Experimental evidence for a spontaneous relaxor to ferroelectric phase transition in Pb(Mg13Nb23)O3 - 10% Ti

Pergamon

Vol. 98, No. 8, pp. 765-769, 1996 Copyright 0 1996 Elsevier Science Ltd

Solid State Communications,

1

Printed in Great Britain. All rights reserved 00381098196 $12.00 + .OO

EXPERIMENTAL

EVIDENCE FOR A SPONTANEOUS

FERROELECTRIC

PHASE TRANSITION

RELAXOR TO

IN Pb(Mgl;3Nb&&

- 10% Ti

0. Bidault *, M. Licheron * 7, E. Husson * t, G. Calvarin 8 and A. Morel1 #

* LPMM, ESEM, rue Leonard de Vinci, 45072 Orleans cedex 02, France t CRPHT-CNRS, s Lab. de Chimie-Physique

45071 Orleans cedex 02, France

du Solide, URA CNRS 453, Ecole Centrale de Paris, 92295 Chltenay-Malabry,

#Thomson

France

CSF, LCR, Domaine de Corbeville, 91404 Orsay cedex, France

(Received 15 November 1995; accepted 10 JQ~ULZ~Y1996 by G. Bastard) From X-ray diffraction, 0.9 Pb(Mgl/3Nb2/3)03 transition.

polarization

This material

ferroelectric

transforms

upon cooling.

Pb(Mgl/3Nbz/3)03

and dielectric

measurements,

it is shown that a

- 0.1 PbTiO3 ceramic undergoes a zero-field rhombohedral spontaneously

from a relaxor

On such an observation,

phase

into a normal

the influence

of titanium

on

based ceramics is discussed.

Keywords: A. ferroelectrics,

D. order-disorder

effects, D. phase transition

1. Introduction

PMN

forms

solid

solutions

with

the normal

ferroelectric lead titanate PbTi@ (PT) allowing a variation Lead

magnesium

(PMN) belongs

niobate

of the permittivity maximum temperature from 265 K up to

Pb(Mgl/3Nby3)03

to the class of the relaxor ferroelectric

766 K. This Ti4+ doping on B site is expected to modify

materials. The frequency dependent broad maximum of the

the chemical order and especially

dielectric permittivity (Tmax = 26.5 K for f = 1 kHz) does

Indeed,

not correspond

PbTi@ contents

becomes

the mean structure remains cubic down to 5 K 1 and PMN

relaxor behavior

persists

is characterized

boundary at a composition containing 35 mol % PT. A few

with a structural phase transition.

by an inability

Indeed

to sustain a macroscopic

the diffuse

observed

for low

sharper and sharper.

In fact a

up to the morphotropic

polarization for temperatures significantly below the permit-

years ago, Cross 8 proposed

tivity maximum. In fact a ferroelectric phase can be induced

superparaelectric

at approximately

fluctuating

evidenced

200 K by applying a field on cooling as

by X-ray diffraction

study ?, Moreover

commonly recognized that PMN, like other relaxors, highly

inhomogeneous

diffraction experiments rhombohedral

material.

Neutron

study of a solid

that these polar regions are

with polarization

between

equivalent

solution

phase

vectors

positions.

thermally

Based on the

of 10 mol % PI

in PMN,

Vielhand and Cross 9,10 suggested that this relaxor ferro-

is a

electric is a polar-glassy

and X-ray

1.3 revealed the existence

polar domains on a nanometric

it is

the polar cluster size ‘.

phase transition

dispersion

of some

of T,,,

with the Vogel-F&her

“freezing temperature”

scale resul-

system. Analyzing (Tr = 291.5

the frequency relationship,

a

K) is determined,

analogous to spin glass. Recently, Tagantsev 11 shows that

ting from short-range correlated atomic shifts. Furthermore high resolution electron microscopy studies 66 evidenced a

such a relationship does not necessarily imply freezing. In a

tendency to 1: 1 ordering of the Mg2+ and Nbs+ ions on the

Pb(Sco._sTao&$

B-site sublattice. These ordered clusters (their size being of

exists with a zerofield

several tens of A 6) are regularly matrix where the polar nanodomains due to the strong polarizability

where a relaxor

macroscopic

behavior

ferroelectric

co-

phase lZ,

the fitted Tf value was found close to the phase transition

spread inside a Nb-rich progressively

ceramic

temperature.

nucleate

On such an observation

the occurence

glassy state in relaxors at Tr has been questioned.

of the Nb5+ ions. 765

of a

In order

EXPERIMENTAL

766 to contribute relaxors,

to the understanding

PI ceramic

undertaken. Indeed, in this compound

of a ferroelectric

present

diffraction,

X-ray

vol. 98, No. 8

of

10’

phase 13. In our

poling

current

give strong evidence

field cubic-rhombohedral

PHASE TRANSITION

were

recent experiments

suggested the occurence

study,

FOR A SPONTANEOUS

of the physics

new studies on a PMN-10%

dielectric measurements

EVIDENCE

and

for a zero-

cu-

transition which escaped earlier

investigations.

lo”

2. Experiment 50

The 0.9 Pb(Mgt/3Nb2&03 was prepared

100

150

200

- 0.1 PbTiO3 sample

at Thomson-CSF/LCR

according

250

300

350

4

T (K)

to the

columbite method in order to avoid a pyrochlore phase. The

Figure 1: (a) Temperature dependence of the real part of the

dielectric measurements

dielectric

(thickness

on ceramic discs

0.2 mm) with gold sputtered

Schlumberger frequency

were performed

electrodes.

A

SI1260 impedance analyser was used with a

ranging from 1 Hz up to 10 MHz. The sample

temperature could be monitored

between 77 to 400 K with

permittivity

frequencies

in the PMN-lO%PT

ranging from 3 Hz (curve

transition

whereas

corresponds

The shoulder

to a macroscopic

the frequency

at

1) up to 3 MHz

(curve 7). Note that the scale is logarithmic. at 280 K (arrows)

ceramic

phase

dependent

diffuse

an accuracy better than 0.05 K. Moreover the experimental

maximum follows a Vogel-Fulcher

set up allows experiments

in (b). The fitting curve (solid line) gives Tt = 294 K, fo =

under dc bias field (0 < E < 10

kV/cm). Cooling and heating rates were 1 Wmin. We also

behavior as illustrated

1.15.10l~HzandE,=46meV.

measured the poling and depoling currents as a function of temperature

(+ 2 Wmin) with a Keithley 617 electrometer

allowing to calculate the polarization

by integration.

The

sample is cooled down to 77 K, possibly under a dc field.

dielectric permittivity maximum (Tarax) can be fitted with the Vogel-Fulcher relationship f = fo exp [-WV,,

At the low temperature the field is switched off and the temperature swept up again. All these measurements

were car-

where

fo,

- Tr)l,

Fa and the “freezing

(11

temperature”

Tt are

ried out on samples freshly thermally annealed at 390 K in order to eliminate the possible remanent polar regions. The

adjustable parameters (figure l(b)). Analysis of the pairs of

X-ray powder diffraction patterns were recorded on a high-

an activation energy I& of 46 meV and a Tt value of 294 K.

accuracy Microcontrole

diffractometer

tion (graphite monochromator)

using CuKa

of a rotating-anode

rator of 18 kW. The low-temperature

(f,Ttnax) gives a preexponential

factor fo of 1.15 lOI2 Hz,

radiagene-

study was performed

in a He cryostat with a stability and precision of 0.1 K.

l.f=

0.12

2. f

0.10

3.f= 4.f= 5.f=

0.03 kHz = 0.3 kHz 3 kHz 30 kHz 300 kHz

3. Results Figure l(a) shows the temperature the real part st of the dielectric permittivity lO%PT ceramic increases

sample.

the maximum

dependence

As the measuring shifts

to higher

frequency

occurs on the low-temperature

side of the peak.

behavior

is characteristic

ferroelectric relaxers and the diffuse dielectric related to a slowing down and a broadening relaxation

in the frequency

0.02;

temperature

whereas a large dispersion This

of

for the PMN-

50

of

100

150

200

250

300

350

4 0

T(K)

anomaly is of a dielectric

space. As observed

by many

authors, the frequency dependence of the temperature of the

Figure 2: Dielectric losses versus temperature at frequencies 0.03 - 300 kHz. For each of these frequencies

a shoulder is

clearly observed at the same temperature, Tpc = 280 K.

EXPERIMENTAL

vol. 98, No. 8

EVIDENCE

FOR A SPONTANEOUS

767

PHASE TRANSITION

For al1 these parameters a good agreement is obtained with earlier published data, especially Vielhandetaf

9 (respectively

291.5 K). Moreover, l(a), a shoulder temperature T,

with those obtained

by

1.03 1012 Hz, 40.7 meV and

from a closer inspection

of figure

in the &t(T) curve can be observed = 280 K which is frequency

at a

independent

(shown by arrows). This anomaly escaped early dielectric

0.08~ 0.071 0.06: 0.05: 0 3 0.04: 0.03 1

investigations. 0.027 The loss tangent

tan B displays

qualitatively

a

similar behavior (figure 2) but the shoulder is more clearly demonstrated.

A peak at a temperature lower than Tmax and

0.01;

viiT,.

I

,....

k=

TW

, O.OO~~~‘P,,~n:r~qm,,,.,,,,,(,,,., 50 100 150 200 250

300

1

kHz

350

400

T (K)

which can be ascribed to the diffuse maximum is observed whereas the additionnal shoulder is detected again at nearly 280 K. It appears close to the Tf value deduced from equa-

Figure 4: Dielectric

tion (1) as earlier noted in Pb(Sco,5Tao5)03

and heating at 1 kHz, which evidence

this shoulder is field dependent.

12. Moreover,

It is well known that in

losses versus temperature

poling and depoling

temperatures,

relaxor materials dielectric properties are very sensitive to a

Temperature

dc field applied during thermal treatments 14215.Particularly

field heating and zero field cooling.

dependence

on cooling

respectively

Tpc and Th.

the Inset:

of the current detected upon zero

a ferroelectric phase can be induced in PMN by applying a field,

E, provided

it reaches

a threshold

threshold field strength for a macro-polarization

value.

This

induction

is found to decrease with increasing titanium doping down to 0 kV/cm for 10% Ti 13. For such a Ti content, ferroelectric

the

phase appears at a temperature Tpc closer and

closer to Tinax, when scanning E from 0 to 3 kVlcm (figure 3). For E = 3 kV/cm, we note that Tp z Tmax.

depolarization

takes place and the sample returns to the

cubic phase. This hysteretic phase transition is manifested also by the poling and depoling

current measurements.

They exhibit a peak during the zero-lield

cycle, on cooling

at 280 K (Tpc) and on heating at 300 K (To,) (inset fig. 4). This observation indicates that the paraelectric-ferroelectric transition occurs spontaneously

in the PMN - 10% Ti cera-

mic, without needing any applied field. The low remanent If the temperature

is swept up to 400 K after a

cooling, a new anomaly can be observed (Th) independently

polarization

value (about 0.32 yC/cm2),

which

is thus

at about 300 K

of the E value (figure 4): a thermal 30 25

20 0.06

15 8 s

iQ 3

h

“& L

0.04

10

5 0

50

100

150

200 250 T(K)

Figure 5: The depolarization

300

350

400

current (at 2 Wmin) and the

Figure 3: Dielectric losses recorded at 10 kHz as a function

polarization,

of temperature during the cooling under different dc electric

under 5 kV/cm down to 77 K and heating without field up

field strengths: 0 (a), 1 (b), 2 (c) and 3 kV/cm (d).

to 370 K.

obtained by integration

from it, after cooling

*

EXPERIMENTAL

768

EVIDENCE

FOR A SPONTANEOUS

deduced, results from an average state over all the polari-

the dielectric measurements.

The change in the permittivity

zation orientations in the different grains. A dc field allows

occurs at lower temperatures

to align the grain polarization

while maintaining

in the same direction.

detected current (figure 5) increases

The

rapidly when increa-

appears spontaneously needed in PMN-PI

existence

of a zero-field

experiment ferroelectric

and CuKaz

radiations,

the (222)

In PMN based ceramics,

understand

stoechiometric

revealing

composition

of the cubic

cell. In

particular, the (222) reflection displays a doublet, which is

growth

well interpreted

expected

with a rhombohedral

symmetry

like that

found in poled PMN 2,16. Indeed, this structural

phase

the tendency

to ordering

(1: 1) of the B site cations seems to be the key parameter to

reflection exhibits two peaks at respectively 20 =Z82.7” and

distortion

in this material whereas a dc field is

solid solution less rich in titanium.

phase is X-ray

82.95” (figure 6). At 210 K, a splitting is clearly observed, a long-range

their physical ordered

properties.

regions

Indeed, these non-

with Pb(Mg1/2Nbt/2)03

and the random fields they induce inhibit the

of the polar domains. to increase

disorder

electron microscopy

studies

The titanium

doping

on PMN-PI

compounds

have shown that there is a gradual disappearance

cubic reflection into two components

superstructure

(222) and (222). The

on heating between

shoulder is still observed, range can be compared

270 K, where a

and 290 K. This temperature

with the values deduced from the

due to B site ordering.

turns from nanoscale

7

of the

This solid solution

ordered regions into random cation

distribution. For 10% Ti, the polar clusters which nucleate at temperature much higher than Tina in the Nb-rich matrix are thus favoured.

depoling current and dielectric measurements.

is

on B site. Transmission

transition is expected to give rise to a splitting of the (222) splitting disappears

of a phase

4. Discussion and Conclusion

that proves the

diffraction. At 290 K, the ceramic is cubic, like pure PMN. Due to the CuKal

character

relaxor. There is no doubt that a true ferroelectric

the polarization saturates at approximately

The more convincing

than the dielectric maximum

the broad dispersive

sing the field strength up to 5 kV/cm. For higher E values, 26 pc/cm’.

Vol. 98, No. 8

PHASE TRANSITION

Their associated

dipole

moment

is

expected to be larger than in pure PMN due to the longer To summarize diffraction

experiments

these experimental evidence

findings,

a macroscopic

X-ray

rhombo-

hedral-cubic phase transition in PMN-10% PI at about 290 K. It corresponds

to the poling/depoling

temperatures

as

observed by the thermal current peaks and the anomalies in

correlation

length.

On cooling,

more and more of the

ceramic volume becomes ferroelectric.

Approaching

T,,,

the cluster size increases whereas the distance between two neighbourgs decreases. The polar regions reach a sufficient degree between

of development neighbouring

dipolar correlations

to involve cluster

a strong correlation

leading

to a long range

needed for a true ferroelectric

transition. These collective

phase

effects due to the preseuce of

some Nb-rich polar clusters can be expected

to become

more evident: the higher the Ti concentration,

the greater

the cluster size and the stronger the correlation effects. The chemical inhomogeneity prevents establishment

due to ordered

regions,

which

of long-range order in PMN, is thus

reduced by Ti doping. Other parameters conditions (sintering temperature,

due to the growth

quenching

rate . ..) may

also influence B site ordering, as observed by Chu etaf (“) 8i.O

83.0

82.5

83

in Pb(Sco,5’lao5)03. phase in PMNlO%

Figure 6: X-ray diffraction 210 K. The 222 reflection CuKal

experiments

(two peaks because

macroscopic electric field.

a splitting

PT escaped earlier investigations.

at 290, 270 and of the

and CuKaz radiations) observed at 290 K displays

at lower temperatures

12

This can explain why the ferroelectric

into a doublet.

This

phase transition occurs without applying any

In summary, the size of the ordered regions, which plays the essential role in the development of the polar clusters, is reduced by Ti doping. Whereas PMN is a relaxor material, which does not undergo any macroscopic

phase

transition, PMN -10% Ti exhibits a cubic to rhombohedral

EXPERIMENTAL

Vol. 98, No. 8

EVIDENCE

FOR A SPONTANEOUS

transition at about 290 K. For such a Ti content, the polar clusters are able to reorient themselves with a true long range

order,

diffraction.

In this compound,

ferroelectric

phase transition

and give a phase

as evidenced

by X-ray

a spontaneous

relaxor to

takes place. The freezing

temperatureTf

PHASE TRANSITION

769

deduced from the Vogel-Fulcher

leads to a value close to the phase transition physical meaning of such an observation

relationship one. The

remains unclear

but have to be taken into account in order to understand the physics of relaxors.

References

1. P. Bonneau, P. Gamier, G. Calvarin, Gavarri,

A. W. Hewat

E. Husson, J. R.

and A. Morell,

J. Solid

State

F. Sauerbier,

Shebakov, Ferroelectrics,

G. Schmidt

and

L. A.

J. R. Gavarri,

A. W. Hewat and A. Morell, J. Phys.: Condens.

Matter,

Bull., 2 3,357

11. A. K. Tagantsev,

M. Chubb

and A. Morell,

Mater.

Res.

J. Am.

Ceram. Sot., 7 2, 593 (1989). 6. C. Boulesteix,

and A. K. Tagantsev,

J. Appl.

(1993).

Bidault, M. Licheron and E. Husson, submitted

to

Ferroelectrics.

(1989). H. M. Chan and M. P. Harmer,

13.0.

(1991).

Phys. Rev. Lett., 7 2, 1100 (1994).

12. F. Chu, N. Setter Phys., 74,5129

(1991).

4. E. Husson,

5. J. Chen,

10. D. Vielhand, J. F. Li, S. J. Jang, L. E. Cross and M. Wuttig, Phys. Rev. B, 43,8316

79, 145 (1988).

3. N. de Mathan, E. Husson, G. Calvarin,

3,8159

76, 241 (1987).

Appl. Phys., 68, 2917 (1990).

Chem., 9 1,350 (1991). 2. H. Amdt,

8. L. E. Cross, Ferroelectrics,

9. D. Vielhand, S. J. Jang, L. E. Cross and M. Wuttig, J.

F. Vamier, A. Llebatia and E. Husson,

14. S. J. Butcher and M. Daglish, Ferroelectrics

Lett., 10,

117(1989). 15. 2. G. Ye and H. Schmid,

Ferroelectrics,

145,

83

J. Solid State Chem., 1 OS, 141 (1994).

(1993).

7. C. A. Randall and A. S. Bhalla, Jpn. J. Appl. Phys.,

16. G. Calvarin, E. Husson and Z. G. Ye, Ferroclectrics,

29,327

165,349

(1990).

(1995).