J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 1, 2 0 0 2, 947 – 949

Influence of tungsten doping on dielectric properties of strontium

bismuth niobate ferroelectric ceramics

YUN WU, S. J. LIMMER, T. P. CHOU, C. NGUYEN, GUOZHONG CAOUniversity of Washington, Materials Science and Engineering, Seattle, WA 98195, USAE-mail: gzcao@u.washington.edu

The effect of doping on various physical and chemi-cal properties of solid materials is commonly known,and this effect has been extensively exploited in piezo-electrics and ferroelectrics to improve their perfor-mance. For example, there have been numerous reportson the effect of doping in the Pb(Zr, Ti)O3 (PZT) sys-tem [1, 2]. Various doping ions, such as La3+, Nd3+ andNb5+, in PZT result in enhanced remanent polarizationand a decreased coercive field. Such ions are commonlyknown as “soft doping” ions [3, 4]. Similar doping ef-fects were also found in bismuth layer-structured fer-roelectrics (BLSFs), such as SrBi2Nb2O9 (SBN) andSrBi2Ta2O9 (SBT) [5, 6]. Millan et al. [7–9] studied thesubstitution of Bi3+ in SBN by Pb2+, Sb3+, and Te4+,and found that the SBN structure could be preserved bysuch a substitution, whereas dielectric properties variedwith the type of doping cations. There are also severalreports on the substitution of Sr2+ in SrBi2(Nb,Ta)2O9(SBTN) by Bi3+, Ba2+and Ca2+ [10–12]. Although alot of research effort has been made in the study ofSBTN ferroelectrics [13–15], there is very limited workon improvement of dielectric and ferroelectric proper-ties of the SBTN through substitution of the B site ions,i.e. Nb5+ or Ta5+.

Recently we reported on the significant enhancementof dielectric and ferroelectric properties of SBN fer-roelectrics through partial substitution of niobium bypentavalent vanadium cations [16, 17]. With 10 at.%vanadium substitution, the remanent polarization in-creased from ∼2.4 µC/cm2 to ∼8 µC/cm2, whereas thecoercive field reduced from ∼63 kV/cm to ∼45 kV/cm.The enhanced ferroelectric properties of the layeredperovskite ferroelectric properties were explained byan enlarged “rattling space” via doping smaller vana-dium ions. Similar to vanadium ions, doping with tung-sten ions could be another approach for improving thedielectric and ferroelectric properties of SBTN ferro-electrics. Tungsten ions do form perovskite-like unitsas a center ion, or in B site, e.g. in Bi2WO6 [18]. In addi-tion to the relatively smaller ionic radius (W6+: ∼64 pmwith a coordination number (CN) of 6) as compared tothat of Nb5+ (∼69 pm with CN = 6), tungsten ions havea higher valence state, which might exert additional in-fluence on the dielectric and ferroelectric properties.In this paper, we present the study of the influence oftungsten doping on the dielectric properties of SBNferroelectric ceramics.

Polycrystalline SBWN ceramic samples doped withtungsten oxide with a composition of Sr1+x Bi2−2/3x -(Wx Nb1−x )2O9 with x ranging from 0 to 0.2 (20 at.%)

were prepared by solid state reaction sintering. Thestarting materials used were SrCO3, Bi2O3, WO3, andNb2O5(Aldrich Chem. Co.), all with a purity of 99%.The chemicals were admixed with desired ratios withan excess of 3 wt% of Bi2O3 to compensate for possibleloss during sintering. For all samples, a two-step firingprocess was applied in this study. The first step, calcin-ing at relatively low temperatures (950–1000 ◦C), wasto promote chemical reactions among the constituentcompounds to form a single-phase layered perovskite.The second step, sintering at relatively high tempera-tures (1150–1200 ◦C), was to achieve high densifica-tion. Because of the high melting temperature of WO3,ceramic samples with tungsten are sintered at 1200 ◦C.After sintering, all of the ceramic samples have a rela-tive density of 90–95%.

X-ray diffraction (XRD, Philips 1830) was used todetermine the formation of layered perovskite phase af-ter both first-step and second-step firing. For the elec-trical property measurements, the sintered pellets werepolished to have flat and parallel surfaces at both sideswith a thickness of ∼1 mm. Pt wires were connectedto the sample surfaces which were electroded with sil-ver paste and heat-treated on hot plate around 500 ◦Cfor 10 min. The dielectric constant and loss tangent asfunctions of temperature up to 600 ◦C and frequencyranging from 20 Hz to 1 MHz were measured by a HPPrecision LCR Meter 4284A.

Fig. 1 shows the XRD spectra of SBN ceramics dopedwith various amounts of tungsten oxide. It is clear thatsingle phase layered perovskites were formed when thedoping content was below 2.5 at.% with no detectablesecondary phase. However, XRD spectra of samples

Figure 1 XRD spectra of the Sr1+x Bi2−2/3x (Wx Nb1−x )2O9 ferroelec-tric ceramics.

0261–8028 C© 2002 Kluwer Academic Publishers 947

(a)

(b)

Figure 2 Dielectric constants (a) and tangent losses (b) as a function oftungsten content measured at 100 kHz.

consisting more than 5 at.% tungsten showed someunidentifiable peaks, suggesting formation of possi-ble unknown secondary phase. The intensity of theseunidentified peaks are also seen to increase as the tung-sten content increased, indicating the increased amountof the unknown secondary phase. Compared to thevanadium doping (single phase being kept at least up to∼30 at.% [19]), tungsten has a much lower solubility inSBN, though the ionic radius of tungsten ions is closerto that of pentavalent niobium ions than that of pen-tavalent vanadium ions. It is not known why tungstenhas such a lower solubility, but it is possible that thehigher valence state (6+) of tungsten may influence thestability of the crystal structure of SBN ferroelectrics.

Fig. 2 shows the dielectric constants (Fig. 2a) andtangent loss (Fig. 2b) as a function of temperature forSBWN ceramics consisting of 0, 1, 2.5 and 10 at.%tungsten, determined at a frequency of 100 kHz witha oscillating amplitude (50 mV). For all the samples,there is a sharp transition in dielectric constant at theirrespective Curie points. It is very clear that tungstendoping resulted in increased peak dielectric constantsand lowered Curie points. Under the measurement con-ditions (100 kHz and 50 mV) used in the current study,the dielectric constant consists of contributions of ionicand atomic polarization only. Since the ionic radiusof W6+ is smaller than that of Nb5+, an increasingamount of W6+ would likely lead to a reduced con-tribution of overall atomic polarization. Therefore, anincreased peak dielectric constant may likely be at-tributable to increased ionic polarization. XRD results(Fig. 1) indicated that there was no noticeable shift ofXRD peaks with tungsten doping, suggesting no appre-ciable change in the lattice constant of SBN perovskite

crystal. The XRD spectra further indicated that only asmall amount of tungsten ions (less than 5 at.%) couldbe incorporated into the perovskite structure. Consider-ing such a low doping level and the structural constraintimposed by the Bi2O2+

2 interlayers, it is very likely thatthe change of crystal lattice constants is very small. Asa result, almost unchanged crystal unit cells accommo-dating smaller ions with higher a valence state wouldlikely lead to an increased ionic polarization and, thus,an increased dielectric constant. In addition, the incor-poration of tungsten ions into the SBN structure wouldlikely introduce some cationic vacancies, so as to main-tain the electroneutrality. Two possible defect chemicalreactions could be simplified as the following:

Null = W•Nb + 1

2V′′

Sr (1)

or

Null = W•Nb + 1

3V′′′

Bi (2)

These cationic vacancies may also have an apprecia-ble contribution to the dielectric constants. Further, anincreased amount of tungsten doping also resulted ina reduced Curie temperature, which implied decreasedstability of perovskite structure. The relationship be-tween the Curie temperature and tungsten doping willbe discussed further later in this paper.

Fig. 2b shows that the loss tangent at a frequency of100 kHz were greatly decreased with a small amountof tungsten doping, this influence becomes more pro-nounced at elevated temperatures. It is not clear whatexactly is the mechanism of the reduction of loss tan-gent with tungsten doping; however, the possible for-mation of cationic vacancies may result in a reducedtangent loss. Similar to La3+ doping in PZT ferro-electrics [3], the formation of cationic vacancies wouldeffectively suppress the formation of oxygen vacanciesand, thus, decrease the electrical conductivity througha reduced amount of charge carriers. Besides, a re-duced tangent loss was also found in the SBN ceramicswith vanadium doping [16]. It is interesting to noticethat both V–O (∼627 kJ/mol) and W–O (∼672 kJ/mol)chemical bonds have weaker bond strength as comparedwith that of Nb–O bond (∼703 kJ/mol) [20].

Fig. 3 shows the tangent loss of SBN doped withvarious amounts of tungsten as a function of frequency

Figure 3 Tangent loss of the four typical samples in the SBWN systemover frequency at room temperature.

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Figure 4 Curie temperature vs. tungsten doping in SBN.

ranging from 20 Hz to 1 MHz at room temperature.The results of four samples consisting of 0, 1, 2.5 and10 at.% tungsten were included in the figure. It is seenthat tangent loss for SBN without tungsten doping re-mained almost constant throughout the frequency rangestudied, whereas other three samples consisting of 1 to10 at.% tungsten showed an appreciable reduction intangent loss as the frequency increased from 20 Hzto 1 kHz. Under the current experimental conditions(50 mV and 20 Hz–1 MHz), there are three possi-ble contributions to dielectric constant: atomic, ionic,and space charge polarization, since the electric field(∼0.5 V/cm) is too small to alter spontaneous polariza-tion (dipolar polarization). Response frequencies foratomic and ionic polarization are 1015 and 1013 Hz,respectively, whereas space charge has a response fre-quency of approximately 1 kHz. The tangent loss islikely due to space charge polarization and domain wallrelaxation. It is reasonable to assume that the spacecharge is the main reason that caused an appreciabledecrease in tangent loss as the frequency increases from20 to 1 kHz. Doping with tungsten is seen to result inincreased tangent loss at room temperature particularlyat frequencies below 1 kHz. The increased tangent lossof SBN doped with tungsten may also be attributed tothe possible introduction of cationic vacancies as dis-cussed before as one possible mechanism, which willneed further verification.

Fig. 4 shows the Curie temperature Tc of SBWN as afunction of the amount of tungsten doping. It was foundthat the Curie temperature decreased sharply with anincreasing amount of tungsten doping at low concen-trations, however, remained constant at higher dopinglevels. With 1 at.% doping, Tc dropped from ∼418 ◦Cto ∼411 ◦C. With higher doping content (>2.5 at.%),Tc decreased to ∼385 ◦C at 2.5 at.% and decreasedmuch slower to ∼383 ◦C at 10 at.%. The results im-plied that the incorporation of tungsten into the SBNcrystal structure reached a saturation point of approx-imately 2.5 at.%. XRD spectra in Fig. 1 indicated theformation of unidentified secondary phase in the sam-ples with tungsten doping levels at and above 5 at.%.Both Figs 1 and 4 suggested a low solubility of tung-sten, as substituting ions, in SBN crystal structure isof approximately 3 at.%. A higher doping content oftungsten would either go to the grain boundary phaseor form a secondary phase.

In summary, although the ionic radius of W6+ wasonly a little smaller than that of Nb5+, the single phaselayered structure could be kept within a very small dop-ing range (∼2.5 at.%). The Curie points decreased from∼418 ◦C for SBN to ∼383 ◦C for SBWN with 10 at.%tungsten. Dielectric constant was found to graduallyincrease with an increasing concentration of tungsten,reach a maximum at approximately 2.5 at.%, and thenreduce with a further increase in tungsten concentra-tion. This phenomenon was explained mainly by theformation of an unknown secondary phase as the tung-sten concentration increases. In addition, it was foundthat a low concentration of tungsten was likely to in-troduce space charge polarization into the system, re-sulting in an appreciable decrease in both dielectricconstant and tangent loss as frequencies reduced from20 Hz to 1 kHz.

AcknowledgmentsY.W. would like to acknowledge nanotechnology fel-lowship and S.L. would like to acknowledge NSFIGERT Fellowship for financial support from the Cen-ter of Nanotechnology at University of Washington.

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Received 9 November 2001and accepted 27 February 2002

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