Effect of PbO-Nonstoichiometry

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The Effect of PbO-Nonstoichiometry

on Dielectric and SemiconductingProperties of PbFe0.5Nb0.5O3-Based

CeramicsI. P. Raevski a , S. P. Kubrin a , S. A. Kovrigina a , S. I. Raevskayaa

, V. V. Titova

, A. S. Emelyanova

, M. A. Malitskayaa

& I. N.

Zakharchenkoa

aInstitute of Physics and Physical Department of Southern Federal

University, Stachki Ave. 194, Rostov on Don, 344090, Russia

Version of record first published: 23 Jun 2010.

To cite this article: I. P. Raevski , S. P. Kubrin , S. A. Kovrigina , S. I. Raevskaya , V. V. Titov , A. S.

Emelyanov , M. A. Malitskaya & I. N. Zakharchenko (2010): The Effect of PbO-Nonstoichiometry on

Dielectric and Semiconducting Properties of PbFe0.5Nb0.5O3-Based Ceramics, Ferroelectrics, 397:1,

96-101

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Ferroelectrics , 397:96101, 2010

Copyright Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 online

DOI: 10.1080/00150193.2010.484738

The Effect of PbO-Nonstoichiometryon Dielectric and Semiconducting Properties

of PbFe0.5Nb0.5O3-Based Ceramics

I. P. RAEVSKI, S. P. KUBRIN, S. A. KOVRIGINA,

S. I. RAEVSKAYA, V. V. TITOV, A. S. EMELYANOV,

M. A. MALITSKAYA, AND I. N. ZAKHARCHENKO

Institute of Physics and Physical Department of Southern Federal University,

Stachki Ave. 194, Rostov on Don, 344090, Russia

Dielectric, X-ray and electric resistivity studies of nonstoichiometric PbFe0.5Nb0.5O3-based ceramics obtained by solid-state reaction route with PbO/(Fe 0.5Nb0.5O2) molarratio differing from 0.6 to 1.2 have been carried out. Deviation from stoichiometric

PbFe0.5Nb0.5O3 composition leads to formation of diphase mixtures containing fer-roelectric perovskite PbFe0.5Nb0.5O3 phase and nonferroelectric PbO or pyrochlorePb3Nb4O13 phases and to decreasing of the dielectric permittivity values. The elec-tric resistivity of PbFe0.5Nb0.5O3 ceramics decreases dramatically when the sinteringtemperature increases. The formation of the pyrochlore phase decreases the room tem-perature resistivity values while the anomaly of the positive temperature coefficient

of resistivity above the ferroelectric Curie temperature gradually diminishes with theincrease of the pyrochlore content.

Keywords Dielectric permittivities; ferroelectric ceramics; lead iron niobate; stoi-chiometry; positive temperature coefficient of resistivity.

Introduction

Ternary perovskite multiferroic PbFe0.5Nb0.5O3 (PFN) is a promising basic material for

ceramic capacitors, piezoelectrics, electrostrictors and PTC resistors exhibiting positive

temperature coefficient of resistivity (PTCR) [15]. Similar to other lead-containing com-

plex perovskites, e. g. PbMg1/3Nb2/3O3 (PMN), the most common problem encounteredduring the synthesis of PFN is the appearance of stable pyrochlore phases [69]. These

phases have a low dielectric permittivity values and it is commonly believed that even

a minor content of pyrochlore phase (about few %) reduces sufficiently of ceramics.

The effect of the pyrochlore phase content on the dielectric properties of PMN has been

thoroughly studied [8, 9], while the similar data for PFN are absent in literature. Besides,

the effect of the pyrochlore phase on the semiconducting properties of PFN has not been

studied yet.

The scope of the present work is the study of dielectric and semiconducting properties

of nonstoichiometric PFN-based ceramics with differing PbO/(Fe0.5Nb0.5O2) molar ratios.

Received September 22, 2009; in final form January 20, 2010.Corresponding author. E-mail: [email protected]

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Effect of Nonstoichiometry on the Properties of PFN 97

The PbO-deficiency is the most probable type of stoichiometry breaking in lead-containing

oxides due to PbO evaporation during sintering. On the other hand, the opposite situation is

also of practical interest as an addition of PbO excess is often used for lowering the sintering

temperature and compensation of PbO losses in ceramics of complex lead-containing

perovskites.

Experimental

The ceramic samples were prepared by conventional solid state reaction route. The

calculated quantities of reagent grade PbO, Fe2O3 and Nb2O5 corresponding to the

PbO/(Fe0.5Nb0.5O2) molar ratios from 0.6:1 to 1.2:1 (these compositions will be referred

further as PbxFe0.5Nb0.5O3 where x varies from 0.6 to 1.2) were mixed thoroughly in agate

mortar in the presence of ethyl alcohol. After drying this slurry, disks of 10 mm in diameter,

were pressed at 100 MPa using polyvinyl alcohol as a binder. Sintering was carried out for

2 hours at 9601160C in a closed alumina crucible. In some cases 1wt.% of Li 2CO3 wasadded before mixing. This addition promotes formation of the perovskite modification of

PFN and reduces its conductivity [9, 10]. Several compositions were calcined at 850C for

4 hours before sintering. The resultant slug was then ground and disc samples were pressed

as described above. The density of ceramics was about 8595% of the theoretical one. For

electrical measurements the samples were grinded and electroded by firing on silver paste

or rubbing In-Ga alloy.

Dielectric studies were carried out in the 0.1 kHz1 MHz frequency range with the aid

of the computer-controlled E7-20 impedance analyzer in the course of continuous heating

at a rate of 23 K/min. Resistivity values were measured under dc electric field of 1 V/cm.

X-ray studies were carried out at room temperature with the help of DRON-3 diffractometerusing FeK radiation.

Results and Discussion

X-ray studies of the ceramics obtained have shown that nominally stoichiometric PFN

ceramics besides perovskite PFN phase usually contain small amounts of pyrochlore

Pb3Nb4O13 phase. The volume percent of pyrochlore phase ,was estimated from the

ratio of relative intensities of the (222) pyrochlore peak (I222)PYRO and the (110) perovskite

peak (I110)PER [8]:

= (I222)PYRO

(I222)PYRO + (I110)PER 100% (1)

For nominally stoichiometric PFN the coefficient is equal 35%. In stoichiometric PFN

with addition of 1 wt.% of Li2CO3 no pyrochlore phase was detected by X-ray studies.

Decreasing of the PbO/(Fe0.5Nb0.5O2) molar ratio below stoichiometric 1:1 value leads to

increasing of the pyrochlore phase content (see inset in Fig. 1). In compositions with x >

1 the PbO phase was detected besides the perovskite PFN phase.

Dielectric studies have shown that the maximal value of permittivity m for the

PbxFe0.5Nb0.5O3 ceramic compositions lowers greatly as x decreases (Fig. 1), however

the diffusion of the (T) peak does not change substantially (Fig. 2a). The latter result

implies that perovskite and pyrochlore phases seem to form a statistical mixture rather than

a matrix system. Indeed reasonably good agreement of the experimental and calculated


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98 I. P. Raevski et al.

Figure 1. (T) dependences at 1 kHz measured on heating for some PbxFe0.5Nb0.5O3 ceramic com-

positions. Numbers at the curves correspond to x values. The inset shows compositional dependence

of the pyrochlore Pb3Nb4O13 phase content for PbxFe0.5Nb0.5O3+1wt.% Li2CO3 compositions with

x 1.

Figure 2. (a) Dependences of permittivity at 1 kHz related to its maximal value m on the reduced

temperature T-Tm for somePbxFe0.5Nb0.5O3 ceramic compositions. (b) Experimental (points) and cal-

culated using the Lichtenecker formula (solid lines) (T-Tm) dependences for some PbxFe0.5Nb0.5O3compositions. Numbers at the curves correspond to x values.


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Figure 3. Dependences of room-temperature resistivity on sintering temperature TS for stoi-

chiometric PbFe0.5Nb0.5O3 ceramics fabricated by: (1) One-step sintering of the mixture of oxides

followed by slow cooling. (2) Sintering of the preliminary calcined mixture of oxides followed by

slow cooling. (3) Sintering of the preliminary calcined mixture of oxides followed by quenching toroom temperature.

(T) curves (Fig. 2b) was achieved by using the Lichtenecker formula [10]:

ln = 1 ln 1 + 2 ln 2 + 3 ln 3, (2)

where 1, 2, 3 and 1, 2, 3 are volume fractions and permittivities of the perovskite

phase, pyrochlore phase and pores respectively. The values of 1 at different temperatures

were taken from the experimental (T) curve for stoichiometric PbFe0.5Nb0.5O3 ceramicswith addition of 1 wt.% of Li2CO3. These data were corrected for porosity using the

Lichtenecker formula. The values of 2 = 100 for pyrochlore phase were taken for all

temperatures [8, 9].

The step in the (T) curve at T 70C, corresponding to the rhombohedral (mono-

clinic) -tetragonal phase transition [57] is well observed only in stoichiometric ceramic

Figure 4. Dependences of room-temperature resistivity on sintering temperature TS for nonstoi-

chiometric PbxFe0.5Nb0.5O3 ceramic compositions fabricated by one-step sintering of the mixture of

oxides followed by slow cooling. Numbers at the curves correspond to x values


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100 I. P. Raevski et al.

Figure 5. Temperature dependences of resistivity for nonstoichiometric PbxFe0.5Nb0.5O3 ceramic

compositions. Numbers at the curves correspond to x values.

compositions. The temperature Tm of the (T) maximum varies only slightly on devia-

tion from stoichiometric (x = 1) PbxFe0.5Nb0.5O3 composition, indicating that PFN phase

maintain its stoichiometry across the whole 0.6


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temperature TS. A lowering of PbO/(Fe0.5Nb0.5O2) ratio decreases TS threshold, above

which low resistivity values are achieved.

Acknowledgment

This study is supported in part by the Russian Foundation for Basic Research (RFBR) grant

09-02-92672 IND a.

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