Materials Chemistry and Physics 75 (2002) 151–156

Effects of dopants on the microstructure and properties of PZT ceramics

Wei Qiu, Huey Hoon Hng∗School of Materials Engineering, Division of Materials Science, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore

Abstract

The effects of Pb(Y1/2Nb1/2)O3 (PYN) content on the microstructure and the electrical properties of Pb(Zr0.53Ti0.47)O3 (PZT) havebeen studied. A Pb-deficient polycrystalline phase and a Y–Zr–Ti-rich phase are observed in all the samples. They are minor phases andare only observed using transmission electron microscopy (TEM). The remanent polarizationPr value is found to increase with increasingamount of Pb(Y1/2Nb1/2)O3, and a highest value of 43.679�C cm−2 is obtained for the sample doped with 1.5 mol% Pb(Y1/2Nb1/2)O3.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: PZT; PYN; Remanent polarization

1. Introduction

Lead zirconate titanate (Pb(Zr0.53Ti0.47)O3, PZT) solidsolutions have achieved wide usage in recent years becauseof their superior properties [1]. For piezoelectric applica-tions, high relative permittivity values and large piezoelectriceffects are necessary. Extraordinary high values are foundat the morphotropic boundary (MPB) corresponding to thecomposition Pb(Zr0.53Ti0.47)O3 [1,2]. In addition, dopantsare often added to PZT to enhance the piezoelectric proper-ties and to modify the microstructure [3–5]. There are twomain types of dopants—donor dopants and acceptor dopants.Donor dopants cause Pb-vacancies by substituting a highervalence ion for Pb2+ or for (Ti, Zr)4+. Examples of donordopants are Nb5+, Ta5+, La3+, W6+, Th4+, B3+ and Sb5+.Acceptor dopants cause O-vacancies by substituting a lowervalence ion for Pb2+ or for (Ti, Zr)4+. Examples of acceptordopants are Sc3+, Mg2+, K+ or Fe3+.

A lot of work has been performed to study the effectsof processing, dopants and microstructures on the proper-ties of PZT [4,6–9]. Most of the microstructure studies areperformed using X-ray diffractometer (XRD) and scanningelectron microscopy (SEM) [6–8]. Studies using the trans-mission electron microscopy (TEM) are emphasized on thedomain structure [9]. Very little work has been done to studythe existence of secondary phases using TEM. In this work,we attempt to study the effect of adding Pb(Y1/2Nb1/2)O3to Pb(Zr0.53Ti0.47)O3 on the microstructure and properties,and paying special attention to the secondary phases.

∗ Corresponding author.E-mail address: ashhhng@ntu.edu.sg (H.H. Hng).

2. Experimental procedures

2.1. Sample preparation

The raw materials were commercially available PbO,TiO2, ZrO2, Nb2O5 and Y2O3 powders. The followingcompositions were chosen:

1. PZT: Pb(Zr0.53Ti0.47)O3.2. PZT-1: 1 mol% Pb(Y1/2Nb1/2)O3 + 99 mol% Pb(Zr0.53

Ti0.47)O3.3. PZT-1.5: 1.5 mol% Pb(Y1/2Nb1/2)O3 + 98.5 mol%

Pb(Zr0.53Ti0.47)O3.4. PZT-2: 2 mol% Pb(Y1/2Nb1/2)O3 + 98 mol% Pb(Zr0.53

Ti0.47)O3.5. PZT-2.5: 2.5 mol% Pb(Y1/2Nb1/2)O3 + 97.5 mol%

Pb(Zr0.53Ti0.47)O3.

The raw materials were mixed according to the corre-sponding compositions and milled in ethyl alcohol. Afterdrying, the powders were calcined at 850◦C for 2 h. Thecalcined slugs were pulverized using a mortar and pestle.In the next step, disks approximately 1 cm in diameter and1 mm in thickness were pressed to reduce the loss of PbOduring calcination. The disks were then heated to 1200◦Cfor 1 h to get the final specimen. An additional 2 wt.% PbOwas added to compensate for the evaporation of the PbOduring calcination.

2.2. Characterization

For electrical measurements, the as-sintered samples werepolished on both surfaces to ensure flat and parallel surfaces.

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0254-0584(02)00045-7

152 W. Qiu, H.H. Hng / Materials Chemistry and Physics 75 (2002) 151–156

They were coated with conductive silver paint on both sur-faces, then baked in an air oven at 500◦C for 10 min for theelectrodes to be completely adhered to the ceramic. TheP–Ehysteresis loops were measured by high voltage ferroelec-tric test system (RT600HVS, Radiant Technologies). X-rayanalysis was carried out using Cu K� radiation (50 kV) on aShimadzu XRD-6000 X-ray diffractometer. Microstructureswere examined by TEM (JEOL JEM-2010) equipped withenergy dispersive X-ray analysis (EDX).

3. Results and discussion

3.1. Ferroelectric hysteresis loop

Hysteresis loops of bulk PZT were measured as a functionof Pb(Y1/2Nb1/2)O3 (PYN) concentration (Fig. 1). Begin-ning with pure PZT,Pr andPsat increased with increasingdopant concentration, while at 1.5% dopant concentration,Pr andPsatreached their maximum values. After that,Pr andPsat decreased with increasing dopant concentration. Basedon the measured results, it can be concluded that doped PZThave better piezoelectric properties than that of undopedPZT. The best piezoelectric properties are obtained whenthe dopant concentration is 1.5 mol%. It is noted that our re-sults are different from those of other researchers [10–13].This is because the hysteresis loop is highly dependent onthe processing method [13].

3.2. XRD result

Fig. 2 shows the X-ray diffraction patterns of pure anddoped PZT samples. Only the Pb(Zr0.52Ti0.48)O3 phase isdetected in our samples. No secondary phases related to thedopants are detected. Tetragonal phase is the only phase

Fig. 1. Hysteresis loops of pure PZT and PYN-doped PZT samples.

Table 1Lattice parameters of the pure PZT and PYN-doped PZT samples

Specimens Lattice parameter Tetragonalityc/a

a (nm) c (nm)

PZT 0.4047101 0.4195414 1.0366PZT-1 0.4046092 0.4193774 1.0365PZT-1.5 0.4040343 0.4185158 1.0358PZT-2 0.4060850 0.4085346 1.0060PZT-2.5 0.4070530 Rhombohedral

present in the pure PZT. As the amount of PYN increases,the lattice parametersa andc, and thec/a ratio of the tetrago-nal phase decrease (Table 1). When the PYN content is morethan 2 mol%, the tetragonal phase converts to the rhombo-hedral phase.

The decrease of lattice parameters is due to ion substi-tution in the perovskite structure [14]. Y3+ (ion radius:0.090 nm) ion does not substitute for Zr4+ (ion radius:0.072 nm) or Ti4+ (ion radius: 0.061 nm) ions because ofits larger ionic radius. Instead, Nb5+ (ion radius: 0.064 nm)mainly substitute for the Zr4+ or Ti4+ ions because of thesimilarity in ionic radii. Because of the stronger Nb–O bondand smaller ionic radius of Nb5+, the O2− will move to-wards Nb5+. Pb2+ will also depart from its normal positiontowards Nb5+ due to Pb–O bond restriction. The perovskitestructure will then contract and the symmetry axis graduallychanges from direction〈0 0 1〉 to 〈1 1 1〉. With the increaseof Nb5+ concentration, the structure deformation becomeslarger, which leads to crystal structure transforming fromtetragonal perovskite to rhombohedral perovskite.

The ion substitution is accompanied by the formation ofPb-vacancies:

2Nb5+Zr4+ = 2NbZr

• + V ′′Pb

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Fig. 2. X-ray diffraction patterns of pure PZT and PYN-doped PZT samples.

The Pb-vacancies are negatively charged, and these can bepaired with Nb5+ ions to form the defect dipoles. Thesedefect dipoles can be aligned when there is spontaneouspolarizationPs or when there is an applied electric field.This will create a larger polarization, and hence explainwhy the doped PZT has a largerPr and Psat than the un-doped PZT observed in our samples. The decrease in thePrandPsat values for samples containing more than 1.5 mol%PYN is due to the tetragonal to rhombohedral phasetransition.

3.3. TEM analysis

The general microstructures of the samples observedusing TEM are similar, showing mainly the PZT domainstructures (Fig. 3). Other than the major PZT phase, two

Fig. 3. Typical PZT domain patterns observed using TEM.

minor secondary phases were also observed in our speci-mens. They are the Pb-deficient polycrystalline phase andY–Zr–Ti phase, which were not detected using XRD. Thereason may be that they are present in small amounts, andare below the detection limit of the XRD.

A bright-field (BF) image of the Pb-deficient polycrys-talline phase is shown in Fig. 4a. The polycrystalline natureof this phase is indicated by the diffraction rings (Fig. 4b).As indicated from the EDX spectrum (Fig. 4c), this phasecontains mainly Zr and Ti elements, without the presence ofPb. This phase is formed due to Pb evaporation during sin-tering. Whether this phase has any relationship with the Nbcontent cannot be determined in this study, as it appears inall the specimens. Furthermore, due to the limited observa-tion area in TEM, the amount of this phase in each specimencannot be quantitatively measured.

154 W. Qiu, H.H. Hng / Materials Chemistry and Physics 75 (2002) 151–156

Fig. 4. (a) BF image; (b) diffraction pattern; (c) EDX spectrum of the Pb-deficient polycrystalline phase (Cu peak is due to the Cu grid used).

Another minor secondary phase (Fig. 5a) is also ob-served in all the doped PZT samples. The surface config-uration of this phase is smooth and is different from theherringbone-like alignment of PZT domain. EDX analysis(Fig. 5b) indicates the presence of Y, Zr and Ti. Electrondiffraction patterns were also obtained for this Y–Zr–Tiphase. Such a phase has not been reported and their diffrac-tion patterns cannot be indexed. Therefore, we used asystematic approach to deduce the crystal structure suchthat consistent indexing of these diffraction patterns wasachieved. The absence of the three-, four- and six-fold sym-metry in the major zone axes indicated that the unit cellwas orthorhombic, monoclinic or triclinic. Moreover, theappearance of 90◦ angles in these major diffraction patternseliminated the possibility of a triclinic lattice. The obser-vation of centered rectangular lattices in reciprocal spacesuggested that the crystal lattice of the secondary phaseis base-centered. Finally, it was found that a tetragonalface-centered lattice, witha = 5.084 Å andc = 5.384 Å

can index all the diffraction patterns. Fig. 6 shows thediffraction patterns, together with the corresponding tiltangles required to go from one zone to another.

Yoon et al. [15] has reported a pyrochlore structureY–Zr-rich phase using XRD analysis in their PZT sam-ples doped with 5 mol% Pb(Y2/3W1/2)O3. However, theydid not specify the actual composition of this phase. Apyrochlore structure is characterized by cubic F lattice ofa = 10.4 Å [16]. It is possible that the phase we observedis a derivative of the pyrochlore phase reported by Yoonet al., since the lattice parameter obtained for our phase isapproximately 1/2 that of a typical pyrochlore phase. Moreinvestigation needs to be performed in order to verify this.

These two minor secondary phases observed using TEMare not ferroelectric phases. We would expect them to causea negative effect on the ferroelectric properties in the dopedsamples. However, such negative effect was not observedin our samples. A possible reason is that these secondaryphases are present in very little amount, since they were not

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Fig. 5. (a) BF image; (b) EDX spectrum of the Y–Zr–Ti-rich phase (Cu peak is due to the Cu grid used).

Fig. 6. Electron diffraction patterns of Y–Zr–Ti-rich phase when assuming a tetragonal lattice.θ is the tilt angle required to go from one zone to another.

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detected using XRD. Their existences were only revealedfrom TEM observation. The improvement in the propertiesthrough PYN doping in our sample is dominated by the ionsubstitution effect.

4. Conclusions

Pure PZT and PYN-doped PZT sintered at 1200◦C werecharacterized by their hysteresis parameters. Their mi-crostructures have been investigated using TEM and XRD.The results are concluded as follows:

1. 1.5 mol% PYN-doped PZT has the highestPr andPsat.Below and above this content,Pr andPsatsimply increaseor decrease with PYN content.

2. A Pb-deficient polycrystalline phase and a Y–Zr–Tiphase existed as minor secondary phases in the dopedPZT. These are non-ferroelectric phases. However, theiramount is too small to pose a significant negative effecton the ferroelectric properties in PZT.

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