neighbors. The first shell iscomposed of six to eight O atoms at two slightly differentdistances, 2.10–2.14 Å and 2.20–2.28 Å. In PZT/O andPZT/Acac sols the coordination is close to eight while thelower octahedral coordination is more probable for the PZT/OAc sol. The second shell of Zr neighbors is composed of Zrand C atoms. The number of Zr neighbors located at3.44–3.49 Å is about 3 for PZT/O and PZT/Acac sols andonly about 1 for PZT/OAc sol.

In the PZT/O sol we observe a major change in the localZr environment22 in relation to the Zr/O sol. In the first shellof neighbors not only the number of O atoms increases from6 to about 8 but also the distribution of distances changes�Fig. 6�a��. The short Zr–O distance at 1.96 Å attributed totwo terminal alkoxide groups �in addition to the four Zr–Obonds at 2.14 Å from bridging and/or coordinated groups� inZr/O is not present in the PZT/O sol, instead there are al-most eight O atoms equally distributed at 2.13 and 2.25 Å. Inthe second shell of neighbors the number of Zr atoms in-creases from �1 for Zr/O to �3 for PZT/O sol. In addition,the number and the distribution of C atom neighbors arechanged: in the PZT/O sol there are less C atoms, located atlarger distances, than in the Zr/O.

The increase of the number of Zr–O bonds and bonddistances leads to the conclusion that the degree of cross-linking of Zr species in PZT/O sol is increased; the shortbond distances stem, namely, from the terminal groups.29 Inaddition, the increased number of Zr–Zr correlations alsosupports increased cross-linking. Segregation of Zr specieshas been quite commonly observed in various amorphous Zrcontaining alkoxide precursors �dried or heat treated at100°C�.11–13,35–37

Our EXAFS results therefore show that for Zr speciesthe major reaction during the PZT/O sol synthesis is theformation of larger oligomeric units. A possible reactionyielding such species could take place during refluxing anddistillation, i.e., at temperatures close to the boiling point ofthe PZT sol. Namely, at such temperatures, polynuclearoxoalkoxides may form by the ether elimination reaction�Eq. �7�� as reported for TM alkoxides including Zr such asZr3O�OiPr�10 or for the reaction between Ba and Tialkoxides.28

�TM – OR + OR – TM � Þ � TM – O – TM � + ROR .

�7�

The summary reaction between Pb acetate and TMalkoxides in 2-methoxyethanol proceeds to some extent asconfirmed by the formation of esters—these are by-productsof the ester-elimination reaction �Eq. �4��—and also by Pb

23

EXAFS that showed the existence of a Pb–Ti correlation.

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We also cannot exclude the possibility of heterometallicbonding below the sensitivity of the EXAFS method.

In the PZT/Acac sol the local environment of Zr atoms iscomposed of about eight O atoms in the first shell distributedat 2.13 and 2.25 Šin a 2/1 ratio and of two to three Zratoms in the second shell22 �Fig. 6�b��. Similarly, as in theunmodified PZT sol, the EXAFS results point to the segre-gation of Zr species.

Note, however, that the distribution of O neighbors inPZT/Acac sol is different from the one in PZT/O sol, indi-

FIG. 6. Number of Zr neighbors �N� at interatomic distances �R� for �a�Zr/O and PZT/O, �b� Zr/Acac and PZT/Acac, and �c� Zr/OAc and PZT/OAc sols deduced from the EXAFS results. Note that only Zr–O and Zr–Zrcorrelations are included, the Zr–C correlations are not shown here. Errorbars represent the uncertainty of the results.

cating that the chemical compositions—the type and amount

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of functional groups bonded to metal atoms in the two PZTsols—are different as confirmed by thermal analysis of therespective as-dried sols.24 Further, clustering of Zr species,possibly via the formation of oxoalkoxides,28 is not hinderedby the presence of acetylacetonate groups.

In PZT/OAc sol, Zr atoms are octahedrally coordinatedwith O atoms distributed in a 4/2 ratio at 2.14 and 2.25 Å.The second shell is composed of about one Zr atom at3.44 Å and of about ten C atoms at two distances.22 Thecomparison of Zr atom environments in Zr/OAc and PZT/OAc sols reveals that the octahedral coordination of Zr isretained upon the synthesis of PZT sol; however, the distri-bution of Zr–O distances is changed. Namely, the short Zr–Odistance of 1.96 Å in the original Zr/OAc is not present inthe PZT/OAc. The Zr–Zr distance in PZT/OAc is smallerthan in Zr/OAc, 3.44 Å instead of 3.54 Å �Fig. 6�c��. Simi-larly, a decrease in the Zr–Zr distance upon modificationwith acetic acid has been observed by Peter et al.31 By modi-fication with acetic acid the original dimeric structure of theZr/OAc precursor is still retained in the PZT/OAc sol, whichis not the case in PZT and PZT/OAc.

Amorphous PZT films

The local neighborhood of Zr atoms in the amorphousPZT/O film consists of about four O atoms at 2.14 Å andabout three O atoms at a much larger distance of 2.67 Å.Three Zr and six O atoms are determined in the second shellof neighbors, at 3.43 and 4.19 Å �Table II�.38 The arrange-ment of O atoms around Zr atoms in the film is significantlyaltered from that in the PZT/O sol: the distribution of Oatoms in the first shell of neighbors is changed and the threeO atoms in the film are located at a much larger distance thanin the sol, 2.67 Å as compared to 2.20 Å. The second shellconsists of Zr and O atoms only, no C atoms could be deter-mined as in the sol �Fig. 7�a��. We attribute this difference tothe decomposition of the organic groups which occurs, ac-cording to thermal analysis of the respective sol,24 between250 and 350 °C. The local structure around Zr atoms is simi-lar to Zr neighborhood in tetragonal zirconia particles.39

In PZT/Acac film the Zr neighborhood consists of aboutfive O atoms at 2.14 Å and about two O atoms at 2.29 Å.The second shell of neighbors consists of about two Zr atomsat 3.39 Å and about ten C atoms at two distances. The dif-ference between PZT/Acac sol and film is not as significantas in the previous case: in the first shell of neighbors the totalnumber of O atoms is decreased. In the second shell, themain difference is in the number of Zr neighbors and thedistances between them: both are lower in the film. In bothcases C atoms are nonequally distributed at two distances�Fig. 7�b��.

In PZT/OAc film, the Zr local neighborhood remainsalmost unchanged as compared to the PZT/OAc sol: the firstshell of neighbors is composed of about four O atoms at2.14 Å and two O atoms at 2.32 Å in the form of a distortedoctahedron. The Zr–O distances in the sol and in the film arethe same within experimental errors. The second shell ofneighbors consists of one Zr atom at 3.44 Å, the distance is

again the same as in the sol �Fig. 7�c��. The number and

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distribution of C atoms in the sol and in the film are similar.Acetic acid therefore successfully contributes to the retentionof small oligomeric Zr units in the transition from the sol tothe amorphous film.

SUMMARY

The sols for Pb�Zr0.53Ti0.47�O3 �PZT� thin films wereprepared by 2-methoxyethanol route from lead acetate, tita-nium n-propoxide, and zirconium n-propoxide, the latter ei-

FIG. 7. Number of Zr neighbors �N� at interatomic distances �R� for �a�PZT/O, �b� PZT/Acac, and �c� PZT/OAc sols and amorphous thin filmsdeduced from the EXAFS results. Note that only Zr–O and Zr–Zr correla-tions are included, the Zr–C correlations are not shown here. Error barsrepresent the uncertainty of the results.

ther unmodified or modified with acetylacetone or acetic acid

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in a 2/1 molar ratio and deposited on sapphire �0001�. Struc-tural changes in the Zr local environment, induced by theaddition of the two modifiers, from the synthesis of the PZTsol and upon transition from the sol to the amorphous filmwere traced by Zr K-edge EXAFS analysis in transmissionand fluorescence mode, respectively.

In Zr propoxide dissolved in 2-methoxyethanol Zr atomsform small oligomeric units, most probably dimers, whichare retained also upon modification with acetic acid. Uponaddition of acetylacetone, Zr monomers are formed.

In the unmodified PZT sol the segregation of Zr speciesresulting in larger oligomeric units is confirmed by EXAFS.The immediate neighborhood of Zr atoms changes markedlyupon transition from the sol to the amorphous film: the localstructure around Zr atoms is similar to the one found intetragonal ZrO2 particles. In acetylacetone-modified PZT sol,clustering of Zr species is observed continuing into the amor-phous film. By modification with acetic acid the originaldimeric units from the Zr sol remain in the PZT sol as wellas in the amorphous film. The changes in the Zr local envi-ronment in the transition from the sol to the film are sche-matically depicted in Fig. 8.

Both modifiers, acetic acid and, to a lesser extent, acety-lacetone suppress the changes in the local Zr atom environ-ment upon transition from the PZT sol to the amorphousfilm. However, while acetic acid is successful in retaining theoriginal dimeric Zr units, the clustering of Zr species occurswhen acetylacetone is used. It is possible that in the case ofthe two modifiers the retention of the Zr local environmentupon transition from the sol to the amorphous film is theconsequence of different processes taking place during thepyrolysis step at 350 °C. Both modifiers induce the higherweight losses of the as-dried sols and they influence, each ina specific way, the thermal decomposition pathways of thesols.24 The crystallization of the perovskite phase on sapphireis enhanced in the acetic-acid modified PZT and to a lesserextent for acetylacetone modified in relation to unmodifiedPZT films. Homogeneous microstructure is characteristic forthe acetic-acid-modified PZT films.40 The improved chemi-cal homogeneity of the PZT films achieved by acetic acidmodification is reflected in their improved functional re-sponse even after low annealing temperatures. The unmodi-

FIG. 8. The scheme illustrates the changes in Zr local environment in theprocess of PZT thin film formation depending on the choice of the modifierthat can be deduced from the EXAFS results.

fied, acetylacetone and acetic-acid-modified PZT films on

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platinized silicon substrates annealed at 600 °C exhibitingthe values of remanent polarization, coercive field, dielectricpermittivity of 17, 21, and 22 �C/cm2, 73, 66, and52 kV/cm, and 1088, 1145, and 1444, respectively.22 Theacetic-acid-modified PZT films annealed at as low as 500 °Creaching almost the same values as at 600 °C while the re-sponse of unmodified PZT is noticeably reduced.24

ACKNOWLEDGMENTS

The work was supported by the Slovenian ResearchAgency �P2-0105 and P1-0112�, the bilateral Project No. BI-DE/03-04-004 by Internationales Buero des BMBF, ECproject Centre for Advanced Processing, Technologies andMaterials for Ceramic Electro and Electromechanical De-vices SICER �Contract no: G1MA-CT-2002-04029�, the EC-Research Infrastructure Action under the FP6 “Structuringthe European Research Area” Programme �through the Inte-grated Infrastructure Initiative “Integrating Activity on Syn-chrotron and Free Electron Laser Science”�, and COST 528Action Chemical solution deposition of thin films. Access tosynchrotron radiation facility of HASYLAB �beamline X1,Project No. II-01-44� is acknowledged. The authors wouldlike to thank Julia Wienold of HASYLAB station X1 forexpert advice on beamline operation.

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