Coondoo I. (ed.) Ferroelectrics

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Theories and Methods of First Order Ferroelectric Phase Transitions

297

temperature dependence of the induced polarization under different electric field is shown

in Fig.10. The polarization

p and temperature t are in dimensionless unit, the numbers

marked in the Fig.10 are the strength of electric field.

The major difference between the temperature dependence of the polarization in PMN (Ye

& Schmid, 1993; Ye, 1998) and Fig.10 appears at lower temperature range. The polarization

of PMN is still difficult to recover by an external field. This suggests that the zero point

energy at lower temperature could be much larger than that in the higher temperature. In

other words, the zero point energy might be increased with decreasing of temperature.

Another evidence of the existence of larger zero point energy can be found from the diffused

phase transition in PMN. For the quantum temperature scale with constant zero point

energy, the temperature dependence of the dielectric constant has been obtained from

Monte Carlo simulation on a Ising model (Wang et al., 2002). As the zero point increases, the

transition temperature or the peak temperature of the dielectric constant shifts to a lower

temperature. As the zero point energy is larger than the critical value, quantum paraelectric

feature is obtained. When the zero point energy increases further, the dielectric constant

decreases at low temperature. Therefore if the zero point energy changes with temperature

and has a larger value only at lower temperature, the dielectric constant will increase as the

temperature increases at lower temperature side, and decreases as the temperature increases

at higher temperature side. A round dielectric peak will be formed around the temperature

of zero point energy dropping down.

From above analysis, a temperature dependent form of zero point energy is proposed in the

following form (Wang et al., 2009)

()

α−

ω=

(44)

where the zero point energy ω changes around temperature T

from ω

at lower temperature

to a relative lower value with crossover rate α. By using the same technique of effective field

approach as in Refs.(Wang et al., 2008; Yuan et al., 2003 Gonzalo, 2006), with the quantum

temperature scale in Eq.(32) and the zero point energy in Eq.(44), the field induce phase

transition and diffused phase transition are obtained and shown in Fig.18. All parameters

are in dimensionless scale,

e stands for the electric field, and k is for the dielectric constant.

From Fig.18 we can see that the basic features of field induce phase transition and diffused

phase transition observed in relaxor ferroelectric are reproduced.

Fig. 18. (a) Field induced phase transition and (b) diffused phase transition. (Wang et al., 2009)

Ferroelectrics

298

The dimensionless quantum temperature scale used in Fig.18 is shown in Fig.19. The solid

line represents the quantum temperature scale, and the dashed line is for the real

temperature scale. The phase transition temperature is marked by the arrow. That means

that the state evolution of relaxor ferroelectric misses the phase transition temperature as the

temperature decreases, and re-entry of the paraelectric state. The kind of non-ergodic

behavior is schematically shown in Fig.19(b) for better understanding.

Fig. 19. (a) Quantum temperature scale and (b) non-ergodic behavior. (Wang et al., 2009)

Overall, relaxor ferroelectrics like PMN are in a tri-critical state of long range ferroelectric

order, thermal fluctuations and quantum fluctuations. This critical state can be described by

a quantum temperature scale. Following features can be understood from this issue: diffuse

phase transition, field induced phase transition, long relaxation time, as well as super-

paraelectric state, micro-macro-domain crossover and nonergodic behavior. However, a

more adequate expression of temperature dependence of zero point energy is still needed

for better description of the physical behavior in the relaxor ferroelectrics.

6. Final remarks

Theoretical methods and models for studying ferroelectric with first order phase transition

are definitely not limited to the contents in this chapter. Hopefully results from these

theoretical techniques can provide useful information for understanding experimental

observations. Heavy-computer relying methods, such as first-principle calculations and

molecular dynamic simulations etc, have been applied to investigate the physical properties

of ferroelectrics. However, fully understanding of the origin of ferroelectrics might need

more efforts from both theoretical and experimental side.

7. References

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uniaxial ferroelectrics,

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Khalatnikov simulations for the effects of external streese on the ferroelectric

properties of Pb(Zr,Ti)O

thin films. Thin Soild Films, 424, 84 -87

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transverse Ising model with a four-spin interaction.

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asymmetric ferroelectric films.

Physics Letters A, 254, 297-300

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simulation of quantum effects in ferroelectric phase transitions with increasing

zero-point energy,

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Gonzalo, J.A. (2008). Electric field induced phase transition in first order

ferroelectrics with large zero point energy.

Physica A, 387, 115-122

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419-421

Electroacoustic Waves in a Ferroelectric Crystal

with of a Moving System of Domain Walls

Vilkov E.A. and Maryshev S.N.

Kotel'nikov Institute of Radio Engineering and

Electronics of Russian Academy of Sciences, Ul'yanovsk Branch,

Moscow Institute of Physics and Technology

Russia

1. Introduction

In addition to conventional applications of ferroelectric crystals that exhibit piezoactivity,

such crystals find wide application in the production of domain electronic devices based on

the interaction between acoustic waves and ferroelectric domains (acousto-domain

interaction) [Esayan et al, 1974]. In the course of the general development of technology of

phonon crystals the ferroelectric with periodic domain structures (PDSs) have attracted a

considerable interest recently [Golenishchev-Kutuzov et al, 2003]. The interest in such

structures with periodically varying elastic, electric, and other physical parameters dates

back to the 1960s since, by that time, the possibilities offered by single-domain crystals in the

realization of the spectral characteristics of elastic and electromagnetic waves had been

exhausted. First of all a ferroelectrics with PDSs differ from usual phonon crystals by ability

of its reorganization under external action due to rather fast moving domain walls(DW)

[Golenishchev-Kutuzov et al, 2003].

The spectra of elastic oscillations of a ferroelectric with a static PDS may significantly differ

from the spectra of a conventional single-domain sample [Li, et al, 1991; Lyubimov &

Sannikov, 1979]. This difference is primarily manifested in the formation of forbidden and

allowed bands, which are absent in the continuous spectrum of waves in a single-domain

crystal. To the full this conclusion extends to oscillation modes of localized at DWs, in

particular – to electroacoustic interfacial waves (EIWs) [Maerfield & Tournois, 1971], which

were widely studied in ferroelectrics with static PDSs, representing a superlattice [Li, et al,

1991; Nougaoui, 1987]. Considerable modification of spectral parameters of EIWs in

superlattices such as boundary localization of oscillation and phase speed was explained in

[Lyubimov & Sannikov, 1979; Li, 1987] as result of interaction between electrosound

oscillations of neighbo(u)ring DW.

At an early stage of research the possibility of the realization of PDSs with various periods

of the lattices of domain walls and the possibility of vary lattice period [Golenishchev-

Kutuzov et al, 2003] have shown the prospects of application of the domain superlattices. It

was found out, in particular, that crystals with PDSs are effective as broadband

piezoconverters of acoustic waves, and with the greater efficiency of transformation, than

have monodomain crystals [Batanova & Golenishchev-Kutuzov, 1997]. Other fields of

application (the acoustic spectroscopy of polydomain ferroelectrics and processing of the

Ferroelectrics

302

signal information) have demanded of studying of effects of a refraction of acoustic waves

from packages of DWs of ferroelectrics. So, in development of results of work [Esayan et al,

1974] it has been shown, that appropriate variations in the period of the domain lattice,

crystal parameters, and conditions of wave propagation make it possible to control the

height, number, and location of Bragg reflections peaks [Shuvalov & Gorkunova, 1999;

Shenderov,1997], thus providing prospects for various applications.

All the results listed above are related to the static case, i.e. they are obtained for domain

lattices with motionless DW. It means that the manipulation of PDSs expressed by moving

DWs occurs not during the process of acousto-domain interaction. The overlap of the

procedure of adjustment PDSs and the process of acousto-domain interaction is interesting

due to a number of circumstances.

First, this overlap demands modeling of a supernumerary situation of work of the acousto-

domain device at "failure" of device domain structure. Second, it is necessary for research of

additional opportunities, for example, scanning of a crystal by means of EIWs on moving

DW [Gulyaev et al, 2000] or realization of ideas put forward by Auld [Auld,1973] of the

Doppler transformation of a spectrum of frequencies of bulk acoustic waves refracted by

moving DW [Shevyakhov, 1990]. Thus, it is expedient to generalize the results of the

research of acoustic properties of static superlattices of ferroelectric to the case of dynamic

superlattices whose distinctive feature is adjustable moving domain boundaries. Below, on

the basis of our works [Vilkov, 2008; Vilkov, 2009; Vilkov et al, 2009] the propagation of

guided EIWs and bulk electrosound waves in a dynamic superlattice of equidistant

uniformly moving 180-degree DW is considered. Let's notice, that until now the research of

acoustic effects in crystals of ferroelectrics with moving DW was limited to cases of non-

collective acousto-domain interactions:a single DW [Shevyakhov, 1990; Gulyaev et al, 2000],

a strip domain (i.e., a pair of DWs) [El’meshkin & Shevyakhov, 2006] and a structure of a

two adjacent strip domains [Bugaev et al, 2005].

2. Basic equations for a shear wave in ferroelectric in the frame of the DW at

rest

In acoustoelectronics, the elastic deformation and electric-field intensity E

normally serve

as independent quantities that determine the state of a piezoactive ferroelectric medium.

The elastic deformation is determined by the deformation tensor

⎛⎞

∂∂

⎜⎟

∂∂

⎝⎠

, (1)

For the adiabatic deformation, the equations of state are written as follows [Sosnin &

Strukov, 1970]

ik iklm lm j ik j

ppqq p

rm rm

Tcu eE

DEeu

επ

=−

(2)

where T

is the mechanical stress tensor, which is a second- rank tensor (as well as

deformation tensor u

given by expression (1)), and c

iklm

, e

j,ik

and ε

are the tensors of

elastic, piezoelectric, and dielectric moduli of a crystal. Note that summation over the

Electroacoustic Waves in a Ferroelectric Crystal with of a Moving System of Domain Walls

303

repeated subscripts is involved in Eq. (2), where D

is the electric induction. The initial

equations are the equation of motion for a crystal

iik

∂∂

∂

(3)

and the Maxwell equations in the quasi-static representation,

∂

−=

∂∂

. (4)

Here, differential operator

∂/∂x

is considered a vector, so that tensor summation over

repeated subscript k is involved in the second line of expression (4);

ϕ is the electric

potential; and

ρ is the crystal density.

For the consistent analysis of Eqs. (1)–(4), we specify the symmetry of the crystal and the

propagation configuration of acoustic waves. We assume that, 4mm ferroelectric has a

crystallographic putting such that, in a crystallographic system the fourth-order symmetry

axis that is parallel to the z axis of laboratory frame xOyz. For the given crystal symmetry

initial Eqs. (1)–(4) can be presented after some transformations in the form of

22 2

1,5 3,3 1,5 3,1 1 3

22 2 22 2

444()

zz z x

uu u u

eeee

zx y

xy z xy z

ϕϕ

πππ εε

∂

⎛⎞ ⎛⎞

⎛⎞

∂∂ ∂ ∂

∂

∂∂ ∂

++ + + += ++

⎜⎟ ⎜⎟

⎜⎟

⎜⎟ ⎜⎟

∂∂ ∂

∂∂ ∂ ∂∂ ∂

⎝⎠

⎝⎠ ⎝⎠

(5)

2222 2 2

11 66 44 12 66 13 44 1,5 3,1

2222

22 2

66 11 44 12 66 13 44 1,5 3,1

2222

() ()( ),

()()( ),

xxxx z

yyyy

uuuu u

ccc cc cc ee

txyz xy xz xz

uuuu

ccc cc cc ee

txyz xy yz yz

∂

∂∂∂∂ ∂ ∂

=++++ ++ ++

∂ ∂ ∂ ∂ ∂∂ ∂∂ ∂∂

∂∂∂∂

∂∂ ∂

=++++ ++ ++

∂∂∂∂ ∂∂ ∂∂ ∂∂

∂

22 2 2 22 2

44 33 13 44 1,5 3,3

2222 222

() .

zz z x

uu u u

c с cc e e

zxz

ϕϕ

⎛⎞

∂

⎛⎞ ⎛⎞

∂∂ ∂ ∂ ∂∂ ∂

⎜⎟

=++++ ++++

⎜⎟ ⎜⎟

⎜⎟

∂∂ ∂ ∂∂∂∂ ∂∂ ∂

⎝⎠ ⎝⎠

⎝⎠

(6)

The received system of the equations (6) together with the equation (5) describes dynamic

behaviour of a piezoelectric material of a class 4mm(6mm,∞m) and often is called as system

of the equations of piezoacoustics. We assume also that, the propagation of acoustic waves

is in the direction perpendicular to the high-order symmetry axis. In these conditions the

solution is not depend on coordinate z and the system of the equations (6) are split into two

independent systems of equations. First of them

222

11 66 12 66

222

66 11 12 66

222

(),

xxx

yyy

uuu

cc cc

txy xy

uuu

cc cc

txy xy

∂

∂∂∂

=+++

∂∂∂ ∂∂

∂∂∂

∂

=+++

∂∂∂ ∂∂

(7)

and describe horizontally polarized waves:

u⊥z. The second system added by the equation

(5)

Ferroelectrics

304

222 22

44 1,5

222 22

22 22

1,5 1

22 22

zzz

uuu

txy xy

xy xy

ϕϕ

πε

⎛⎞⎛⎞

∂∂∂ ∂∂

=+++

⎜⎟⎜⎟

∂∂∂ ∂∂

⎝⎠⎝⎠

⎛⎞⎛⎞

∂∂ ∂∂

+= +

⎜⎟⎜⎟

∂∂ ∂∂

⎝⎠⎝⎠

(8)

describes horizontally polarized waves:

u||z.

Waves of horizontal and vertical polarization do not interact among themselves. At the same

time, elastic displacements in vertically polarized waves are not accompanied by the

generation of the electric field, so that the crystal behaves toward these waves as a

conventional elastic medium in the absence of the piezoelectric effect. In the further analysis,

we disregard vertically polarized waves owing to the absence of the acousto-domain

interaction. Horizontally polarized waves with the particle displacements along the z axis are

piezoelectrically active and accompanied by the electric field in the propagation plane. (see

Fig. 1).

•

the plane of wave propagation

Fig. 1. The geometry of the propagation of a shift wave of horizontal polarization

For final writing of the equations (8) we consider, that the piezoelectric modulus changes

the sign in the neighboring domains divided by 180-degree DWs because of the antiparallel

orientation of the polar directions, i.e. the plane of wave propagation

() 1

15 15

1,0

eee

−>()

. (9)

where j= 1 and 2 stand for domains of the “+” and “–” types with positive and negative

piezoelectric moduli, respectively. As a result we write down of the equation (8) in the form of

() ()

22 22

44 1

jj jj

cue eu

ϕπ εϕ

∂

=∇ +∇ ∇ =∇

∂

, (10)

Electroacoustic Waves in a Ferroelectric Crystal with of a Moving System of Domain Walls

305

where ∇

=(∂

/∂x

+∂

/∂y

) is the Laplace operator, and shift displacement u

have designated

as u

according number of domain j.

Domain walls can move in ferroelectrics under specific external actions. In this case, to

construct a solution in domains, it is necessary to consider the equation (10) in the frame of

the DW at rest

0xyz



. Because V

<< c (c is the velocity of light, V

is the velocity of DW), we

employ the Galilean transformation for the connection of coordinates:

,,.

Vt t t

=− =





(11)

Using the relationships between the differential operators according to (11):

xx tt

∂

∂∂ ∂ ∂∂ ∂

↔↔↔−

∂

∂∂ ∂ ∂∂ ∂



 

, (12)

and expressions (11) and (12), we can easily represent Eqs. (10) as

0, 0

Djj

∗

⎡⎤

⎛⎞

∂∂

⎢⎥

−

−∇ = ∇ =

⎜⎟

⎢⎥

∂∂

⎝⎠

⎣⎦





(13)

In these equations,

44 44 15 1

4/cc e

=+ , and Φj is the component of total potential

in the

jth domain,

()

jjj

=− + , (14)

which corresponds to the electric field induced from the boundaries

nnD

yVt=−



piezopolarization charges [Lyubimov & Sannikov, 1979].

3. The electroacoustic bulk propagating waves in a ferroelectric with a

system of moving periodic domain structure

3.1 The statement of the problem

In this section we consider the influence of a uniform motion of domain boundaries forming

a dynamic superlattice of the tetragonal ferroelectric, on spectral properties of bulk

electrosound waves (EW). The comparison of the phonon spectra of static and dynamic

superlattices allows a more detailed description of specific wave processes realized in

periodic structures. The results obtained indicate that even slow DW motions cause

significant (detectable) variations in the wave spectra.

The schematic of the problem is presented in Fig. 2. We assume that, in a crystallographic

system with the fourth-order symmetry axis that is parallel to the z axis of laboratory frame

xOyz , a 4mm ferroelectric represents a superlattice of 180-degree DWs with period 2d (d>>Δ

is the distance between neighboring DWs and Δ is the DW thickness). The superlattice

uniformly moves at the velocity V

⎜⎜ y [010]⎜⎜. Positive velocity V

corresponds to the

superlattice motion codirectional with the y axis. To eliminate the significant structural

sensitivity of DWs to the regime of motion related to the generation of both spontaneous

polarization and spontaneous deformation in domains [Sosnin. & Strukov, 1970; Vainshtein,

Ferroelectrics

306

1988], we restrict the consideration to subsonic DW velocities. We also assume that the

ferroelectric is far from the phase transition. Then, the DW motion is realized at V

= const

and the current coordinates are given by y

t+nd, where t is time; n=0, ±1, ±2,… and

DWs are geometrically thin and structureless objects (kΔ << 1, where k is the EW wave

number).

Fig. 2. Schematic representation of a domain superlattice with an electroacoustic eigenwave.

The dashed lines indicate that two elementary cells are reproduced in the positive and

negative directions of the Y axis and form an infinite superlattice. Vectors k’ and k’’show

the direction of the EW propagation. The arrow shows the direction of the lattice motion.

Here, Е

is the electric-field intensity in domains

It is known [Vainshtein, 1988] that the above conditions are valid for DWs in BaTiO

-type

ferroelectrics at ultrasonic frequencies. It is commonly accepted [Maerfield & Tournois, 1971;

Auld,1973] that, in this case, the ferroelectric can be considered as a piezoelectric crystal that

is periodically twinned with respect to the planes y = y

. With regard to the analysis of

propagation of only horizontally polarized shear waves in the (001) crystal plane, we

assume that antiparallel polarizations in the neighboring domains are related to the sign

alternation of the piezoelectric modulus according equation (9). At that, there are in eq.(9)

j = 1 for (n-1)d + V

t < y< nd+ V

t and j = 2 for nd + V

t< y <(n+1)d+ V

t (n = 0, ±2, ±4,…).

The remaining characteristics of domains are identical.

Then, we assume that the electroacoustic eigenwaves with the wave vector = k’=(k’sinθ’,

k’cosθ’, 0) propagate in the XOY plane at angle θ’ relative to the normal to the plane of 180-

degree DWs in the positive direction of the y axis (Fig. 2). For the electroacoustic eigenwaves

propagating in the XOY plane at angle θ" to the normal to the DW plane in the negative

direction of the y axis, we employ another wave vector, = k’’=(k’’sinθ’’, k’’cosθ’’, 0) in

accordance with another expected Doppler shift [Shevyakhov, 1990].