147

RELAXOR PROPERTIES OF THE BARIUM TITANATE

FERROELECTRIC

E. Dul’kin, A. Patron, E. Mojaev and M. Roth

Department of Applied Physics, The Hebrew University of Jerusalem, Jerusalem 91904,

Israel

ABSTRACT

Phase transitions in the BaTiO3 (BT) single crystal, a classical ferroelectric

material, have been studied using acoustic emission (AE) in conjunction with the

standard dielectric method. It has been found that BT exhibits also relaxor properties,

which is manifested by the appearance of AE peaks at Td = 550 K and T* = 506 K,

namely temperatures much higher that the cubic-tetragonal ferroelectric phase transition

temperature, Tc = 403 K. Td is the Burns temperature at which dynamic polar nanoregions

(PNR) are formed, which is a characteristic feature of relaxors. T* is also a relaxor

feature indicating the onset PNR merging and growth. Another relaxor feature found is

the existence of a threshold external electric field at which the Tc reaches a minimum, and

the AE activity - a maximum. Studies of Fourier transform power spectra of individual

AE pulses during the ferroelectric phase transition have reveled activity in the MHz

frequency range indicating that the transition is of the martensite-type and occurs by

twinning due to the dislocation movement.

1. INTRODUCTION

Barium titanate (BaTiO3, or BT) is a perovskite structure oxidel known for over

six decades as a prototypical ferroelectric (FE) material characterized by a switchable

macroscopic polarization [1]. It has been shown [2] that the ferroelectricity originates

essentially from hybridization between the titanium 3d states and the oxygen 2p states

within the [TiO6] octahedron (see Fig.1), whereas the interaction between barium and

oxygen is completely ionic, stabilizing the compound in the rhombohedral structure

(ground state) at low temperatures. Upon heating, BT undergoes a succession of

structural phase transitions, from the slightly distorted rhombohedral to orthorhombic to

tetragonal ferroelectric phases, belonging to the 4mm, mm2 and 3m point group

symmetries respectively, to the high-temperature paraelectric (PE) cubic phase belonging

to the simple m3m point group symmetry [3]. Figure 1 shows the structures of the cubic

phase stable above 403 K and the tetrahedral phase stable between 278 and 403 K and

exhibiting a spontaneous polarization along, say, the z axis. The FE-/PE phase transition

has been traditionally recognized as displacive [4], yet many features observed are not

compatible with a merely displacive behavior. The observed diffuse X-ray shows that the

Ti ions are disordered and occupy one of the eight equivalent off-center sites along the

<111> cubic directions producing a nonzero local dipole moment in each unit cell [5].

The existence of strong quasielastic neutron scattering above the FE transition

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temperature (> Tc = 403 K) is associated with critical relaxor-type polarization

fluctuations in the BT crystal [6].

Fig. 1. High-temperature nonpolar (PS = 0) cubic perovskite structure (left) and lower

temperature polar tetragonal structure (right) of the BT compound; a, c - lattice constants.

A key feature that appears to be common to all of the relaxor-FE compounds, and

which is believed to play a fundamental role in producing the enhanced piezoelectricity,

is the formation at high temperature of tiny regions of local and randomly-oriented

electric polarization just several unit cells in size, also known as polar nanoregions (PNR)

[7]. The PNRs are often attributed to chemical disorder, random bond–random field

appearance, or local symmetry lowering, and lately it has been also shown that the huge

intrinsic nonlinearity of ferroelectrics gives rise to spatially limited excitations of discrete

breather type, which interact strongly and self-consistently with the remaining lattice [8].

In most relaxors, the PNR appear several hundred degrees above the FE transition

temperature, Tc, or above the maximum of permittivity, at the so called Burns

temperature, Td. In the paraelectric phase of BT, a clear deviation of the refractive indices

(at three different wavelengths) below about 550 K has been observed [9]. Measurements

of many other physical properties of BT, such as the dielectric permittivity,

electrostrictive strain and thermal expansion [10] and Raman scattering [11] have

revealed anomalous behavior above the Tc as well. Upon cooling below Td, the

paraelectric (cubic) crystal structure transforms into an ergodic relaxor state where PNRs

with randomly distributed directions of dipole moments exist and start growing until they

reach a critical value [12] and at a temperature T* transform under local strain fields into

polar nano-domains, PNDs, with permanent polarization fluctuations [13]. Further

cooling slows down markedly the dynamics of PNRs, and at a low enough temperature,

Tf, the PNRs become frozen into a nonergodic state, while the average symmetry of the

crystal still remains cubic. The process of freezing in of the dipole dynamics is associated

with a large broad peak in the temperature dependence of the dielectric constant, ε. This

peak is highly diffuse and its temperature Tm (> Tf) shifts with frequency due to the

dielectric dispersion.

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The nonergodic relaxor state existing below Tf can be irreversibly transformed

into a FE state by a sufficiently high external electric field. This is an important

characteristic of relaxors which distinguishes them from typical dipole glasses. This also

indicates the existence of some kind of critical behavior. Indeed, it has been shown [14]

in the case of the best known Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) relaxor-FE that in the

electric field-temperature-composition phase diagram a first-order paraelectric-

ferroelectric phase transition terminates in a line of critical points where the piezoelectric

coefficient is maximum. Near the critical point, the threshold electric field (Eth) needed

for inducing the ferroelectric polarization rotations is rather small. We have performed

recently acoustic emission (AE) measurements on the prototypic PMN-PT relaxor-FE

under external electric field [15]. We have found that the Tm values are field-dependent in

this material; Tm goes through a minimum at the Eth, while a concurrent maximum is

observed in the corresponding AE activity. The reasons for such behavior in various

relaxor-FE are currently under extensive investigation.

AE is one of the new efficient and nondestructive methods of in situ studies of the

onset of a local polarization and the various characteristic temperatures typical for all

relaxor-FE. In the present work, we summarize the efforts aimed at revealing the relaxor

properties of the ferroelectric BT crystal in general, and in particular its critical behavior

under an externally applied electric field. A new attempt will be made to elucidate the

mechanisms underlying the maximal release of the elastic strain energy during the

ferroelectric phase transition using AE. Such transition is known to be martensitic-like in

BT, namely being accompanied by dislocation movement generating elastic waves.

Therefore, individual pulses of the AE signal will be analyzed in order to identify the

waveform shapes responsible for the dislocation movement and other kinetic parameters

associated with ferroelectric phase transition in the BT material.

2. EXPERIMENTAL

Two BaTiO3 single crystal samples were used in the present study, both

commercially grown from top-seeded solutions, but by different vendors. Sample 1 was

[100]-cut, while sample 2 was [001]-cut. The samples were of equal shapes and sizes,

namely plates 5×5 mm2 in area and 1 mm in thickness. They were characterized using the

standard dielectric method of capacitive measuring the dielectric permittivity and the

dedicated AE technique [11]. With the latter, not only the AE activity (dN/dt, or N s−1

),

or number of pulses per unit of time, but also the waveforms of the individual pulses

were analyzed in order to get a deeper insight into the kinetics of the ferroelectric phase

transformation in BT.

For dielectric measurements, air-dried silver paint was applied to the samples.

The dielectric permittivity, ε, was measured using an automated system that consisted of

an LCR meter (HP-4284A, Hewlett-Packard Inc.) in connection with a custom-made

temperature chamber and a sample holder adapted for high temperature measurements.

The capacitance and dissipation factors of sample were measured at 100 Hz and

temperature varied between 290 and 450 K. A heating rate of 3 K/min was used during

the measurements. In the AE measurements, each sample was coupled with a heat

conducting silicon fluid to the polished side of a fused silica acoustic waveguide. A PZT-

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19 piezoelectric sensor was attached to the rear end of the waveguide. The sensor was

electrically coupled to a 500 kHz band-pass low noise (≤1 µV) variable (up to 40 db)

preamplifier linked to a detector-amplifier (40 db). A Ch-Al thermocouple junction was

glued to the waveguide near the sample. The lower part of the acoustic waveguide and

the sample attached to it were mounted in a resistance element tube furnace. Both the

thermocouple and amplifier outputs were interfaced with a PC for a coupled readout. AE

activity measurements were performed in the same temperature range as the dielectric

measurements at an average heating rate of about 1-3 K/s. The basic setup for the AE

pulse counting is shown in the left part of Fig. 2.

Fig. 2. Acoustic Emission measurements setup (pulse sampling and waveform analysis).

The setup components in the right side of Fig. 2 mainly serve for capturing single

AE pulses. The main component is the TDS 1002 oscilloscope operating in the ‘single

sequence’ mode while receiving a signal from the preamplifier/amplifier block. In fact, a

package of 2,500 pulse waveforms is recorded in course of 25 s, namely with a 0.1 s

duration between consequent samplings. The oscilloscope was connected to a PC through

the RS232 interface, and the PC was equipped with the Wave Star 6.0 software capable

of online interaction between an external electrical signal source and the computer, and a

Y-t (signal amplitude in arbitrary units versus time) output is thus reproduced for each

acoustical signal waveform. Fig. 3 shows an example of such a waveform recorded in

course of the FE phase transition in BT.

The setup components in the right side of Fig. 2 mainly serve for capturing single

AE pulses. The main component is the TDS 1002 oscilloscope operating in the ‘single

sequence’ mode while receiving a signal from the preamplifier/amplifier block. In fact, a

package of 2,500 pulse waveforms is recorded in course of 25 s, namely with a 0.1 s

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duration between consequent samplings. The oscilloscope was connected to a PC through

the RS232 interface, and the PC was equipped with the Wave Star 6.0 software capable

of online interaction between an external electrical signal source and the computer, and a

Y-t (signal amplitude in arbitrary units versus time) output is thus reproduced for each

acoustical signal waveform. Fig. 3 shows an example of such a waveform recorded in

course of the FE phase transition in BT. (The exact measurement temperature was

determined using the temperature controller employed in dielectric measurements).

Fig. 3. Acoustic emission (single pulse) waveform durin FE phase transition in BT.

3. RESULTS AND DISCUSSION

3.1 BT relaxor-like properties – acoustic emission study

We have detailed above the succession of ferroelectric phase transitions in BT.

Only one of them, namely the cubic-tetragonal, or PE-FE, appears above room

temperature at around 403 K [3]. Figure 4 displays the results of concurrent

measurements of the dielectric permittivity and the AE activity performed in the 370-440

K temperature range on cooling using sample 1. The sharp maximum of the = (T)

curve corresponds to the Tc = 403 K value. The shape of this curve is consistent with the

behavior of the first order ferroelectric phase transition described by the Ginzburg-

Landau mean field theory for long-range Coulomb interaction of multiple dipoles [1].

The Curie-Weiss law [1], (T) ~ C/( Tc), is fulfilled for T > Tc (C is a constant). A weak

( N = 5 s−1

), but distinct and sharp peak of the AE activity is observed exactly at T = Tc.

We presume that such activity may arise from elastic waves generated by dislocation

movement in course of a martensite-type ferroelectric phase transition. The initial

reasoning behind this suggestion is as following. Even when the ferroelectric strain

associated with the ferroelectric distortion is small, like in BT [2], its effect on the phase

transition is strong [16]. It has been also shown that ferroelectric microdomains of

tetragonal symmetry with very low domain energies are formed through the FE phase

transition as a lamellar structure of head-to-tail, or 90° twins; they can be treated within

the conventional theory of martensitic transitions [17]. Twinning assisted by dislocation

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movement be may be the core feature underlying the martensite-type ferroelectric phase

transformation in the BT crystal. We will address this issue in more detail below, when

analyzing the experimental shapes of individual AE pulse waveforms of BT sample 1.

Fig. 4. Temperature dependences of dielectric permittivity and AE intensity (sample 1).

Martensitic phase transformations are first- order processes. They typically

exhibit an appreciable temperature hysteresis in the values of physical parameters related

to the transformation. We have verified this by monitoring the AE activity during a

heating-cooling cycle. Figure 5 displays the AE activity of sample 2 in the 350 - 650 K

temperature range. An approximately 7 K wide hysteresis is indeed observed in around

400 K (actual peaks at 407 K on heating and 400 K on cooling) for the FE phase

transition, which is somewhat larger then that reveled in dielectric measurements [18].

The AE is more intense on cooling ( N = 80 s−1

) than on heating ( N = 50 s−1

), which we

will attempt to explain at a later stage.

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Fig. 5. Tc, T* and Td detected in course of thermal cycling of a BT crystal (sample 2); full

squares – heating, empty squares – cooling.

Two additional AE peaks appearing at higher temperature are of particular

importance. The broad peak in the 530-570 range (especially broad on cooling)

corresponds well to the Burns temperature, Td 550 K [9], at which the dynamic PNRs

are formed – a characteristic feature of relaxors. The associated AE signal is weak ( N =

17 s−1

on cooling) and without any noticeable thermal hysteresis. However, this is a

common feature for all relaxor ferroelectrics, e.g. for Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT)

[12]. The intermediate peak at about 506 K is also a relaxor feature. It represents the T*

temperature at which the PNRs turn static, namely begin to freeze in cooperatively by

undergoing a local nanolscale FE phase transition associated with random fields [12, 13,

19]. By definition, this intermediate peak appears only on cooling with an appreciable AE

intensity ( N 25 s−1

).

In order to support the suggestion that the T* = 506 K anomalous peak is related

to strains caused by local cooperative freezing, we have investigated the field dependence

of the peak position. External electric fields of up to 2 kV/cm along the [001]-direction

were applied to sample 2, and the results of T* peak positions are shown in Fig. 6,

together with similar results for the Td and Tc peaks. The T* peak position varies notably

with the field increase (the experimental slope is dT*/dE = 10.5 K∙cm/kV), which can be

explained by field-induced polarization rotation from <111> to the [001] direction. The

associated tetragonal strain may then ease the repulsion between the Ti and O atoms

within the TiO6 octahedra [20], and the cooperative PNR freeze may occur earlier, or at a

higher temperature. Interestingly, the Burns temperature practically does not vary with

the field, which may be understood in terms of the prevalent role of fast dynamic dipole

moment fluctuations at around Td ~ 550 K. The Tc does change under an applied field, but

less dramatically then the T*, since much stronger fields are needed for influencing the

long-range Coulomb dipole-dipole interection. From Fig. 6, the corresponding slope,

dTc/dE = 1.5 K∙cm/kV, is several times smaller in comparison with T* (in good

agreement with the 1.4 K∙cm/kV value obtained from dielectric measurements [21]).

154

Fig. 6. Electric field dependence of the Tc, T* and Td peak values determined by AE

(sample 2).

We have studied separately the field dependence of the AE intensity at the Tc. The

appropriate N = N (E) dependence is shown in Fig. 7. The number of counts decreases

linearly with the externally applied field and almost vanishes at E = 2 kV/cm. We believe

that this is another relaxor-like property mainly associated with the system approaching

the critical point where the line of first-order transitions ends [14], and no discontinuity in

any parameter (including strain) is expected. The results of a more detailed inspection of

the Tc peak values and AE activity variations under low externally applied electric fields

is presented in the next section.

Fig. 7. Electric field dependence of the AE count rate at T = Tc.

3.2 Critical behavior

AE measurements presented above show unambiguously that the BT crystal in its

paraelectric phase shows a relaxor-type behavior exhibiting the two characteristic

temperaturs, Td and T*, associated with existence of PNRs. However, the Tc behavior of

BT does not seem to be in consonance with the characteristic Tm temperature of relaxors.

We recall that on cooling a relaxor from Td the dynamics of PNRs is enormously slowed

down, and at a low enough temperature (typically hundreds of degrees below Td), the

PNRs become frozen into a nonergodic state, while the average symmetry of the crystal

still remains cubic. The process of freezing of the dipole dynamics is associated with a

large broad peak in the temperature dependence of the dielectric permittivity, ε. This peak

is highly diffuse and its temperature (Tm) shifts with frequency due to the dielectric

dispersion. We did not observe such Tc frequency shift with our BT crystals in the radio-

frequency range used, up to 500 kHz, for the dielectric and AE measurements. It now

known that the reason for that is the much fast dipole dynamics in the PNRs of BT - it is

in the THz range [22]. Yet, the critical Tc behavior under low external electric fields may

resemble that of the Tm in relaxors, namely a minimum in the Tc peak value and a

155

corresponding maxiumum in the AE activity at a threshold field, Eth, may be sought.

Recently, we have reviewed such effects in both perovskite structure (PMN-PT) and

tungsten bronze (SrxB1-xNb2O6, or SBN) relaxor ferroelectrics [23].

Fig. 8. Field dependences of the BT Tc and maximum AE activity at Tc (sample 1).

We have used sample 1 of BT for similar measurements. The external electric

field was thus applied in the [100] direction. Figure 8 presents the field dependences of

both the Tc, detected by means of AE, and the corresponding AE count rate at the Tc.

Apparently, the Tc(E) curve exhibits a characteristic V-shape similar to those previously

detected for relaxor-FEs [23]. The Tc starting from the zero-field 403 K value initially

increases slightly and then gradually decreases down to 399.5 K where it attains a

minimum at the threshold field Eth = 0.256 kV/cm. As E is further enhanced, the Tc starts

increasing with a slope of 1.5 K∙cm/kV, in excellent agreement with earlier data [21],

when the BT crystal behaves as a normal ferroelectric. The AE count rate at the varying

Tcs initially rises to a plateau at very low fields, then drops prior to reaching a maximum

value of N = 9 s−1

at E = Eth and diminishes as field increases further. Such behavior of

the AE activity, except for the initial plateau, is typical also for all relaxor-FEs studied.

This provides an additional confirmation of the fact that BT exhibits tangible relaxor-FE

properties. Moreover, the nontrivial Tc(E) field dependence points at the existence of

polar short-range order (presumably emanating from the chemical short-range order) in

the paraelectric phase. Fluctuations of such order on a nanoscale create heterogeneities

with disordered local fields, often called random fields, and the latter couple with the FE

degrees of freedom generating PNRs with collective dipole moments [24]. The presence

of PNRs in BT has been discovered recently by means of Brillouin light scattering and

birefringence measurements [25]. The PNRs are explained thereby as being created due

to off-center motions of correlated Ti ions, and they may be distinguished at temperatures

quite higher than Tc, but lower than T*. On cooling, the PNRs grow substantially,

presumably due to the merging of smaller PNRs [7], and they attained micron sizes at Tc

[25]. We presume that such merger of small PNRs into large ones may be the reason for

weak AE responses occasionally recorded around the Tc (Fig. 4). Finally, we attempt to

explain the lack of a prominent N maximum at E = Eth. In fact, the existence of a sharp

FE phase transition at the Tc in BT, like in normal ordered ferroelectrics, instead of the

156

diffuse and frequency dependent Tm peaks in relaxor-FEs implies weaker cooperative

coupling between the PNRs in the former. The latter does not allow the PNRs to prevent

efficiently the occurrence of the long-range ordered FE phase transition.

3.3 Single AE pulse analysis

The waveforms of single AE pulses have been probed in course of heating and

cooling BT sample 1 from about 600 K down to room temperature. Pulses of measurable

amplitudes have been recorded in four different temperature ranges: 545-555 K, 515-525

K, 470-480 K and 400-405 K. The first range encompasses the Burns temperature, Td,

while the last range covers the high-temperature (paraelectric) side of the ferroelectric

transition at Tc. Averages of typically ten pulses have analyzed for each range by

subjecting them to Fourier transform. Descrete Fourier transform has been used, since

each pulse (using the Wave Star 6.0 software) was build of 2,500 discrete segments of 0.1

s duration each and tailored into one waveform. The squares of the amplitudes have

been calculated simultaneously using the Origin software in order to obtain the power

spectra of the pulses in the regions of interest. Figure 9 shows the power spectra for all

four temperature intervals mentioned above with the frequencies spanning from 1 to 5

MHz – typical range for defect associated elastic waves in solids. Clearly, the only

temperature segment producing pulses of appreciable power is the one adjacent to the Tc

(and not around Td or T*). This is the region where merging PNRs already grow into

large ferroelectric domains, and the incoherent boundaries between them and paraelectric

matrix generates substantial mechanical stresses at the PNRs’ surfaces. In order to

minimize the accumulated elastic energy, the domains may break down into twinned

domains for stress accommodation [17].

1x106

2x106

3x106

4x106

5x106

0.0

1.0x10-7

2.0x10-7

3.0x10-7

4.0x10-7

5.0x10-7

6.0x10-7

555K - 545K

Frequency (Hz)

Po

wer

(a.

u.)

1x106

2x106

3x106

4x106

5x106

0.0

1.0x10-7

2.0x10-7

3.0x10-7

4.0x10-7

5.0x10-7

6.0x10-7

525C - 515K

Frequency (Hz)

Po

wer

(a.

u.)

1x106

2x106

3x106

4x106

5x106

0.0

1.0x10-7

2.0x10-7

3.0x10-7

4.0x10-7

5.0x10-7

6.0x10-7

480K - 470K

Po

wer

(a.

u.)

Frequency (Hz)

1x106

2x106

3x106

4x106

5x106

0.0

1.0x10-7

2.0x10-7

3.0x10-7

4.0x10-7

5.0x10-7

6.0x10-7

405K - 400K

Po

wer

(a.

u.)

Frequency (Hz)

157

Fig. 9. Fourier power spectra of average AE waveforms in four temperature ranges.

Twinning produces changes in the internal strain field accompanied by generation

and movements of dislocations at the PNR-matrix boundary, giving rise to elastic waves

propagating within the crystal, or presumably to AE. The few MHz acoustic frequency

range is typical indeed for movement of dislocations. The suggested dislocation

mechanism is similar to twinning of polarized embryos inside a paraelectric matrix

through the martensite-like ferroelectric phase transitions in normal ferroelectrics [26].

The transition-related stress is relaxed by breaking up these embryos into head-to-tail

arranged 90°-domain lamellar tetragonal twins in the cubic paraelectric matrix. We have

analyzed such process earlier, for the case of the PZN-PT relaxor ferroelectric [12], and

will use now a similar approach for assessing such process in BT.

In order for the 90°-twins to remain stabilized down to room temperature, the

phase-mismatch-induced stresses must exceed the yield stresses, or the associated strain

leaps must be equal or larger than certain values, 0

s s . The latter is electrostrictive

in nature being due to the existence of spontaneous structural polarization. There are

three components of such strain leaps in the tetragonal state, and they can be expressed

by [27] 0 0

1 2 12s s sQ P and 0

3 11s sQ P , (1)

where 0

1s , 0

2s and 0

3s are the induced transverse and longitudinal strain

components respectively, Q12 and Q11 are the relevant electrostrictive coefficients and Ps

is the polarization along the <001>-direction. The appropriate phase-mismatch-induced

strains are defined as

1 2 1s s t ca a and 3 1s t cc a , (2)

where ac is the cubic and at and ct are the tetragonal lattice constants. In the case of BT,

the cubic-to-tetragonal (ferroelectric) transformation takes place at a critical polarization

Ps = 0.02 C/m2 and Q12 = -0.033 m

4/C

2 [28], and using the first of eqs. (1) we obtain

0

1s 6.6104

. Data on the BT lattice parameters just above Tc are also available [28]

(ac = 4.00Å, at = 3.99Å), and the first of eqs. (2) yields 1s 2.5∙103

. Therefore, the

criterion 0

s s for 90°-twinning is fulfilled, like in most ferroelectrics.

Twinning during the ferroelectric phase transition is facilitated by dislocation

movement. Therefore, the next question is whether such movement can produce tangible

acoustic emission. Our preliminary results based on the available elastic constants of BT

and sizes of twins observed in BT crystals indicate that the AE intensity is very sensitive

to the dislocation density in the material.

4. CONCLUSIONS

The AE method is shown to be a powerful nondestructive tool for studying

relaxor properties of BaTiO3 crystals widely considered as classical ferroelectrics.

Concurrent dielectric and AE measurements show that the cubic-tetragonal ferroelectric

phase transition at Tc = 403 K is indeed sharp as in normal ferroelectrics. However,

additional AE peaks are observed at higher temperatures, Td = 550 K and T* =506 K. The

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former is the Burns temperature, at which dynamic PNRs are formed, which is a

characteristic feature of relaxors, and the associated AE signal is weak ( N = 17 s−1

) and

shows no thermal hysteresis. The latter peak is also a relaxor feature; it represents the

temperature at which the PNRs turn static, namely begin to freeze in cooperatively by

undergoing a local nanolscale FE phase transition associated with random fields.

Naturally, this intermediate peak appears only on cooling with a higher AE intensity

( N 25 s−1

). In addition, by monitoring the AE count rate under externally applied

electric fields , we have been able to detect the strain relief, and thus the propagation of

elastic waves, due to reorientation of random fields (polarization) associated with local

heterogeneities in the polar order. A threshold field of 0.26 kV/cm known to correspond

to a minimum in the peak value, Tm, of the diffuse dielectric permittivity temperature in

relaxor FE is also found in BT, but in relation to the Tc. A characteristic maximum

intensity of the AE at Tc is observed as well. This relatively low critical field is known to

be responsible for the easy polarization switch during the first order ferroelectric

transitions. Fourier analysis of the single AE pulse during the ferroelectric phase

transition in BT has revealed signals in the MHz range typical for dislocation movement.

The latter may be the cause for the AE activity detected during the martensite-type

ferroelectric transition in BT by 90°-twinning. Our calculation show that the elastic

properties of the BT crystal allow for such mechanism to take place.

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