Double Doping Effects

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Double doping effect on the structural and dielectric properties of PZT ceramics

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 37 (2004) 31743179 PII: S0022-3727(04)78701-X

Double doping effect on the structural and

dielectric properties of PZT ceramicsPuja Goel1, K L Yadav1,3 and A R James2

1 Department of Physics, Indian Institute of Technology, Roorkee 247 667, India2 Solid State Physics Laboratory, Lucknow Road, Timarpur, Delhi 110 054, India

E-mail: [email protected] and [email protected]

Received 5 April 2004, in final form 2 September 2004Published 28 October 2004Online at stacks.iop.org/JPhysD/37/3174doi:10.1088/0022-3727/37/22/019

AbstractPolycrystalline samples of the modified Pb(Zr1x Tix)O3 (PZT)composition, with representative formulaPb0.92(LazBi1z)0.08(Zr0.65Ti0.35)0.98O3 (PLBZT), a family of relaxorferroelectrics, were prepared via the chemical route with z = 0.3, 0.6 and0.9. Crystalline phases of powders calcined at different temperatures and themicrostructure of the sintered pellets were investigated by x-ray diffraction(XRD) analysis and scanning electron microscopy, respectively. XRDconfirms the result obtained by differential scanning calorimetry. The XRDprofile shows that the samples having z = 0.9 and 0.6 do not exhibit apyrochlore phase, whereas the samples with z = 0.3, have 3% of thepyrochlore phase. Microstructural analysis suggests that the shape of grains

and intergranular residual pores are modified upon La doping. The dielectricconstant and dielectric losses were measured as a function of frequency atroom temperature for different frequencies starting from 0.1 kHz to 1 MHz.The dielectric constant was found to be strongly influenced by frequencywhereas the Curie temperature remained almost the same. Finally, weconclude that the dielectric constant, loss and activation energy of PLBZTstrongly suggest that these compounds are suitable for the preparation ofhigh value capacitors and may be good candidates for device applications.

1. Introduction

There has been continued interest in lead-based ABO3-type perovskites, particularly in Pb(Zr1xTix)O3 (PZT) based

materials for a variety of applications such as transducers,

sensors and dielectric ceramics for capacitor applications

particularly in the field of nonvolatile memories [17].

Depending upon the specific requirement for different

applications, various compositions of the PZT system with

various Zr/Ti ratios may be chosen. There are several

reports [811] concerning the synthesis and characterization

of modified PZT synthesis by a solid-state reaction method

with various Zr/Ti ratios. La doping is one of the most

widely used strategies to tailor the dielectric and optical

properties of ferroelectric materials. It also enhances the

breadth of diffuse phase transition (DPT) due to the promoted

3 Author to whom any correspondence should be addressed.

B-site compositional inhomogeneity, thus enlarging the

utilization temperature range of multi-layer ceramic capacitors

(MLCCs) and holding promise for applications like precisiondisplacement sensors [12,13]. Much work has been done in

the past to study the effects of substitution of single isovalent,

supervalent or subvalent dopants with varying concentrations

in the A and B sites of the PZT [1424] for different device

applications, but not much work has been done to report the

effect of doubledoping (ofdifferent radii)at thePb site. Hence,

powder samples of Pb0.92(LazBi1z)0.08(Zr0.65Ti0.35)0.98O3(PLBZT) (where z = 0.3, 0.6, 0.9) were synthesized by

the chemical co-precipitation method because of their better

purity, better homogeneity and enhanced reactivity leading to

the development of quality ceramics. Our goal in the present

work is to investigate the effect of supervalent double doping

(La+3

and Bi+3

) at the Pb site on the structural and dielectricproperties and the nature of the ferroelectric phase transition

in chemically synthesized PZT compounds.

0022-3727/04/223174+06$30.00 2004 IOP Publishing Ltd Printed in the UK 3174
http://stacks.iop.org/jd/37/3174


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Structural and dielectric properties of PZT ceramics

2. Experiment

Powder samples of the PLBZT system were prepared

accordingtotheformulaPb1x (LazBi1z)x (Zry Ti1y )(1x/4)O3via the chemical route for x = 0.08, y = 0.65 and z = 0.3,

0.6 and 0.9. Compositions are denoted as 30/70, 60/40,90/10 according to the La and Bi ratio. Nitrate salts (AR

grade) of component atoms [Pb(NO3)2, La(NO3)3 6H2O,

Bi(NO3)3 5H2O, ZrO(NO3)2 2H2O] were dissolved in dou-

ble distilled water. The resulting solution was stirred on a

magnetic stirrer for 23 h in order to get a clear transparent

solution. Titanium isopropoxide was added drop-wise to this

solution. The added titanium hydrolysed into an intermedi-

ate Ti(OH)4 and slowly redissolved. A few drops of nitric

acid were added to dissolve Ti(OH)4 completely and to get a

clear solution. A white precipitate was observed immediately

after adding some amount of ammonia into the solution. To

obtain the desired powder, the solution was vacuum filtered

and washed several times with distilled water. The filteredpowder cake was then kept in an oven (for 24 h) at 150C for

drying.

Thermal analysis, such as differential thermal analysis

(DTA), thermogravimetric analysis (TGA) and differential

thermogravimetry (DTG)of the oven-dried powder was carried

out in a thermal analyser (Perkin Elmer, Pyris Diamond) at a

heating rate of 5C min1. Oven-dried powders of the three

different compositions prepared were calcined at 800C for

4 h. These powders were cold pressed into discs (pellets) at

a pressure of 2 108 N m2 using a uniaxial hydraulic press.

The pellets were sintered at 1100C for 6 h. In order to prevent

PbO loss and to maintain the stoichiometry of the compounds,

the pellets were placed in a covered alumina crucible with leadzirconate titanate powder during sintering. The densities of

sintered pellets were calculated by Archimedes method.

The formation and quality of the PLBZT compound in

calcined powder as well as in sintered pellets were studied

using an x-ray diffractometer (PW 1140/90), using Cu K( = 0.154 18 nm) in a wide range of Bragg angles (20

2 60) at room temperature. To find the temperature at

which the pure phase wasformed, the first composition (30/70)

washeated at differenttemperatures for2 h starting from 200C

up to 800C in steps of 100C and x-ray diffraction (XRD) was

done.

Before measuring the dielectric properties, the sintered

pellets were coated with high purity silver paste (to work as anelectrode) and dried at 200C for 2 h. The dielectric constant

() and dielectric loss (tan ) of the samples as a function of

temperature (room temperature to 410C) were obtained using

a capacitance measuring assembly (HP-4284A) with a three

terminal sample holder. Microstructures of the samples were

analysed by using scanning electron microscope (SEM) (LEO

435 VP).

3. Results and discussion

TGA, DTGand DTA curves of theas-prepared PLBZT powder

(oven dried at 150C) are shown in figure 1. The DTA study

reveals two endothermic peaks at 446C and 460C, which aredue to the decomposition of Pb(NO3)2 into PbO [25]. An

onset of weight loss has been observed at 386C in TGA, and

Figure 1. DTA, DTG and TGA curves of as-prepared PLBZTpowder for z = 0.3.

Figure 2. XRD of PLBZT at different temperatures for z = 0.3:

() TiO2, (

) Pb(NO3), ( ) La2O3, () Bi2O3, () ZrO2.

the weight becomes almost constant after 500C, which gives

information about the complete decomposition of complex

oxyhydroxides at this temperature and also confirms that this

temperature is sufficient to obtain the desired powder through

this process; this was also confirmed by XRD analysis.

The XRD patterns (figure 2) of the PLBZT (30/70)

powders heat treated at temperatures ranging from 200C to

800C for 2 h, were obtained to find the different phases during

compound formation. As indicated, the XRD pattern of the

as-prepared powder (calcined at 200C) has mixed peaks of

the oxidesof precursor compoundssuch as La2O3, ZrO2, TiO2,

and Pb(NO3)2. Therhombohedral phase of PLBZT is observedafter calcinations at 400C. A comparative analysis of both

XRD and DTA shows that the reaction is completed at about

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Table 1. Structural and electrical parameters of PLBZT ceramics.

Lattice constant Electrical properties at 1 kHzComposition(z) a (nm) max Tc (C) Ea (eV)

Density(gm cm3)

0.3 0.402 89.72 3980 360 1.88 5.96 7.35

0.6 0.404 86.99 4982 286 1.77 2.02 7.550.9 0.404 89.80 9440 214 2.08 0.88 7.68

450C. The XRD profile reveals that complete crystallization

of PLBZT could be achieved at calcinations around 500C

and this is confirmed from the TGA (figure 2) as no weight

loss is observed after 500C. Diffraction peaks become sharper

upon calcinations at temperatures higher than 500C due

to better crystallization of the material. Lattice parameters

(table 1) were calculated using the dhkl values calculated from

XRD patterns of pellets sintered at 1000C. These values are

found to be consistent with the reported values [26]. From

the diffraction pattern of the PLBZT and calculated latticeparameters(a and values),no changecouldbe observed in the

basic crystal structureof PZTafter(La, Bi)doping (for z = 0.3,

0.6, 0.9 at the A site), except that a pyrochlore peak is observed

neartheperovskite(110)peakforz = 0.3. The relativeamount

of the pyrochlore phase to the perovskite phase was estimated

using the following peak area ratio equation [27]:

%Pyrochlore =Apyrochlore

Apyrochlore + A(110) 100,

where Apyrochlore and A(110) are the areas under the pyrochlore

peak and the (110) perovskite phase. The pyrochlore value is

3%, which is small and in agreement with the value reported

[28] for doped PZT. Hence, this composition can also beconsidered as a single phase. The measured linear particle size

ranged from 20 to 27 nm, of the powders calcined at different

temperatures (400800C), calculated from XRD patterns by

making use of Scherrers equation [29] P = K/(1/2 cos )

where K = 0.89 and 1/2 is the full width at half maximum

intensity.

SEM micrographs of the pellets prepared from PLBZT

powder and sintered at 1100C are shown in figures 3(a)(c).

The sintered pellets have been found to have a grain size of the

order of (1.2 m) and uniform grain distribution, which is in

accordance with the high-density value, as can be seen from

table 1.

The frequency variation of the dielectric constant () and

the dielectric loss (tan ) of the sample are plotted in fig-ures 4(a) and (b), respectively. The frequency variation of

is like that of normal ferroelectric materials. The pattern of

dielectric losses is irregular up to a frequency of 100 kHz for

all the compositions and then starts increasing continuously.

This increase can be explained on the basis of the dipole mech-

anism [30] of losses given by = 1, which is the condition

for the maximum of dielectric losses in a polar dielectric at

a given temperature. Here, the frequency () corresponds to

such a ratio between the period of an external field and the

time of the relaxation of the dipole as is needed to observe

the greatest loss of energy to overcome the resistance of the

viscous medium by the dipoles.

To determine the natureof the ferroelectric phasetransition,the variation of and tan of the samples with tempera-

ture was studied. It is apparent from figures 5(a)(c) that

Figure 3. SEM micrographs of PLBZT pellets (a) z = 0.3,(b) z = 0.6 and (c) z = 0.9 sintered at 1100C.

the variation of with temperature (40410C) at different

frequencies (0.1 kHz to 1 MHz) is a broadened curve rather

than a sharp peak (as in normal ferroelectrics) around Tc,

which is one of the most important characteristics of a dis-ordered perovskite structure with a diffuse phase transition.

This broadening is believed to be due to the compositional

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(a)

(b)

Figure 4. (a) Frequency dependence of dielectric constant at roomtemperature for (a) z = 0.3, (b) z = 0.6 and (c) z = 0.9compositions of PLBZT. (b) Frequency dependence of dielectriclosses at room temperature for (a) z = 0.3, (b) z = 0.6 and(c) z = 0.9 compositions of PLBZT.

fluctuations, and/or substitutional disordering in the arrange-

ment of cations in one or more crystallographic sites of the

structure, which leads to microscopic or nanoscopic hetero-

geneity in the compounds, with different local Curie points

[31, 32]. These fluctuations occur when Pb+2, Ti+4 and Zr+4

are replaced by soft dopants La+3 and Bi+3. The changes are

ascribed to the lead vacancies and the resulting increase in

domain wall mobility [33]. It is also clear that an increase

in frequency causes a decrease in the maximum value of the

dielectric constant, whereas only a small shift in Curie temper-

ature could be observed.

The degree of diffuseness ( ) in the dielectric peak of the

material was estimated by using the expression

1

1

max

(T Tc)

,

where and max are the dielectric constants at temperature

T (T > Tc) and max (maximum value of at the transition

temperature Tc), respectively. This large value of (table 1)confirms the second-order phase transition and the high degree

of disorderliness in the material [18, 28].

Figure 5. Variation of dielectric constant with temperature

(at different frequencies) for (a) z = 0.3, (b) z = 0.6 and (c) z = 0.9compositions.

Figure 6. Variation of % TC for (a) z = 0.3, (b) z = 0.6 and(c) z = 0.9 as a function of temperature, at 1 kHz frequency.

The temperature coefficient of permittivity (TC) is

defined as

TC = 100T RT

RT,

where T is the permittivity at any temperature T and RT is its

room temperature value. The relative variation of the dielectric

permittivity with temperature between room temperature and

400C for all the compositions are shown in figures 6(a)(c).

Curve (c) is significantly broader than curve (a). This indicates

that PLBZT (90/10) possesses a more diffuse phase transition

than the rest of the compositions.

From the above dielectric analysis we have found that

the peak dielectric constant increased with an increase in La+3

concentration, which implies that the substitution of La+3 ions

at the A site is more effective in increasing the dipole moment

of the lattice [34], which enhances the peak dielectric constant

(figure 7).The variation of dielectric loss with temperature

(40410C) at frequencies starting from 0.1 kHz to 1 MHz is

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P Goel et al

Figure 7. Variation of dielectric constant (at room temperature (RT)and at Tc(max) and Tc as a function of composition (z).

shown in figures 8(a)(c). The dielectric loss was found to

be very small and nearly the same at all the frequencies in

the temperature range 40125C. Above 125C, a continuous

increase in tan was observed, and was greater at lower

frequencies. As a general rule, tan increases appreciably

when the temperature rises. This growth in tan is brought

about by an increase both in the conduction of residual current

and the conduction of absorption current. In actual fact, therise in temperature and the resulting drop in viscosity exert

a double effect on the amount of losses due to the friction

of the rotating dipoles, on the one hand, and the increase in

the degree of dipole orientation, on the other hand; there is

a reduction in energy required to overcome the resistance of

the viscous medium. In a polar substance, apart from dipole

losses, losses due to electrical conduction, which increase with

an increase in temperature, arepresent [30]. Theobserved tan

of our samples were nearly the same for all the compositions

at lower temperatures but started rising on raising the

temperature.

Figure 9 is the plot of log( ) with respect to the inverse

temperature (T1). The activation energy Ea can be calculated

by making use of this plot and the relation given below:

= 0 exp

Ea

KBT

.

The conductivity is given by = 0 tan , where 0 is the

vacuum dielectric constant, is the angular frequency and KBis the Boltzmann constant. The calculated values of activation

energy at 1 kHz frequency for all the compositions are given

in table 1. The value of Ea decreases with increasing La+3 as

can be seen from figure 9. This type of conductivity has been

confirmed as p-type [35]. Substitution of supervalent cations

such as La+3 or Bi+3 for Pb+2 contributes electrons and thusreduces the hole conductivity according to the law of mass

action. This is believed to be due to the formation of Pb site

Figure 8. Dielectric loss variation as a function of temperature ofPLBZT (a) z = 0.3, (b) z = 0.6 and (c) z = 0.9 compositions atdifferent frequencies.

Figure 9. Conductivity vs temperature characteristics of PLBZT(a) z = 0.3, (b) z = 0.6 and (c) z = 0.9 compositions at 1 kHzfrequency.

vacancies (here, with La doping) as in the reaction

0.01La2O3 + Pb(ZrTi)O3 (Pb0.97La0.020.01)(ZrTi)O3

+0.03PbO

taking place during sintering.

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4. Conclusions

In this work we have reported the effect of supervalent double

doping (donor type) on the structural and dielectric properties

of a composition of PZT near the morphotropic phase

boundary (MPB) with fixed Zr/Ti (65/35) ratio prepared bythe co-precipitation method. The chemical route allows a low

temperature synthesis (450C) and ensures a molecular level

of homogeneity. The results can be summarized as follows:

XRD studies confirm the formation of a rhombohedral phase

of all the studied samples at 450C. The density increases with

the increasing concentration of La+3, which is also confirmed

by the SEM analysis. There is a variation in the grain size

with different concentrations of La+3 and Bi+3. In dielectric

studies, thetransition temperature is found to decrease whereas

is found to increase with the increasing concentration of

lanthanum. It is finally concluded that the dielectric constant,

loss and activation energy of PLBZT strongly suggest that

these compounds are suitable for the preparation of highvalue capacitors and may be good candidates for device

applications.

Acknowledgments

The authors thank Professor M R Maurya (Department of

Chemistry, IIT Roorkee) for his help in material preparation

and the Head of the Physics Department and Head of

Metallurgy and Materials Engineering for their kind help and

encouragement. PG thanks MHRD for providing a research

fellowship.

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