Chemical nature of giant strain in Mn-doped 0.94(Na0.5Bi0.5)TiO3–0.06BaTiO3

Chemical nature of giant strain in Mn-doped 0.94(Na 0.5Bi 0.5)TiO 3–0.06BaTiO 3lead-free ferroelectric single crystals

Haiwu Zhang,a ,b ,?Hao Deng,a ,b Chao Chen,a ,b Long Li,a ,b Di Lin,a Xiaobing Li,a

Xiangyong Zhao,a Haosu Luo a ,?and Jun Yan c

a

Key Laboratory of Inorganic Functional Materials and Devices,

Shanghai Institute of Ceramics,Chinese Academy of Sciences,215Chengbei Road,Jiading,Shanghai 201800,China

b

Graduate University of Chinese Academy of Sciences,Beijing 100049,China

c

Shibei Power Supply Branch,Shanghai Municipal Electric Power Company,Shanghai 200072,China

Received 25October 2013;accepted 15November 2013

Available online 22November 2013

Giant bipolar and unipolar strains i.e.S max >0.5%,e max /E max >1000pm V –1have been observed in Mn-doped Na 0.5Bi 0.5TiO 3–6BaTiO 3single crystals after being annealed.Temperature-dependent impedance spectra were studied and activation energies of oxygen vacancies were calculated accordingly.The two di?erent binding energies present in X-ray photoelectron spectra for Na and Bi were assigned to di?erent coordinate environments.However,titanium exhibits only one oxidation state (e.g.Ti 4+).The site occupation and valence ?uctuation of Mn were characterized by electric paramagnetic resonance spectra.Ó2013Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.

Keywords:Ferroelectric;Single crystals;Annealing;Electrical properties

Colossal e?ects,such as giant dielectric and elec-tromechanical responses,make relaxor ferroelectrics suitable for use in advanced devices such as high-perfor-mance super capacitors and high-power transducers.Recently,large electric-?eld-induced strains have been found in Na 1/2Bi 1/2TiO 3-based materials,which were considered promising environmental friendly alterna-tives to the toxic Pb-based counterparts [1–4].Around the morphotropic phase boundary (MPB),the average structure of Na 1/2Bi 1/2TiO 3–BaTiO 3(NBBT)as well as

Na 1/2Bi 1/2TiO 3–BaTiO 3–K 1/2Na 1/2NbO 3

(NBT–BT–KNN)is pesudocubic [5–7].However,short-range anti-phase (a Àa Àa À)and in-phase (a 0a 0c +)oxygen octa-hedral tilted polar nanoregions (PNRs)coexist in the point view of local structure [8].These PNRs will pro-mote the development of long-range ferroelectric order under the application of an electric ?eld (E):when E is applied along the <001>crystallographic direction,the coherent length of the anti-phase (a Àa Àa À)tilted re-

gions increases and phase transition from pseudocubic to tetragonal structures occurs;along the <111>direc-tion,the in-phase (a 0a 0c +)oxygen octahedral tilted re-gions expand,resulting in an induced transition from pseudocubic to rhombodedral structures [9].Therefore,polarization extension was proposed to explain the large strain in NBT-based ferroelectrics [4,9,10].This is quite di?erent from the Pb-based ferroelectrics,where polari-zation rotation via the bridging phases determines the electromechanical properties [11–13].

The electrical properties of perovskite ferroelectrics are typically tailored by doping with di?erent elements,especially in the case of acceptor doping [14–16].Models such as “volume e?ect ”,“domain-wall e?ect ”and “grain boundary e?ect ”have been proposed to explain the con-?guration of defect dipoles and their interaction with domain walls [17–19].In particular,the defect dipoles align preferentially and generate a restoring force for domain switching during aging [20–22].However,the ef-fects of chemical defects on electrical properties in NBBT have not been systematically studied.Besides,the defect structures in NBBT are sensitive to processing parameters.Noguchi et al.[23]found that the concen-tration of Bi and oxygen vacancies depends on the O 2?ow rate during crystal growth processing.A slight

1359-6462/$-see front matter Ó2013Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved./10.1016/j.scriptamat.2013.11.017

?Corresponding author at:Key Laboratory of Inorganic Functional

Materials and Devices,Shanghai Institute of Ceramics,Chinese Academy of Sciences,215Chengbei Road,Jiading,Shanghai 201800,China.Tel.:+862169987759;fax:+862159927184;e-mail addresses:zhw3789@ ;hsluo@

Available online at

ScienceDirect

Scripta Materialia 75(2014)

50–53

/locate/scriptamat

deviation in the Ba/Ti ratio will result in a valence?uc-tuation between Ti3+and Ti4+in BaTiO3,the end mem-ber of NBBT[24].It has been found that Mn doping is an e?ective way to reduce leakage current and enhance the piezoelectric constant[14,25].However,the underly-ing mechanism is still not clear,including its site occupa-tion and valence state.Thus,clarifying the crystal’s chemical nature,i.e.the elements’oxidation states, chemical defects and ion binding energies,is crucial for revealing the microscopic origin of NBBT’s excellent properties.

In present work,the electrical properties and defect chemistry of Mn-doped0.94Na0.5Bi0.5TiO3–0.06BaTiO3 (Mn:NBBT6)single crystal have been studied in detail. Field-induced bipolar and unipolar strains have been measured.The temperature-dependent conductivity was studied by impedance spectra.The oxidation states and binding energy of Na,Bi,and Ti were studied by X-ray photoelectron spectra(XPS).The site occupation and valence?uctuation Mn ions were investigated by paramagnetic resonance spectra(EPR)spectra.The de-fect structures and their relation to the improved electri-cal properties have also been discussed.

Mn:NBBT6single crystals were grown by a carefully controlled top-seeded solution growth method.Induc-tive coupled plasma atomic emission spectrometry mea-surements show that the concentrations of Ba2+and Mn2+ions are6and0.1at.%,respectively.The as-grown single crystals were cut along the pseudocubic (001)/(100)/(010)planes with dimensions of 5Â5Â0.5mm3.The samples were then annealed under di?erent conditions:(i)as-grown,labeled as“AS”;(ii) 20h at600°C in O2atmosphere,labeled as“OS”;and (c)20h at600°C in a vacuum,labeled as“VS”.To characterize the electrical properties,gold electrodes were sputtered on both of the main faces.The bipolar strain curves(S–E)and unipolar strain curves(e–E) were measured using a ferroelectric test system (aixACCT TFanalyzer1000).Temperature dependence impedance spectra were measured using an Agilent 4294impedance analyzer.The X-band(9.8GHz) EPR measurements were performed on a Bruker EMX spectrometer(Karlsruhe,Germany)at ambient temperature.

Figure1a shows bipolar strain curves(S–E)for Mn:NBBT6single crystals after di?erent annealing pro-respectively.The obvious increases in the coercive?eld for OS and VS reveal that the potential energy of ?eld-induced phase transformation from pesudocubic to tetragonal increases after annealing[9,26].The increase in the remanent strain(S rem)and the develop-ment of negative strain(S neg)indicate that the stability of the?eld-induced ferroelectric order is also enhanced [27].Similar behavior was also observed in unipolar strain curves(e–E),as shown in Figure1b.The values of normalized strain e max for OS and VS samples were 1086and1361pm V–1,respectively,much larger than that of the AS samples.The electrical properties of Mn:NBBT6single crystals were compared with previ-ously reported data and are summarized in Table1.It is clear that controlling chemical defects is an e?ective way to tailor the electromechanical properties.

Figure2a–c shows complex impedance spectra of Mn:NBBT6single crystals from40to1MHz at di?erent temperatures.A single semicircle was observed for all samples,indicating that a single localized relaxation mechanism dominates the impedance in the measured temperature range.Upon heating,the semicircles gradu-ally contract and shift to the real axis(Z0).This behavior can be assigned to the activation of the weakly trapped charge carriers,which results in the increase in conduc-tivity(r).The impedance was simulated as an equiva-lent circuit consisting of parallel connection of resistance(R)and capacitance(C).By extrapolating the low-frequency intercept of the simulated semicircles with real axis,the values of R were obtained.The relation between the activation energy(E a)and r can be denoted by the Arrhenius equation:

r¼r0expðÀE a=kTÞð1Þwhere r0is a constant,k is the Boltzmann constant and T is the absolute temperature.The logarithms of r vs. 1000/T are plotted in Figure2d.The solid lines are the best least-squares?tting of Eq.(1).The values of activa-tion energy for AS,OS and VS are evaluated to be1.3, 1.38and1.2eV,respectively.In ABO3perovskite mate-rials,the values of E a for A-and B-site cations are around4and12eV,respectively[29].For oxygen vacancies,it varies from0.5to2eV,depending on their concentration[30].Therefore,it is reasonable to suggest that oxygen vacancies dominant the conductivity in this temperature range.The enhanced E a reveals the reduc-tion in oxygen vacancies in OS,while the suppressed E a in VS indicates it has experienced an increase in the concentration of oxygen vacancies.

Table1.Electrical properties of Mn:NBBT6single crystals as com-pared to NBT–BT–KNN and PZT ceramics.

Material P r

(l C cm–2)

E c

(V mm–1)

S max

(%)

e max/

E max

(pm V–1)

Reference

AS30.7512500.23372This work OS11.8520000.721086This work VS25.8018320.521361This work NBT–BT-5KNN33.00–0.30–[1]

NBT–BT-2KNN––0.41560[2]

Hard PZT,PZT8–––150[28]

Soft PZT,PZT5H––400–590[28]

Figure1.(a)Bipolar strain curves and(b)unipolar strain curves of

[001]-oriented Mn:NBBT6single crystals measured under5kV mm–1

H.Zhang et al./Scripta Materialia75(2014)50–5351

Figure3shows the X-ray photoelectron spectra for Na1s,Bi4f and Ti2p of Mn:NBBT6single crystals. The shifts of core-level spectra due to the charging e?ect were corrected by calibration with the C1s peak set at 285eV.The spectra were deconvoluted using Lorentz-ian–Gaussian functions.The Na1s XPS spectra were?t-ted by two components at binding energies(BEs)of 1072and1073.5eV,respectively.The intensity of the peak located at the lower BE decreases for OS.How-ever,it increases for VS.For Bi4f7/2,two components were?tted at BEs of164and164.8eV.For Bi4f5/2, the BEs were?tted to be158.8and159.3eV,respec-tively.No obvious change was observed in the Bi4f spec-tra after annealing.The two peaks in the Ti2p spectra correspond to the angular momentum of the titanium

electrons.With the shape of the Ti2p3/2and Ti2p1/2 peaks and the?tting results,the conclusion can be drawn that only Ti4+exists in the crystal lattice.There-

fore,the positive charge of V

O is not compensated by

the reduction of Ti4+to Ti3+.

According to the charge potential model suggested by Siegbahn et al.[31],the binding energy mainly depends on the average electron density of the element and the neighboring environment.The di?erence in the BEs of Na+and Bi3+is the consequence of the chemical disor-der,which results in di?erent e?ective electron densities of elements.Similar behavior has also been observed in PMN,where the presence of anomalous Pb4+was as-signed to the the coexistence of two unequivalent Pb–O bond lengths[32,33].The di?erent changes in the Na1s and Bi7f line intensities upon annealing reveal the di?erence in the chemical binding natures of Bi and Na.According to Jones and Thomas[34],Bi3+is underbonded in NBT,with a32%de?ciency from its ideal values–much higher than the de?ciency(8%)ob-served for Na+.The Bi3+ions displace and interact with oxygen,resulting in a much“harder”Bi–O bond than the Na–O bond.Therefore,the binding energy of Na+ ions is more sensitive to the change in the coordinate environments.

Figure4shows the X-band EPR spectra for Mn:NBBT6single crystals measured at ambient temper-ature.The AS samples exhibit a hyper?ne structure, with six prominent lines of typically isolated Mn2+, where Mn2+substitutes Ti4+ions at the B-site and acts as an acceptor[35,36].Oxygen vacancies(V

O

)will be created simultaneously for charge compensation:

Mn MnþTiÂ

Ti

!Mn00

Ti

þTiþV

O

A superimposed signal originating from Mn2+and Mn4+was observed in the EPR spectrum of OS,indicat-ing that Mn2+ions were partly oxidized to Mn4+:

Mn00

Ti

þ2h !MnÂ

Ti

According to impedance spectra in Figure2,this pro-cess is accompanied by absorption of oxygen into the oxygen vacancy sites as a way of compensating electrical neutrality:

V

O

þ

1

2

O2!OÂ

O

þ2h

The EPR spectrum of VS exhibits axial symmetry, indicating the presence of nearest-neighbor oxygen vacancies.This is because the long-time annealing in a vacuum favors the mobility of oxygen vacancies,which are preferentially“trapped”on these isolated Mn00

Ti

defect centers and form½Mn00

Ti

ÀV

O

defect dipoles.

Based on these results,the giant?eld-induced strain

plex impedance spectra of Mn:NBBT6single crystals as

function of temperature from40to1MHz:(a)AS;(b)OS;(c)VS;

(d)Arrhenius plot of r dc conductivity.

Figure3.XPS pro?les of Mn:NBBT6single crystals:(a)Na1s;(b)Bi4

(c)Ti2p.X-band EPR spectra for Mn:NBBT6single crystals.

Materialia75(2014)50–53

will result in the?attening of the free energy pro?le and increase the distortion of[(Ti,Mn)O6]octahedral(i.e. Jahn–Teller distortion)by lengthening the o?-center dis-placement in the perovskite structure.However,the im-proved?eld-induced strain in VS cannot be attributed to this mechanism because the size of Mn3+(0.58A?)ions is close to Ti4+(0.61A?)ions.The“charge e?ect”model is proposed to explain the enhanced?eld-induced strain in

VS samples.The formation of½Mn00

Ti ÀV

O

defect di-

poles will introduce a tetragonal crystal?eld and en-hance the tetragonality,i.e.the c/a ratio.Upon the application of an external?eld,these defect dipoles act as anisotropy centers and promote the development of a0a0c+in-phase octahedral tilted regions[38].Therefore, when a su?cient high external?eld is applied,a large ionic displacement accompanied by?eld-induced phase transition can be expected.

In summary,the chemical nature of the giant?eld-in-duced strain of Mn:NBBT6single crystals was systemat-ically studied.The concentrations of oxygen vacancies in OS and VS samples decrease and increase,respectively. XPS results reveal that di?erent local coordination envi-ronments for Na+and Bi3+ions coexist in the crystal lattice.However,only one oxidation state presents for titannate,i.e.Ti4+.The Mn2+ions substitute Ti4+at the B-site and act as acceptors in the as-grown single crystals.They were partly oxidized to Mn4+in OS,

and form½Mn00

Ti ÀV

O

defect dipoles in VS.Based on

these results,“size e?ect”and“charge e?ect”models were proposed to explain the improved?eld-induced strains in OS and VS samples,respectively.

This work was?nancially supported by the Ministry of Science and Technology of China through 973Program(Nos.2013CB632902-3,2009CB623305, 2013CB6329052and2013CB632906),the Natural Science Foundation of China(Nos.51332009,51372258, 11304333,61001041,11090332and51272268),Science and Technology Commission of Shanghai Municipality (No.12DZ0501000),Shanghai Rising-Star Program (No.11QA1407500),Open Project from Shanghai Insti-tute of Technical Physics,CAS(No.IIMDKFJJ-11–08), the Fund of Shanghai Institute of Ceramics(No. Y29ZC4140G and Y39ZC4140G)and Shanghai Muni-cipal Electric Power Company(No.52091413502W).

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