Electric field-induced effect on the optical, dielectric, and ferroelectric properties of Pb(Fe_2/3W_1/3)O₃ single crystals

Article

Reference

Electric field-induced effect on the optical, dielectric, and ferroelectric properties of Pb(Fe

W

)O

single crystals

YE, Zuo-Guang, SCHMID, Hans

Abstract

Single crystals of the complex perovskite Pb(Fe2/3W1/3)O3 (PFW), synthesized by high-temp. soln. growth, have been studied by optical and dielec. characterizations at zero field and under an elec. bias field. At zero elec. field, the PFW crystals remain optically isotropic down to 10 K, and that the diffuse max. of the permittivity near 178 K results from a typical dielec. relaxation with strong frequency dispersion. Application of an elec. field can induce a macroscopically polar phase, which remains metastable when removing E at low temps. Reversing the polarity of the applied field leads to a reversal of the induced polarization Pind, giving rise to a dielec. hysteresis loop of ferroelec. character. Thus, the ferroelectricity in PFW results in reality from the induction of the macropolar phase. The coercive field for the establishment of the induced polarization increases sharply with decreasing temp. Switching of the induced polar states have been evidenced and the induced birefringence measured by means of simultaneous optical examns. with polarized light microscopy. It is expected that the magnetoelec. effect can be [...]

YE, Zuo-Guang, SCHMID, Hans. Electric field-induced effect on the optical, dielectric, and ferroelectric properties of Pb(Fe_2/3W_1/3)O₃ single crystals. Ferroelectrics, 1994, vol. 162, no.

1-4, p. 119-133

DOI : 10.1080/00150199408245097

Available at:

http://archive-ouverte.unige.ch/unige:31316

Disclaimer: layout of this document may differ from the published version.

1 / 1

Ferroelectrics. 1994. V o l 162, pp. 119-133 Reprints available directiy from the publisher Photocopying permitted by liccnsc only

© 1994 O P A ( O v e r s e a s Publishers Association) A m s t e r d a m B . V . Published under license hy G o r d o n and Tireach Science Publishers S A

Printed in the United States of A m e r i c a

E L E C T R I C F I E L D I N D U C E D E F F E C T O N T H E O P T I C A L , D I E L E C T R I C A N D F E R R O E L E C T R I C P R O P E R T I E S O F P b ( F e 2 / 3 W i / 3 ) 0 3 S I N G L E C R Y S T A L S

Z . - G . Y E and H . S C H M I D

Department of Minerai^ Analyticaî and Applied Chemistry, Vniversity of Geneva, CH-1211 Geneva 4y Switzerland

(Received September 13, 1993; in final form June 10, 1994)

Single crystals of the complex perovskite Pb(Fe2/3W,/3)03 [PFW], synthesized from high température solution growth, have been studied by optical and dielectric characterizations at zéro field and under an electric bias field. It was found that at zéro electric field the PFW crystals remain optically isotropic down to 10 K , and that the diffuse maximum of the permittivity near 178 K results from a typical dielectric relaxation with strong frequency dispersion. Application of an electric field can inducc a macroscopically polar phase, which remains metastable when removing E at low températures. Reversing the polarity of the applied field leads to a reversai of the induced polarization P^^^, giving rise to the dielectric hystérésis loop of ferroelectric character. Thus the ferroelectricity in PFW results in reality from the induction of the macropolar phase. The coercive field for the establishment of the induced polarization increases sharply with decreasing température. The switching of the induced polar states have been evidenced and the induced biréfringence measured by means of simultaneous optical ex- aminations with polarized light micruscopy. It is expected that the magnetoelectric effect can be detected in such an induced ferroelectric phase in PFW.

Keywords: Pb(Fe2,3Wy2)Oy, induced polar phase, biréfringence, dielectric relaxation, ferroelectricity.

1. I N T R O D U C T I O N

As most of the Pb-contained complex perovskites, lead iron tungstate Pb(Fe2/3Wi/3)03 PFW] was considered to be "ferroelectric" according to the spread maximum of the dielectric permittivity occurring at the so-called Curie température Te = 178 K.^"^ It was also announced to be the first "ferroelectric antiferromagnetic" ma- terial/ because an antiferromagnetic ordering was reported to take place below the Néel température Tyy = 363 K , at which the température dependence of the magnetic susceptibility x showed an inflexion point, corresponding to the para- magnetic-antiferromagnetic phase transition. I n the paramagnetic région far above r^v, the Curie-Weiss law x = CI{T — 6) was satisfied.^ Appearance of the anti- ferromagnetic ordering can be understood by taking into account the fact that a considérable number (66.7%) of the octahedral sites in P F W are occupied by the magnetic ions Fe^ +.

The existence of a magnetic ordering in P F W gives an interesting peculiarity to this material in contrast to the other complex perovskites like Pb(Mgi/3Nb2/3)03 [PMN] or Pb(Sni/2Tai^)03 [PST] which do not contain magnetic ions. I n such a crystal, the magnetoelectric effect which consists in an induction of a polarization P (or magnetizalion M) by appHcation of a magnetic field H (or electric field E ) ,

f467]/I19

1 2 0 / [ 4 6 8 1 Z . - G . Y E and H . S C H M I D

may a fortiori be expected to occur as in the case of some ferroelectric/weakly ferromagnetic boracite crystals at low températures," because a redistribution of mean charge density gives rise to a change of distribution of mean spin density and vice versa.

The crystal structure of P F W shows a B-site disordered perovskite lattice in which the Fe^^ and W^"^ ions are distributed at random in the octahedral positions, leading to the diffuse character of the maximum of dielectric permittivity. On the other hand, a séries of compounds with the gênerai formula Pb(B?^Wi/2)03, such as Pb(Mgi/2Wi/2)03 and Pb(Coi/2W 1 ^ ) 0 3 were considered to have an ordered per- ovskite structure i.e. the B^^ and W*"*" ions occupy the octahedral position so that each B^"*^ ion has only W^^ ions as the nearest neighbours in the octahedral sublattice, and vice versa for each W^"^, forming thus a superstructure. A certain number of solid solutions on the basis of P F W were studied with a view to under- standing not only the structural ordering modifications by increasing the amounts of an ordered perovskite, but also the change of dielectric and magnetic properties by adding a second compound containing magnetic ions or not, e.g. Pb(Fe2/3Wi/3)03 [ P F W ] - P b ( M g i,2W,,2) 0 3 [ P M W ] , ^ P F W — P b ( Y b i,2Nbi;2) 0 3 [ P Y N ] , « PFW—Pb(Mgi^Ta2^)03 [PMT],' PFW—Pb(Coi^W,^)03 [PCW],io PFW—BaTiOa"

and PFW—Pb(Fei;2Nbi;2)03 [PFN].^^

It was found that the degree of ordering of P F W increased when increasing the amounts of an ordered perovskite. In the case of the solid solution (1 - x)PFW -I- jcPMW System,' for example, an ordering of B-ions in octahedral positions was

aiready observed for x ^ 0.2, with a doubling of the crystallographic and magnetic unit cell parameter ( V — 8 x alyUa being the cubiccell parameter without ordering).

In ail other soHd solution Systems the dielectric permittivity showed a maximum, which became more and more broadened with the increase of the additive com- pound. Depending upon the second compound, the température of the e^-maxi- mum, which was postulated to be the ferroelectric "Curie" température Te, in- creased (for PFW-PYN« and PFW-BaTi03^i) or decreased (for PFW-PMW^ and PFW-PCW^°) with the increasing amounts of the substituent, whereas in the P F W - PMT System^ "Te" presented a minimum for x = 0.4. Several phase diagrams delimiting the para-, ferro- or "antiferro-electric" régions were proposed in thèse Systems. The magnetic properties were modified by the substitution of compounds with or without magnetic ions, leading to para-, antiferro-, ferri- or weakly ferro- magnetic behaviours.''"'*'

Dielectric properties of P F W ceramics were studied with a relaxational model at various frequencies^^ and under pressure up to 7 kbar.*** According to the tem- pérature and frequency dependences of the permittivity and the dissipation factor tan ô, the dielectric polarization was suggested to be of relaxation nature.** The température and pressure dependences of the permittivity, the polarization and the critical exponent y were explained in terms of a phenomenological theory using statistical treatments based on a gaussian distribution of the local Curie température of the diffuse phase transition. The values of the "spontaneous polarization" and the coercive field were also deduced from the dielectric hystérésis loop, but without reaching the saturation.

So far ail physical characterizations of P F W have been performed on ceramics only, because the synthesis of the single crystals encountered some difficulties.

F I E L D I N D U C E D E F F E C T I N Pb(Fe2,3W,/3)03 [469]/121

However, single crystals of P F W have recently been successfully grown in our Laboratory. The purpose of this work was therefore to study the optical and dielectric properties of P F W single crystals in an electric bias field and under simultaneous optical control by means of polarized light microscopy» with a view to characterizing the electric field induced effect on the optical, dielectric and ferroelectric properties.

2. E X P E R I M E N T A L T E C H N I Q U E S 2.1 Crystal Sample Préparation

Single crystals of Pb(Fe2/3Wi/3)03 were synthesized from high température solution growth using PbO + B2O3 flux.'^ The grown crystals exhibited a regular cubic morphology, black and semi-metallic luster and a good optical isotropy, without inclusions or internai stress. Platelets were eut parallel to (lOO)^^^. (110),.ub or

( l l l ) c u b planes with a thickness of 30 to 50 fim, below which they become trans- parent with red absorption colour. They were finely polished with 0.25 \Lm diamond paste and electroded by déposition of semi-transparent A u - C r layers permitting to realize the simultaneous visual control of the domain state and the measurement of optical biréfringence, and to minimize the mechanically induced stress due to surface contacts. Gold wires (4> = 40 |jim) were fixed by silver/epoxy at the centre of the platelets.

2.2 Characterization Techniques

The electroded crystal samples were mounted on a rotating rod of a spécial optical He-flow cryostat (Oxford Instr.) in conjunction with a polarized light microscope.

The thermal treatments were carried out between 10 K and 300 K . Températures were measured with the help of a caiibrated Carbon-Glass Resistor fixed close to the sample. The dielectric properties of the P F W crystals were studied at frequencies ranging from 1 kHz to 10 M H z by means of an HP4192A Impédance Analyzer.

The oscillation level was fixed to 1 Vrms. A n internai D C bias field (with maximum voltage limited to 35 V ) was applied during the measurements.

A high voltage supply and a protecting resistor of 100 k l î were used as bias source when studying the switching of the induced polarization. The relation be- tween the electric displacement (polarization) and the electric field was determined by using the well known Sawyer-Tower circuit. Dielectric hystérésis loops were displayed by an oscilloscope (HP141B) and photographed on the screen. A standard capacitance of = 22 |xF was connected in séries with the crystal. A Synthesizer/

Function Generator (HP3325A) was used for providing a saw-toothed A C electric field. Low frequency (<10 H z ) was chosen in order to avoid the fatigue-failure of the crystal due to the field-induced destructive ferroelastic domain switchings (see Section 5). Optical biréfringence was measured with a tilting compensator (Leitz M ) .

Various thermal treatments or combinations of treatment have been used for char- acterizing the crystals: Zero-Field-Coohng (or -Heating) [ZFC(H)]; Field-Coohng

122/[470] Z . - G . Y E and H . S C H M I D

[FC]; Zero-Field-Heating-after-Field-Cooling [ Z F H af. F C ] ; Field-Heating-after-Field- Cooling [FH af. F C ] ; Negative-Field-Heating-after-Fositive-Field-CooIing [ ( - ) F H af.

( + ) F C ] . The coohng and heating runs were performed at a rate of 3 K/min.

3. C R Y S T A L O P T I C A L C H A R A C T E R I Z A T I O N

3.1 Optical Isotropy

The (100)^„b, (llO)cub and ( l l l ) c u b platelets of P F W crystals remained isotropic upon a cooling run without electric bias field, since no biréfringent domains, which should indicate a symmetry breakdown due to a structural phase transition into a ferroelectric/ferroelastic phase, have been observed under crossed Niçois down to 10 K (Figure l a ) . Therefore for longer cohérence length probing radiations like polarized light and X-ray, the P F W crystals show a mean cubic symmetry down to low températures, i.e., far below the température of the maximum dielectric con- stant near 178 K (see Section 4). This behaviour was also revealed in some other disordered complex perovskite hke Pb(Mgi/3Nb2/3)03 [PMN].*^

3.2 Induced Biréfringence

Application of an electric field E (//(lll)cub) can induce, upon a field cooling run, a biréfringence on a (lll)cub plane with the higher index (n'y) axis oriented along (llO)cub) as shown in Figure I b . The red interefence colour results from the char- acteristic absorption edge of Fe^"^ ions. On a (lOO)cub plane, however, an electric field E parallel to (lOO)cub gave rise to an induced uniaxial biréfringence, with the révolution axis of the optical indicatrix parallel to E . The (lOO)cub platelet remains therefore isotropic when observed along {100)cub (Figure 2a). It becomes weakly biréfringent in the diagonal position when rotating the crystal around the main axis of the rod, which was perpendicular to the electric field and lay in the plane of the platelet, as shown in Figure 2b. The appearance of this biréfringence is due to an inclination of the isotropic axis ( / / E ) with respect to the observation direction.

<11Ô>c

F I G U R E 1 Photographs of a ( l l l) c u b platelet of PFW crystal (thickness = 40 ^.m, area = 0.56 mm^) under polarized light: (a) Optical isotropy observed upon Zero-Field-Cooling down to 10 K; (b) Biré- fringence induced upon Field-Cooling with E = 8.75 kV/cm (T - 10 K ) . See Color Plate V I I .

F I E L D I N D U C E D E F F E C T IN Pb(Fe2/3W,;3)03 I 4 7 1 J / 1 2 3

F I G U R E 2 Photographs of a ( 1 0 0 ) , „ b platelet of PFW crystal (thickness = 4 0 \Lm, area = 0 . 7 2 mm^) under crossed Niçois upon Field-CooIing with £ = 1 5 kV/cm applied along ( 1 0 0 ) c „ b . showing an electric field induced state with uniaxial optical indicatrix {T = 1 0 K ) : (a) Isotropic when observed along the révolution axis, i.e., //{lOO)^^^,; (b) Biréfringent when rotating the platelet by 3 0 ° along an axis perpen- dicular to E , as indicated by the arrow. See Color Plate V I I I .

giving rise to an anisotropic cross section of the indicatrix. The higher index (n'^) is perpendicular to the rotating axis (Figure 2b), indicating an optically positive indicatrix of the induced phase. Therefore the application of an electric field along

{100)cu5 induced an anisotropic phase with the uniaxial tetragonal symmetry. The fact that with E parallel to < l l l ) c u b no isotropic section was obtained on ( l l l ) c u b

plane, indicates a preferential orientation along {100)cub of the induced state (po- larization). According to the polar character of this induced state (see Sections 4 and 5), the point group 4mm is suggested to be the most plausible.

3.3 Thermal Variation of the Biréfringence

The température dependence of the induced biréfringence has been studied upon various thermal treatments. Figure 3 shows such a variation on a ( l l l ) c u b at an electric field of 8.75 kV/cm under various conditions. Upon an initial field cooling run, the induced biréfringence An set up at about 180 K and increased to a value of 3.8 X 10~^ upon further cooling down to 10 K . In the case of F H af. F C , the induced biréfringence decreased reversely upon heating and vanished near 180 K . It decreased much more rapidly, however, upon Z F H af. F C and disappeared at about 95 K , indicating a free release of the induced state. The biréfringence re- mained metastable when removing the electric field at low températures, showing well the field induced character. In the case of ( — ) F H af. ( + ) F C , the application of an electric field with ( - ) polarity, opposite to that of the initial poling field ( + ) , gave rise at first to a decrease of An to 3 x 10~^ at 10 K and afterwards to a sharp fall of the biréfringence upon heating. An vanished at 50 K and increased anew upon further heating to a maximum value at about 85 K , before decreasing and vanishing again near 180 K as in the case of F H af. F C .

It can be seen that the onset and the disappearance of the initially induced biréfringence An occurred progressively under a bias field or at zero-field, indicating a phase transformation of 2nd order-type. Application of an electric field can be suggested to give rise, by means of ion displacements, to an anisotropic state with

124/(472] Z . - G . Y E and H . S C H M I D

0.5-

o OO FH af FC o QO ZFHaf.FC 0.4-

isrà-ù- {-)FHal(+)FC CE = 8.75kV.cm"M

0

• '^1

100

aoo

F I G U R E 3 Température dependence of the electric field induced biréfringence measured on the

( n i) « . b plate upon (i) FieW-Heating-afier-Field-Cooling [FH af. F C ] , (ii) Zero-Field-Heating-after- Field-Cooling [ Z F H af. F C ) and (iii) ( - )FieId-Heating-after-( + )Field-Coohng [ ( - ) F H af. ( + ) F C ] with E = 8.75 kV/cm.

appearance of the induced biréfringence. Such an induced phase is found to be polar (Sections 4 & 5). The decrease of the A« in Figure 3 which is quicker upon Z F H af. F C than upon F H af. F C , corresponds to the release of the induced state.

The temporary vanishing of the biréfringence at 50 K shows an isotropy due to a counterbalance of the initially induced state by the application of a field with opposite polarity, which induced by retum an anisotropic state above 50 K , but with a reversed induced polarization.

3.4 Electric Field Dependence of the Biréfringence

The value and the variation as a function of température of the induced biréfrin- gence was found to dépend strongly upon the applied field strength. Figure 4 shows the température dependence of An upon F H af. F C and Z F H af. F C at E = 5, 8.75 and 15 kV/cm, respectively. Higher electric poling field resulted in bigger value of the biréfringence, which set up and vanished at higher température upon F C and F H af. F C runs. When removing the electric field at 10 K , the induced biréfringence subsisted, showing a metastable behaviour of the induced state. A decrease of An was measured right after the removal of E at 10 K , which is more pronounced with higher field strength (see Figure 5). Upon the subséquent Z H F af. F C , the curves of An converged to a zéro biréfringence at about 95 K , inde- pendently of the initial field strength.

The field dependence of the induced biréfringence at 10 K was presented in

F I E L D I N D U C E D E F F E C T I N P b ( F e 2 „ W„3) 0 [473J/125

F I G U R E 4 Température dependence of the induced biréfringence of the ( l l l) < . u b pïate measured upon (i) Field-Heating-after-Field-Cooling [FH af. F C ] and (ii) Zero-Field-Heating-after-Field-Cooling [ZFH af. F C ] at field sirengths £ = 5, 8.75 and 15 kV/cm.

0.6

0 4 C K

<3

02- T=10K

0 after FC CE=15KV.cm-')

©FC (E:=5,a75 ond ISkV.cm"'') 5 10 , 1 5

F I G U R E 5 Electric field strength dependence of the induced biréfringence on the ( l l l) c u t > plate at 10 K; (1 ) After F C at £ = 15 kV/cm, the variation of An corresponding to the réversible electro-optical effect; (2) Upon F C down to 10 K at £ = 5, 8.75 and 15 kV/cm, respectively.

Figure 5. The curve 1 shows a nearly Unear variation with E of the biréfringence previousiy induced upon a F C run at £ = 10 kV/cm, which corresponds to the contribution of the réversible electro-optical Fokels effect. The curve 2 gives the values of the biréfringence at 10 K induced upon F C at E = 0, 5, 8.75 and 15 kV/cm, respectively.

1 2 6 / ( 4 7 4 ] Z . - G . Y E and H . S C H M I D

1 10 100 1,000 lopoo

Frequency (kHz)

F I G U R E 6 Frequency dispersion of the real part of the dielectric permittivity and the dissipation factor tan ^S at ^{2 9 3} of the ( 1 0 0 ) , „ b platelet of PFW.

4. D I E L E C T R I C P R O P E R T I E S

4,1 Dielectric Relaxation

Figure 6 shows the frequency dispersion of the real part of the dielectric permittivity g; and the dissipation factor tan 5 of a (lOO)^^^ platelet of the P F W crystal at 293 K . remained stable in a large frequency range up to 2 M H z . A sharp decrease of the permittivity was measured f o r / > 2 MHz» the sign of which changed at 6.3 MHz where the dielectric losses diverged because of an electro-mechanical réso- nance, as in the case of PMN.^^ A t zéro electric field the température dependence of the dielectric permittivity shows indeed a very diffused maximum, the tempér- ature of which dépends upon the measuring frequency. Figure 7 gives the variation of the dielectric constant and the dissipation factor of the (lOO)cub plate at various frequency upon zéro field heating. The value of the maximum of was found to be higher than 10^ around 178 K . The température corresponding to e^^^x at 1 kHz, i.e., 178 K , has been considered so far to be the so-called ferroelectric Curie température T^^^'-^ However, the maximum of the dielectric permittivity was found to displace to higher températures with increasing frequency, i.e., 178 K at 1 kHz, 182 K at 10 kHz, 187 K at 100 k H z and 195 K at 1 M H z . Such a frequency dispersion of the dielectric properties indicates a typical dielectric relaxation be- haviour, rather than an évidence for a spontaneous ferroelectric ordering, as was confirmed by the optical isotropy of the crystals far below ' T e . " Platelets of (llO)cub and ( l l l ) c u b eut exhibited a similar dielectric dispersion at zéro bias field.

The statistical composition fluctuations in Fe:W concentration due to disordered B-site ions and the présence of very small polar micro régions in nano-scale, which

F I E L D I N D U C E D E F F E C T I N Pb(Fe2,jW,,3)03 [475]/127

10-

'o X

5

0

ZFH

IKHz 10 kHz /

lOOkHz II ' 1MH2

I I I '

0 100 200 300 TEMPERATURF (K)

F I G U R E 7 Température dependence of the real part of the dielectric permittivity e; of the (lOO)cub platelet of PFW at various frequencies without bias field (Zero-Field-Heating-after-Zero-Field-Cooling).

The frequency dispersion of the maximum of the dielectric permittivity is characteristic of a dielectric relaxor.

TEMPERATURE (K)

F I G U R E 8 Température dependence of the dielectnc constant of the (lOO)cub platelet upon (a) Field-Cooling {E = 8.75 kV/cm) and (b) Zero-Field-Heating-after-Field-Cooling. A strong atténuation of the dielectric relaxation was measured under the bias field upon F C .

128/[476] Z . - G . Y E and H . S C H M I D

have been postulated to be the origin of the dielectric relaxation phenomena in the disordered complex perovskites like PMN,*^~^^ can be suggested to be respon- sible for dielectric relaxor properties in P F W crystals.

measuring frequencies, which was visible upon différent thermal treatments (see Figures 7-10). This anomaly appeared independently of the applied electric field and can be attributed to a magnetic ordering due to the super-exchange of

—Fe^"^—O—W—O—Fe^+— type. It has been shown that such a super-exchange usually gives rise to a magnetic ordering at low températures in the ordered per- ovskites.^^

0.3

02

c a 0.1

0.0 0 (a)

(a) F C

(E = 8.75kV.cm"')

1MHz

100 200 Température (K) 300

03

02 -

c 01 -

0 0 (b)

(b) ZFH Qf. FC (E=8.75kV.cnn-M

100 200 Température (K)

300

F I G U R E 9 Température dependence of the dissipation factor of the (lll) c u b plate upon F C (a) and Z F H af. F C (b) at £ - 8.75 kV/cm.

F I E L D I N D U C E D E F F E C T IN Pb(Fe2,3^^ [477]/129 8-

6-

><

_ w CO

2-

0 100 200

TEMPERATURE (K)

{-) FH af. {+) FC 1KHz

(E = a75kV.CTn^) ^1^^ / / /

IMHï / / ^

50K J^y^

300

F I G U R E 10 Température dependence of tlie dielectric constant of the (11 \\^^ plate upon ( - )Field- Heating-after-( + )Field-Cooling, showing a weak inflexion near 50 K and an anomaly at 18 K.

of e^, making the diffused maximum more broadened (Figure 8a). The high di- electric constant around " r ^ " at zéro bias field can be suggested to resuit from a dielectric relaxation due to the response of the electric dipoles in the form of micro polar régions to the external exciting field. This dielectric relaxation is strongly hindered by the action of a bias field, which tends to orient thèse dipoles along the direction of E and hence to weaken their contribution to the dielectric per- mittivity, leading to a smaller value of and an atténuation of relaxation.

When removing the bias field at low température, the température dependence of the dielectric constant (Figure 8b) upon Z F H af. F C showed the same shape as in the case of Z F H (see Figure 7), indicating the reestablishment of the relaxation régime around "Te" = 178 K . The effect of electric bias field on dielectric prop- erties appears thus to be thermodynamically réversible, as justifies the dipole con- tribution to the dielectric relaxation mechanism. Upon F C , a broad maximum of 8^ occurred at about 180 K for various frequencies, which corresponds to the onset of the induced biréfringence measured on a (lll)^^,, plate (see Figure 3). No anomalies can, however, be revealed around 95 K upon Z F C , where the induced biréfringence was found to vanish. Figure 9 gives the température dependence of the dissipation factor of a ( l l l ) c u b plate at various frequencies upon F C and Z F H af. F C . The dielectric losses were also attenuated upon cooling under a bias field and released upon heating without bias field.

Upon ( - ) field heating after ( + ) field cooling, an inflexion point was measured at 50 K in the température dependence of the dielectric constant at various frequencies (Figure 10), corresponding to the counterbalance of the initially induced polarisation by the field of reversed polarity. This is consistent with the vanishing of the induced biréfringence in Figure 3. It can be noted that at lower températures, an anomaly of the dielectric permittivity and the dissipation factor was revealed near 18 K for ail

130/[478] Z . - G . Y E and H . S C H M I D

F I G U R E 11 Dielectric hystérésis loops of the (lOO)^.^^ platelet of PFW displayed at various tempér- atures, indicating a ferroelectric behaviour of the induced metastable polarization ( / = 2 Hz for a, b, and c, / = 3 Hz for d, Scale: E = 8.33 (kV/cm)/div., P = 14.3 (M-C/cm^ydiv.).

F I G U R E 12 Break of the (lOO)^ub platelet of PFW (see Figure 2) having undergone a séries of the destructive ferroelastic/ferroelectric switchings of the induced polarization.

measuring frequencies, which was visible upon différent thermal treatments (see Figures 7-10). This anomaly appeared independently of the applied electric field and can be attributed to a magnetic ordering due to the super-exchange of

—Fe^+—O—W—O—Fe^^— type. It has been shown that such a super-exchange usually gives rise to a magnetic ordering at low températures in the ordered per- ovskites.^^

5. I N D U C E D F E R R O E L E C T R I C I T Y

5.1 Dielectric Hystérésis Loops

The induction of a macro polarization in PFW crystals by the application of an electric field, as above suggested from the optical biréfringence and the dielectric properties.

F I E L D I N D U C E D E F F E C T IN Pb(Fe,;3W„3)03 [479|/131

50 ^{100 150}

Température (K)

200

F I G U R E 13 Variation as a function of température of the coercive field strength and the induced polarization P,,,^ of PFW crystals measured on a (lOO),,^^ platelet.

was evidenced by studying the relation between the electric displacement (or the induced polarization P,„J and the electric field strength. Dielectric hystérésis loops were thereby displayed under an A C field at différent températures and shown in Figure 11, which indicate a ferroelectric appearance. Such a ferroelectric behaviour resuUs in reality from the induced polarization, which, once established, subsists when removing the electric poHng field and can be switched by reversing the polarity of E , leading to the hystérésis loop between P^^ and E . A saturation of the polarization was obtained at various températures below 125 K .

This 180°-reversal of the induced polarization gave rise to the switching of ferroe- lastic/ferroelectric domains, which was observed by a simultaneous optical control in polarized light. On a (lOO)cub platelet with E perpendicular to the plane, the 18(f- switching was monitored by the intermittent and periodic flickers of the crystals, due to the appearance of a transient biréfringence. This biréfringence was suggested to be induced by the mechanical stress generated at the phase boundary of the antiparallel polar states during the 180°-switching. Therefore the switching of the induced polar- ization (or ferroelectric domains) was coupled to the ferroelastic déformation in the PFW crystals. At the end of a séries of switching, the platelet of P F W was found to be broken (Figure 12), indicating thus a destructive ferroelastic/ferroelectric switching of the induced state. A mechanical vibration of the sample was observed, which appeared synchronousiy with the period of the apphed field during the switching procédure, suggesting a strong electrostrictive effect for the crystals.

5.2 Coercive Field and Induced Polarization

The température dependence of the coercive field ( E ^ ) necessary for reversing the induced polar state and the value of the rémanent induced polarization (F^^^) was determined from the dielectric hystérésis loops. Figure 13 shows that E ^ increases

132/(480] Z . - G . Y E and H . S C H M I D

exponentially with decreasing température, indicating a higher electric rigidity of the crystal lattice at lower températures. P,„^ decreases with increasing température starting from a maximum value of about 20 fxC/cm~^. Thèse behaviours présent some similarities compared with those of a spontaneous ferroelectric phase.

It should be noted that the electric conductivity of PFW increased, hence the effective field in the crystals decreased rapidly with increasing température. There- fore the electric induced effect became less effective. Higher frequencies were needed to display the saturated hystérésis loops at températures above 100 K because of the losses of electric charges by conduction. No évidence of a field induced polarization and its switching can be measured above 200 K , the P-E relationship becoming Hnear with a very large electric displacement. The variation of Pi„j was thus affected by the increase of the conductivity upon heating. The values of Pf„^ (considered to be spontaneous polarization) and E^ found in this work are higher than that ones measured on ceramic samples, because no saturation of the polarization could be obtained in ceramics.^** A direct measurement of the induced [>olarization by means of the thermal depoling method showed also an anomalous increase of charges above 230 K due to an increase of electric conduc- tivity. A slight change of slope in the température dependence of the dielectric properties was also revealed near 230 K due to the same effect (see Figures 7-10).

6. DISCUSSION A N D CONCLUSIONS

Single crystals of the complex perovskite Pb(Fe2;3Wi/3)03 have been characterized by the optical and dielectric measurements under a bias field. A n electric field induced polar state has been evidenced by the optical anisotropy, the dielectric permittivity and the ferroelectricity of the P F W crystals. The appearance and dis- appearance of the induced biréfringence, as well as its magnitude, were found to dépend upon the applied field strength, Higher electric field strengths gave rise to a higher biréfringence which vanished at higher températures.

The typical dielectric relaxation around the maximum of the dielectric permit- tivity was found to be strongly attenuated by the bias field. No sharp anom- alies have, however, been revealed, which would indicate the onset of a field in- duced phase transition. This is in contrary to the case of complex perovskite Pb(Mgi/3Nb2/3)03, in which an electric field induced Ist order phase transition has been clearly evidenced by a sharp peak of the dielectric constant and the poling/

depoling current.^^ The relatively high electric conductivity in P F W is considered to have weakened the electric field induced effect, leading to the broadened anom- alies of the dielectric permittivity.

The induced polar state has been obviously revealed by the rémanent induced polarization P,^^. Once having been induced, the polar state remained metastable and can be reversed by application of an electric field with opposite polarity. The 180**-switching of the induced polarization gave rise to dielectric hystérésis loops of ferroelectric behaviour. Therefore the ferroelectricity displayed in the P F W crystals results in reality from the electric field induced polar phase. It is expected

F I E L D I N D U C E D E F F E C T IN Pb(Fe2/3W,/3)03 [481J/133

that the magnetoelectric effect, at least the one quadratic in the magnetic field, can be detected in such an induced ferroelectric phase in Pb(Fe2/3W ^1/3)03 crystals.

The magnetic ordering at low températures should be studied in détail by a S Q U I D magnetometer and by neutron diffraction.

A C K N O W L E D G E M E N T

The authors would like to thank Dr. J.-P. Rivera for many helpful discussions, R . Cros, R . Boutellier and E . Burkhardt for their technical help. The support by the Fonds National Suisse de ia Recherche Scientifique is gratefully acknowledged.

R E F E R E N C E S

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(a) (b)

C O L O R P L A T E V L See S. Ishihara, Figure 3.

FERROELECTRICS,Vo\ume 162(1-4).

C O L O R P L A T E V I I . See Z . - G . Y e and H . Schmid, Figure 1 FERROELECTRICS, Volume 162(1-4).

(ÏOO)cub

C O L O R P L A T E V I I I . See Z . - G . Y e and H. Schmid, Figure 2.

FERROELECTRICS,\o\ume 162(1-4).

C O L O R P L A T E X . See H . Rabe et al. Figure 2a.

FERROELECTRICS, Volume 162(1-4).