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Materials Chemistry and Physics 132 (2012) 87– 90

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

he influence of defects on ferroelectric and pyroelectric properties ofb(Mg1/3Nb2/3)O3–0.28PbTiO3 single crystals

iao Wua,b,∗, Linhua Liua,c, Xiaobing Lia,c, Xiangyong Zhaoa, Di Lina, Haosu Luoa, Yanlin Huangb

Key Laboratory of Inorganic Functional Material and Device, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, ChinaCollege of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, ChinaGraduate School of Chinese Academy of Science, Beijing 100049, China

r t i c l e i n f o

rticle history:eceived 11 February 2011eceived in revised form 25 October 2011ccepted 31 October 2011

a b s t r a c t

Relaxor-based ferroelectric single crystals 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 (PMN–0.28PT) were grownby a modified Bridgman technique. The direct current (dc) conductivity was investigated and corre-sponding conduction mechanisms were discussed. V ′′

Pb − VO•• defects are dominant from 245 ◦C to 650 ◦C.

The ferroelectric properties of [1 1 1]-oriented PMN–0.28PT were systematically investigated, with the−1 −2

eywords:MN–0.28PT single crystalsonductivityxygen vacancyyroelectric properties

coercive field (Ec) of 5.2 kV cm and remnant polarization (Pr) of 37.8 �C cm at room temperature.Moreover, the dielectric and pyroelectric performances of PMN–0.28PT were measured and the inte-grated pyroelectric performances greatly enhanced after annealing in oxygen at 500 ◦C for 20 h. Thisis due to the decrease of oxygen vacancies in the single crystals when being annealed in the oxygen-rich atmosphere. These make [1 1 1]-oriented PMN–0.28PT crystals a promising candidate for infrareddetectors and thermal imagers used at room temperature.

. Introduction

Nowadays pyroelectric detectors are playing an important rolen many civilian and military applications due to their useful combi-ation of sufficiently high sensitivity, room-temperature operation,road-spectral response, and low cost [1,2]. Pyroelectric materialss the core of pyroelectric detectors, have attracted more and morettention. Compared with conventional bulk ceramics or thin films,uch as lithium tantalate (LiTaO3) and barium strontium titanateBST), pyroelectric single crystals offer many advantages, such asomogeneity as well as compositional and microstructural con-rollability.

Relaxor-based ferroelectric (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3PMN–xPT) single crystals are complex perovskite solid solution

aterials [3]. It is well known that in perovskite ferroelectrics,xygen vacancies could be formed during the sintering processf the samples due to the volatilization and/or charge compen-ation processes [4]. The electro-migration of oxygen vacancies

s usually suggested as the main cause of conductivity [5]. Fur-hermore, A-site vacancies (such as lead vacancies), space chargelectrons, or other impurities are also main defects in the crystals

∗ Corresponding author at: Key Laboratory of Inorganic Functional Material andevice, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai01800, China. Tel.: +86 21 69987759; fax: +86 21 59927184.

E-mail address: sunquene@163.com (X. Wu).

254-0584/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.matchemphys.2011.10.055

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

[6–8]. These defects including oxygen vacancies are responsiblefor the dielectric relaxation at the high-temperature region [4].However, many studies have been carried out on the techniqueof crystal growth, domain structure, ferroelectric and piezoelec-tric properties of PMN–xPT single crystals [9], and the natureand distribution of crystal defect is mainly focused on themorphology and color of crystal so that the conduction mecha-nisms of the single crystals have not been systematically studied[10].

PMN–xPT single crystals show excellent pyroelectric per-formances [11,12]. The most suitable composition lies in0.26 ≤ x ≤ 0.29 for PMN–xPT with 〈1 1 1〉 orientation and rhombohe-dral phase. For the Mn-doped PMN–0.26PT crystals, their dielectricloss could reduce to 0.0005 and the detectivity figures of merit (Fd)reach up to 40.2 × 10−5 Pa−1/2, which is the highest value so farreported among all intrinsic pyroelectric materials with ferroelec-tric rhombohedral to tetragonal phase transition temperature (TRT)greater than or equal to 90 ◦C [13].

What will be present in this paper are defects, conduction mech-anisms and chemical equilibrium of PMN–0.28PT single crystals.The ferroelectric, dielectric and pyroelectric properties of [1 1 1]-oriented PMN–0.28PT are investigated to look for the potentialapplication in pyroelectric detectors. The effects of oxygen anneal-

ing on the integrated pyroelectric performances are also mainlydiscussed. These investigations will provide a better understandingof the microstructure–property relations in this complex per-ovskite material.

ghts reserved.

88 X. Wu et al. / Materials Chemistry and Physics 132 (2012) 87– 90

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Fig. 1. PMN–0.28PT single crystals grown by a modified Bridgman technique.

. Experimental

0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 (PMN–0.28PT) relaxor-ased ferroelectric single crystals were grown by a modifiedridgman technique [14]. Fig. 1 shows the PMN–0.28PT singlerystals with the size of Ø 50 mm × 70 mm. The initial startingaterials included Pb3O4, MgO, Nb2O5 and TiO2, with high purity

etter than 99.99%. X-ray diffraction analysis reveals that theingle crystals are of a pure perovskite structure, which can beeen in Fig. 2. The observed peaks could be indexed to R3m spaceroup with trigonal symmetry (Rhombohedral). The as-grownrystal was cut as [1 1 1] wafers because high values of pyro-lectric coefficients appear along the spontaneous polarizationirection 〈1 1 1〉 [15]. Then the samples were cut into dimensionsf 4 mm × 4 mm × 0.5 mm and the surfaces were polished withl2O3 and fine diamond polishing powders to achieve flat andmooth. The samples were electroded with gold film by JS-1600ons Sputtering Apparatus, and then poled along 〈1 1 1〉 directionn silicon oil at 120 ◦C. For the annealing experiment, the samples

ere annealed in oxygen atmosphere at 500 ◦C for 20 h.The polarization hysteresis loops were measured by an aix-

CCT TF1000 ferroelectric measurement system. The resistance

as measured by the Keithley 6517A high resistance meter

nd tube furnace, and dc conductivity was calculated accord-ng to resistance. The dielectric measurements were carriedut using the Agilent 4294A precision impedance analyzer. The

Fig. 2. XRD pattern of PMN–0.28PT single crystals.

Fig. 3. The dc conductivity of PMN–0.28PT sample versus the inversed temperature(1/T).

pyroelectric coefficients were measured by a dynamic technique[16] using sinusoidal temperature change with amplitude of 1 ◦Cat a frequency of 45 mHz.

3. Results and discussion

The conductivity in mixed B-cation perovskites can beattributed to the following types of defects: VO

••, V ′′Pb, V ′′

Pb − VO••

and space charges. The dominant defect structure is the one whoseactivation energy is the lowest, which is also the controlling factorfor conductivity [17]. Computational simulations of the ionic trans-port in perovskite oxides [18] indicate that the activation energy foroxygen migration is around 1 eV. On the other hand, the activationenergies for the A- and B-sites cation transport are around 4 and12 eV, respectively. Thus, oxygen ions will show a certain motionin perovskite materials [19].

The dc conductivity of PMN–0.28PT samples versus the inversedtemperature (1/T) is plotted in Fig. 3. The activation energy Ea iscalculated according to the Arrhenius relation:

�dc = �0 exp(−Ea

kT

)(1)

where �0 is the constant and k is the Boltzmann constant.At lower temperature (T < 245 ◦C), Ea of PMN–0.28PT is about

0.17 eV. It could be reasonably suggested that electrons or holescontrol the conduction process. At the higher temperature region(245–650 ◦C), a larger activation energy value (Ea = 1.72 eV) canbe shown. This value is assigned to a mixed conduction mecha-nism by doubly ionized oxygen vacancies (VO

••) and lead vacancies(V ′′

Pb) diffusion. At room temperature, lead vacancies are quencheddefects and difficult to activate; while above 300 ◦C, lead vacan-cies become mobile and will play an important role in conductionmechanism. We consider V ′′

Pb − VO•• as the dominant conduc-

tion mechanism in PMN–0.28PT between 245 ◦C and 650 ◦C. Thesimilar conclusion can be found in (Mn, Nb) co-doped PZT ceram-ics reported by B. Guiffard [20]. At much higher temperature(T > 650 ◦C), the gold electrode will be ruined and the conductivitycannot be obtained.

The formation of the dominant point defects proposed abovecan be described by the equations:

Oo ⇔ V ′′O + 2e′ + 1

2 O2 (gas) (2)

PbPb(sd) + Oo ⇔ V ′′Pb + VO

•• + PbO (3)

X. Wu et al. / Materials Chemistry and Physics 132 (2012) 87– 90 89

Table 1Summary of pyroelectric parameters comparing with the unannealed and oxygen annealed samples of PMN–0.28PT, and Mn doped PMN–0.26PT [13] at room temperature.

Samples p (10−4 C m−2 K−1) εr @1 kHz tan ı @1 kHz Fi (10−10 m V−1) Fv (m2 C−1) Fd (10−5 Pa−1/2)

0.000.000.00

W

K

K

TPTAd

K

K

wsSEc

[

[

PVctpi

ttc1d

Ff

Unannealed PMN–0.28PT 9.79 625

Annealed PMN–0.28PT 10.09 604

Mn doped PMN–0.26PT 17.2 660

ith the equilibrium constants at constant temperature:

1 = [VO••]n2PO2

1/2 (4)

2 = [V ′′Pb] · [VO

••]. (5)

he electroneutrality condition is given by n = 2[VO••], where n and

O2 represent an electron and oxygen partial pressure, respectively.he formation of VO

•• in PMN–PT is the same as that in TiO2 [21].ctually, Eq. (2) is an intrinsic reaction and Eq. (3) is a Schottkyefect reaction, which are both influenced by temperature:

1 = Krea(T) exp(

−�Hrea

kT

)(6)

2 = Ks(T) exp(

− Es

kT

)(7)

here Krea(T), �Hrea, Ks(T) and Es represent the equilibrium con-tant forming VO

••, reductive enthalpy, Schottky constant andchottky energy, respectively [22]. Combining with Eqs. (4) and (6),qs. (5) and (7), the concentration of VO

•• and V ′′Pb in PMN–0.28PT

an be deduced:

VO••] = 4−1/3K1

1/3PO2−1/6 (8)

V ′′Pb] = 41/3K1

−1/3K2PO21/6 (9)

Eqs. (8) and (9) can also be used in PMN–xPT with differentbTiO3 contents, especially at a temperature range where V ′′

Pb −O

•• conduction mechanism is dominated. The equations of con-entration of VO

•• and V ′′Pb will provide theoretical basis to discuss

he defects in other lead-containing perovskite materials. And theroperties of materials can be optimized through tuning the defects

n them.In order to reveal the ferroelectric properties, the (P–E) hys-

eresis loops of [1 1 1]-oriented PMN–0.28PT as a function of

emperature were illustrated in Fig. 4. Three temperatures werehosen to compare: 30 ◦C as room temperature, 100 ◦C as TRT and30 ◦C as Curie temperature (TC). Fig. 5 shows the temperatureependence of coercive field (Ec) and remnant polarization (Pr). At

ig. 4. (P–E) hysteresis loops of [1 1 1]-oriented PMN–0.28PT single crystals at dif-erent temperatures.

371 3.78 0.068 8.34222 3.9 0.073 11.3105 6.88 0.12 40.2

room temperature, PMN–0.28PT shows “hard” ferroelectric prop-erties with Ec = 5.2 kV cm−1 and Pr = 37.8 �C cm−2 and the excursionof Ec(+) and Ec(−) is about 0.2 kV cm−1 because of formation ofinternal electric field caused by VO

•• and V ′′Pb. With the temperature

increase, Ec and Pr both decrease but there is an abrupt rise for Ec ataround 100 ◦C, which is due to the phase transition from rhom-bohedral to tetragonal phase. The domain configuration changefrom macro-domain to micro-domain and the effect of phase tran-sition is greater than that of temperature. When the temperatureincreases to TC, PMN–0.28PT shows weak ferroelectricity becauseat this temperature phase transition induced by electric field maytake place.

The dielectric and pyroelectric properties of [1 1 1]-orientedPMN–0.28PT are also investigated. To evaluate the potential ofpyroelectric materials, figures of merits (FOMs) for current respon-sivity Fi = p/Cv, for voltage responsivity Fv = p/(Cvε0εr), and fordetectivity Fd = p/[Cv(ε0εr tan ı)1/2] are required, where p, Cv, ε0,εr, and tan ı are pyroelectric coefficient, volume specific heat, per-mittivity of free space, relative dielectric constant and dielectricloss, respectively [1]. The common wisdom concerning the selec-tion of materials for pyroelectric devices is to maximize the FOMs.The samples were annealed in oxygen at 500 ◦C for 20 h, and thenelectroded with gold and poled along 〈1 1 1〉 direction to comparethe properties with the unannealed samples. Table 1 summarizesthe pyroelectric parameters of unannealed and oxygen annealedsamples of PMN–0.28PT, and Mn doped PMN–0.26PT [13] at roomtemperature.

After being annealed in oxygen, the pyroelectric coefficient ofPMN–0.28PT increases slightly while dielectric constant decreasesa little. However, the dielectric loss decreases to 0.00222, by 40%compared with the unannealed one (0.00371) so that the FOMswere greatly enhanced (reach up to 11.31 × 10−5 Pa−1/2). We con-sider the annealing process in oxygen can improve the pyroelectricperformances of [1 1 1]-oriented PMN–0.28PT. In the oxygen-richatmosphere, less oxygen vacancies appear in the crystals and the

integrality of crystals will be improved, so that the dielectric losswill decrease intensively. The comparison of electrical properties ofPMN–0.28PT and Mn doped PMN–0.26PT are also shown in Table 1.

Fig. 5. The temperature dependence of coercive field and remnant polarization of[1 1 1]-oriented PMN–0.28PT single crystals.

9 stry a

Woss

4

btprcibwaTtldpsi

A

S2(

[[[[

[[[[

[[[20] B. Guiffard, E. Boucher, L. Eyraud, L. Lebrun, D. Guyomar, J. Eur. Ceram. Soc. 25

0 X. Wu et al. / Materials Chemi

ith larger pyroelectric coefficient and much lower dielectric lossf Mn doped PMN–0.26PT, the highest Fd can be obtained. In conclu-ion, the optimizing of FOMs will push the application of PMN–xPTingle crystals in pyroelectric detectors.

. Conclusions

Relaxor-based ferroelectric PMN–0.28PT single crystals haveeen obtained by the modified Bridgman technique. The defects inhe crystals have been studied by dc conductivity. V ′′

Pb − VO•• hop-

ing is the dominating conduction mechanism at the temperatureegion of 245–650 ◦C. Through the equations of defect reactions, theoncentration of VO

•• and V ′′Pb in PMN–0.28PT can be deduced. The

nfluence of defects on ferroelectric and pyroelectric properties haseen discussed. PMN–0.28PT shows “hard” ferroelectric propertiesith Ec = 5.2 kV cm−1 and Pr = 37.8 �C cm−2. The excursion of Ec(+)

nd Ec(−) is due to the internal electric field caused by VO•• and V ′′

Pb.he dielectric and pyroelectric performances have also been inves-igated. The results indicate that after being annealed in the oxygen,ess oxygen vacancies appear in the crystals and the dielectric lossecrease intensively. So that the FOMs related to the pyroelectricerformances were enhanced greatly, which will provide more pos-ibility for [1 1 1]-oriented PMN–0.28PT single crystals to be appliedn pyroelectric detectors.

cknowledgements

This work was financially supported by the Ministry ofcience and Technology of China through 973 Program (No.009CB623305) and National Key Technology R&D ProgramNo. 2010BAK69B26), the Natural Science Foundation of China

[[

nd Physics 132 (2012) 87– 90

(Nos. 60837003 and 50777065), Science and Technology Commis-sion of Shanghai Municipality (Nos. 10520712700, 10JC1415900and 10dz0583400), and the Innovation Fund of Shanghai Instituteof Ceramics (Nos. O99ZC4140G and O99ZC1110G).

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