Science - USA (2022-02-11)

PIEZOELECTRICS

Breaking symmetry for piezoelectricity

ByFei Li

P

iezoelectricity, the ability of a ma-

terial to generate an electric field

in response to applied mechani-

cal stress, has been widely used for

state-of-the-art electronics, such as

medical ultrasound machines, un-

derwater microphones, and vibration and

pressure sensors ( 1 , 2 ). As a basic prerequi-

site, only materials with a noncentrosym-

metric microstructure can potentially be

piezoelectric. Thus, the search for piezo-

electric materials has been mainly limited

to noncentrosymmetric materials. On page

653 of this issue, Park et al. ( 3 ) report a

record-breaking piezoelectric performance

in a centrosymmetric oxide. Instead of

starting with a noncentrosymmetric mate-

rial, the symmetry of the oxide was broken

by inserting oxygen vacancies—a type of

point defect—and then manipulating these

vacancies with an electric field.

Most high-performance piezoelectrics

are ferroelectrics ( 4 – 10 )—materials that

have a spontaneous electric polarization.

The piezoelectric coefficient (d 33 ) is per-

haps the most important measure for a

piezoelectric material. It quantifies the

change of mechanical strain when a piezo-

electric material is subject to an electric

field and is often expressed in picometers

per volt. The piezoelectric coefficient of

ferroelectrics is determined by several pa-

rameters. Two of these parameters—dielec-

tric permittivity and spontaneous polariza-

tion—are what materials scientists focus

on. Dielectric permittivity is the measure

of how much electric polarization is in-

duced in a material by an external electric

field, and spontaneous electric polariza-

tion is the measure of the material’s natu-

rally existing polarization in the absence of

an external field.

The general approach to enhance the

piezoelectric coefficient of ferroelectrics is

to enlarge the dielectric permittivity by in-

ducing structural instability. For example,

through the design of ferroelectric phase

transition and chemical disorder to en-

hance structural instability, Pb(Mg1/3Nb2/3)

O 3 -PbTiO 3 (PMN-PT) and rare earth ele-

ment–doped PMN-PT crystals can exhibit

ultrahigh piezoelectric coefficients ranging

from 1000 to 4000 pm/V ( 9 , 10 ). However,

there is a long-standing issue presented

in these ferroelectrics—their operational

temperature range is greatly limited by

Curie temperature. Above this tempera-

ture, these materials would go from being

ferroelectric to paraelectric and, with this

transition, would lose their spontaneous

polarization and piezoelectricity. To gener-

ate piezoelectricity in ferroelectric materi-

als above the Curie temperature, one of the

most promising approaches is to induce

polarization for the materials by applying

a direct current (DC) electric field.

Previous efforts that looked for piezo-

electric materials using this DC electric

field approach focused on centrosymmet-

ric perovskite oxides. However, because

the dielectric permittivity generally de-

creases as the DC electric field increases

for perovskite oxides, there is a limitation

to the piezoelectricity induced by a DC

electric field, with the current bottleneck

around 1500 pm/V ( 11 ). Looking beyond

perovskites, Park et al. used a DC electric

field on a thin film of Gd-doped CeO2–x

(CGO) to induce even higher piezoelec-

tricity. By applying a DC electric field of 1

MV/cm, the authors obtained an ultrahigh

piezoelectric coefficient of 200,000 pm/V

at a frequency of 10 mHz—about 50 times

as high as that of the current best piezo-

electric oxides ( 10 ).

Park et al. discovered that the oxygen

vacancies in the CGO thin film can be re-

arranged by the DC electric field, which

leads to symmetry breaking and thus in-

corporating piezoelectricity in the film (see

the figure). They measured the piezoelec-

tric coefficient by applying an alternating

current (AC) electric field on top of the DC

field. The AC field pushes oxygen vacan-

cies toward the top electrode when it is in

the same direction as the DC field, which

expands the material, and it does the op-

posite when it is in the opposite direction

of the DC field.

The authors attributed the ultrahigh

piezoelectricity of the thin film to the

large dielectric permittivity caused by the

greatly enhanced defect migration under

the DC electric field. Additionally, the au-

thors observed that the redistribution of

oxygen vacancies could induce a cubic-to-

tetragonal structural transition, which is

Record-breaking piezoelectricity is achieved in oxides with symmetry-breaking defects

618 11 FEBRUARY 2022 • VOL 375 ISSUE 6581

GRAPHIC: C. BICKEL/

SCIENCE

science.org SCIENCE

INSIGHTS | PERSPECTIVES

Electric field–

induced expansion

DC electric field o DC electric field on DC + AC electric field on

EDC

Electrode

Oxygen

vacancy

CGO thin film

EDC

–EAC

+EAC

Oxygen

vacancies

vibrate

Thickness

change

Electronic Materials Research Lab, Key Lab of Education

Ministry and State Key Laboratory for Mechanical Behavior

of Materials, School of Electronic Science and Engineering,

Xi’an Jiaotong University, Xi’an 710049, China.

Email: [email protected]

Breaking symmetry aids piezoelectric materials

A direct current (DC) electric field (EDC) induces the redistribution of oxygen vacancies within a Gd-doped

CeO 2 – x(CGO) thin film, turning the material piezoelectric. The piezoelectricity is then measured using

an additional alternating current (AC) electric field (EAC), which further drives the oxygen vacancies up and

down in synchronization with the physical expansion and contraction of the thin film.