This article was downloaded by: [TOBB Ekonomi Ve Teknoloji]On: 21 December 2014, At: 17:42Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

FerroelectricsPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/gfer20

Grain size dependence of relaxorcharacteristics in La-modified leadzirconate titanateQi Tan a & Dwight Viehland aa Department of Materials Science and Engineering, andMaterials Research Laboratory , University of Illinois atUrbana-Champaign , Urbana, IL, 61801, USAPublished online: 26 Oct 2011.

To cite this article: Qi Tan & Dwight Viehland (1997) Grain size dependence of relaxorcharacteristics in La-modified lead zirconate titanate, Ferroelectrics, 193:1, 157-165, DOI:10.1080/00150199708228329

To link to this article: http://dx.doi.org/10.1080/00150199708228329

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoeveras to the accuracy, completeness, or suitability for any purpose of the Content. Anyopinions and views expressed in this publication are the opinions and views of theauthors, and are not the views of or endorsed by Taylor & Francis. The accuracy of theContent should not be relied upon and should be independently verified with primarysources of information. Taylor and Francis shall not be liable for any losses, actions,claims, proceedings, demands, costs, expenses, damages, and other liabilitieswhatsoever or howsoever caused arising directly or indirectly in connection with, inrelation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms& Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Frrroeircrricr, 1997. Vol. 193, pp. 157-165 Reprints available directly from the publisher Photocopying permitted by license only

@;I 1997 OPA (Overseas Publishers Association) Amsterdam B V. Published in The Netherlands under

license by Gordon and Breach Science Publishers Printed in India

GRAIN SIZE DEPENDENCE OF RELAXOR CHARACTERISTICS IN La-MODIFIED LEAD

ZIRCONATE TITANATE

QI TAN and DWIGHT VIEHLAND

Department of Materials Science and Engineering, and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, I L 61 801 U S A

(Received I 1 May 1996: In final form 16 July 1996)

Studies have been performed on a series of La-modified lead zirconate titanate (PLZT) speci- mens with grain sizes between 0.6 and 6 prn by dielectric spectroscopy and transmission electron microscopy. These investigations have revealed an increase in the degree of relaxor characteristics with decreasing grain size for tetragonal-structured PLZT. In addition, small grain sizes were found to favor tweed-like structures and polar nanodomains, over normal micron-sized ferroelectric domains.

Keywords: Grain size; relaxor; domain; PZT

A number of interesting studies concerning the influence of grain size on the dielectric proper tie^['-^-^], optical propertiesc4], and structural changesL51 have been reported for ferroelectrics. Nearly all of these investigations have fo- cussed on conventional ferroelectric systems such as BaTiO,. An internal stress model has been proposed by Buessem et al. for BaTiO, fine-grained ceramics[21. According to this model, grain boundaries act as constraining surfaces, which may then serve as nucleation sites for twinning. However, at some critical grain size with decreasing average size, twinning does not occur due to the development of large internal stresses, rather each grain contains a single domainr21.

At much smaller average grain sizes, studies of sol-gel derived nanocrystal- line ferroelectric films with an average grain size below a critical limit have shown no evidence of ferrelectricityr6.’J. It is generally believed that ferroelec- tricity is destroyed with decreasing grain size in this range. KanzigcS1 found

157

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014

158 Q. TAN AND D. VIEHLAND

that the transition temperature (T,) of BaTiO, increased with decreasing grain size, while more recent investigations have found that the transition tempera- ture decreases with decreasing grain size", 5,9 ], exhibiting a broadening of the transformation. Ishikawa et al.['"' have investigated the temperature depend- ence of the soft phonon mode in PbTiO, as a function of grain size using Raman spectroscopy. They found that the transition temperature was nearly size independent until - 0.05 pm, and then decreased markedly at smaller sizes.

Nevertheless, limited investigations have been performed concerning the grain size dependence of ferroelectric phase transformations in (Pb, - 3,2x La,) (Zr, -yTiy)03 (PLZT lOOx/(l - y ) / y ) ceramics. In particular, little is known about how the relaxor characteristics of PLZT are influenced by grain size. Relaxor ferroelectrics are characterized by a frequency dispersive dielectric response whose maximum value shifts to higher temperatures with increasing measurement frequency["], and by the presence of polar nanodomains or clusters['*]. The microstructural and dielectric properties of ferroelectric PLZT 100x/65/35 and x/40/60 large grained ceramics have recently been systematically studied as a function of La-c~ntent"~-'~]. A common sequence of domain-like states was observed with increasing La-content, including: (i) normal micron-sized domains, (ii) tweed-like structures, and (iii) polar nanodomains or clusters. The purpose of this investigation was to study changes in the relaxor characteristics and domain-like structures with changes in grain size for PLZT. Investigating were performed by dielectric spectroscopy. Sawyer-Tower polarization (P-E) methods, and microstructural analysis. The compositions chosen for study were PLZT 5/40/60 and 12 140 / 60.

Dense specimens were obtained using a mixed oxide method followed by hot pressing at various temperatures (960, 1120, 1150°C) under a pressure of 60 MPa. To achieve larger grain sizes, specimens were then annealed at 1300°C for various times in a PbO excess environment. The complex dielec- tric permittivity was measured using a Hewlett-Packard 4284A inductance- capacitance-resistance (LCR) meter which can cover a frequency range between 20 and lo6 Hz. Low temperature measurements were made by placing samples in a Delta Design 9023 test chamber. The P-E behavior was characterized with a computer-controlled, modified Sawyer-Tower cir- cuit using a measurement frequency of 50 Hz. The TEM studies were done on a EDS-attached Phillips EM-420 microscope operating at an accelera- ting voltage of 120 kV. A Hitachi-800 SEM was utilized to check grain morphology and average grain sizes.

The grain size dependence of the ferroelectric phase transformation for 5/40/60 is shown in Figure 1. A decrease of the permittivity and an increase

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014

GRAIN SIZE DEPENDENCIES IN La-MODIFIED PZT 159

n m 12

v

150 200 250 300 350 400 Temperature ("C)

FIGURE 1 grain sizes. The measurement frequency are lo3, lo4, lo5 Hz (from top to the bottom curves).

Dielectric constant as a function of temperature for PLZT 5/40/60 at various

in the transition temperature can be seen with decreasing grain size. Also, a broadening of the temperature dependent characteristics and an increase in the frequency dispersion were observed with decreasing grain size. Electron microscopy for this composition did not reveal any apparrant changes in the domain morphology with grain size. Also no observed compositional inhomogeneity was found for these samples by EDS. Though the general domain morphology did not change, the domain size was significantly de- creased with decreasing grain size. The increase of the fraction of grian boundaries probably results in different domain intersection with grain boundaries, as well as a decrease of polarization in the smaller grain. We believe this effect to be responsible for the change of dielectric response.

For 12/40/60, relaxor behavior was apparent in the dielectric response, in addition a spontaneous relaxor-normal ferroelectric transition was also observed at temperatures significantly below that of the dielectric maximum (Tmax). The influence of grain size on the dielectric responses is illustrated in Figures 2(a)-(c). With decreasing grain size, the dielectric constant was de- creased and the dielectric peak broadened, resulting in a decrease in the relaxor-normal transition temperature. It should be noticed at small grain sizes that the diffuseness in the temperature dependent characteristics was

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014

160 Q. TAN AND D. VIEHLAND

16, . . . . , . . . . , . . . . , . . . . ,

O 2 0 . . . . 100 a - . . . 150 ' . . 200 ' . . . * ' 250

Temperature (%)

FIGURE 2 Dielectric constant as a function of temperature for PLZT 12/40/60 at various grain sizes. (a) 5 pm, (b) 2.7 pm and (c) 0.64 pm. The measurement frequencies used were lo2, lo3, lo4, lo5 and 5 x lo5 Hz (from the top to the bottom curves).

increased. These results indicate that normal ferroelectric behavior is weakened and relaxor behavior enhanced with decreasing grain size for 12/40/60.

Finite grain size effects were also evident in the P-E response for 12/40/60, as shown in Figure 3. Inspection of this figure will reveal with decreasing grain size that the remanent and saturation polarizations de- creased. Figures 4(a)-(c) show the grain morphologies for 12/40/60 speci- mens with various grain sizes. A homogeneous grain distribution and phase

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014

GRAIN SIZE DEPENDENCIES IN La-MODIFIED PZT 161

-40 -20 0 20 40 Electric Field (kV/crn)

FIGURE 3 various grain sizes.

Room-temperature polarization behaviors for PLZT 12/40/60 specimens with

uniformity can be seen throughout the specimens. Microstructural analysis excluded further study of specimens with grain sizes below 0.64 pm. X-ray diffraction (XRD) and energy dispersive analysis by x-rays (EDS) on finer- grained specimens revealed an incomplete reaction of titania and La, result- ing in strongly compositional and phase nonuniformity for grain sizes below 0.6 lm .

A change in the stable domain structure with decreasing grain size was observed for 12/40/60, as shown in Figures 5(a)-(c). For an average grain size of 0.64 pm (Fig. 5(a)), submicron-sized domains, tweed-like subdomain structures and nanodomains can be seen to coexist throughout the specimen. However, the stable domain structure for a 5 pm average grain size specimen (Fig. 5(b)) was observed to be micron-sized ferroelectric domains at room temperature. Comparisons of Figures 5(a) and (b) will demonstrate that de- creasing grain size favors decreasing degrees of domain evolution, consistent with the enhanced relaxor-like characteristics and broadening of the tempera- ture dependent dielectric response shown in Figures 1 and 2. Moreover, the stable domain-like state in Figure 5(b) was found to be sensitive to changes in electron beam intensity, while that in fine-grained specimen was not. In coarser-grained specimen, the micron-sized ferroelectric domains transformed to nanodomains with increasing beam intensity (Fig. 5(c)), however the

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014

162 Q. TAN AND D. VIEHLAND

FIGURE 4 SEM micrographs of PLZT 12/40/60 after various heat treatment procedures. (a) Hot-pressing at 1120°C for 2 hours followed by annealing at 1300°C for 2 hours, (b) hot- pressing at 1120°C for 2 hours and (c) hot-pressing at 960'C for 4 hours followed by annealing at 1000°C for 1 hour.

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014

GRAIN SIZE DEPENDENCIES IN La-MODIFIED P Z T 163

FIGURE 5 Bright-field images for a PLZT 12/40/60 specimen with various grain sizes. (a) 0.64 pm, (b) 5 pm, taken under weak beam conditions and (c) 5 pm taken under strong bcam conditions.

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014

164 Q. TAN A N D D. VIEHLAND

micro-sized domains were recovered after the beam was weakened. These changes probably resulted from changes in the local temperature induced by electron beam heating, resulting in a relaxor-normal ferroelectric transition.

It is known that the composition 12/40/60 is near a cross-over between relaxor and normal ferroelectric behaviorsc13 I. This composition exhibits unique behaviors which are significantly different from those of either nor- mal ferroelectrics or relaxors, i.e., the evolution of ferroelectric order through a sequence of domain-like states. At high temperatures, polar clus- ters form which then coarsen on cooling. Near the temperature of the dielectric maximum, polar clusters begin to organize into tweed-like struc- tures, which then spontaneously organize into normal micron-sized do- mains on further cooling. The organization of cluster into tweeds and then tweeds into domains is driven by long-range interactions. Theories of premartensitic behavior have previously shown how long-range strain inter- actions between low-temperature precursor states within the high-tempera- ture parent phase result in the stabilization of tweed-like structures[' 6*1 71.

Similarly, the evolution of order through the sequence of domain-like states for PLZT would seemingly be driven by long-range dipolar and/or quadru- polar interactions. In the case of finite grains, tweed-like structures would seemingly be favored over micron-sized domains, possibly due to the inter- ruption of long-range interactions by the increased particle surface area. As a result, enhanced degrees of relaxor characteristics may be observed with decreasing grain size.

In summary, dielectric spectroscopy and electron microscopy studies have been performed on PLZT 5/40/60 and 12/40/60 ceramic specimens with grain sizes in the range between 0.6 and 6 pm. These studies have revealed that finite grain sizes decrease the dielectric permittivity. In addi- tion, decreasing grain size tends to enhance relaxor behavior, which can be explained in terms of the domain refinement and weakening of long-range ferroelectric interactions.

This work was supported by the Office of Naval Research (ONR) under contract No. N00014-95-1-0805 and by Naval Undersea Warfare Center contract No. N66604-95-C-1536. The use of the facilities in the Center for Microanalysis in Materials Research Laboratory at the University of II- linois at Urbana-Champaign is gratefully acknowledged.

References

[l] Arlt, G., Hennings, D. and de With, G. (1985). J. Appl. Phys., 58, 1619. [Z] Busseum, W. R., Cross, L. E. and Goswami, A. K . (1966). J . Am. Cer. SOC., 49, 33.

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014

GRAIN SIZE DEPENDENCIES IN La-MODIFIED PZT 165

[3] Lee, B. W. and Auh, K. H. (1995). J . Mater. Res., 10, 1418. [4] Okazaki, K. and Nagata, K. (1973). J . Am. Ceram. Soc., 56, 82. [5] Caboche, G. and Niepce, J. C. (1990). 1990 IEEE 7th International Symposium on Appli-

cations of Ferroelectrics, Ed. by S . B. Krupanidhi and S . K. Kurtz, IEEE Catalog number: 90CH2800-1, Urbana.

[6] Frey, M. H. and Payne, D. A. (1993). Appl. Phys. Lett., 63, 2753. [7] Tani, T. (1993). PhD Dissertation, University of Illinois, Urbana, IL. [S] Anliker, M., Brugger, H. and Kanzig, W. (1954). Helu. Phys. Acta, 27, 99. [9] Kanata, K., Yoshikawa, T. and Kubota, K. (1987). Solid state Commun., 62, 765. :lo] Ishikawa, K., Yoshikawa, K. and Okada, N. (1988). Phys. Rev. B 37, 5852. :11] Smolenski, G. and Agranovskaya, A. (1960). Sou. Phys. Solid State, 1, 1429. :12] Haertling, G. and Land, C. (1971). J . Am. Ceram. Soc., 54, 1. .13] Dai, X. H., Xu, Z. and Viehland, D. (1994). Phil. Mag., B 70, 33. :14] Dai, X. H. and Viehland, D. (1994). J . Appl. Phys., 76, 3701. :15] Li, J. F., Dai, X. H., Chow, A. and Viehland, D. (1995). J . Mater. Res., 10, 926. 1161 Bratkovsky, A., Salje, E. K. H. and Heine, V. (1994). Phase Transitions, 52, 77. 1171 Semenovskaya, S . and Khachaturyan, A. G. (1993). Phys. Rev., B 47, 12182.

Dow

nloa

ded

by [

TO

BB

Eko

nom

i Ve

Tek

nolo

ji] a

t 17:

42 2

1 D

ecem

ber

2014