Properties of Nd-substituted SrBi4Ti4O15 ferroelectric ceramics

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 39 (2006) 370–374 doi:10.1088/0022-3727/39/2/019

Properties of Nd-substituted SrBi4Ti4O15ferroelectric ceramicsWei Wang1, Jun Zhu1, Xiang-yu Mao1 and Xiao-bing Chen1,2,3

1 College of Physics Science and Technology, Yangzhou University, Yangzhou 225002,People’s Republic of China2 National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210008,People’s Republic of China

E-mail: xbchen@yzu.edu.cn

Received 29 May 2005, in final form 13 October 2005Published 6 January 2006Online at stacks.iop.org/JPhysD/39/370

AbstractCeramic samples of SrBi4−xNdxTi4O15 (SBNT-x, x = 0.00–1.00) havebeen prepared by the conventional solid-state reaction method. As the Nddoping content increases from 0.00 to 0.18, the remnant polarization (2Pr)increases steadily and reaches a maximum value of 25.8 µC cm−2, which isalmost 56% higher than that of the non-doped sample. However, 2Prdecreases with further substitution. The variation of 2Pr is dominated by therestraint of space charge and the relief of structural distortion. The coercivefield (Ec) remains almost unchanged at a value of ∼80 kV cm−1 in the caseof x = 0.00–0.18, then decreases with further Nd-doping, which is probablydue to the formation of anti-phase boundaries induced by the modification.As doping content exceeds 0.75, the specimens exhibit relaxor behaviourdue to a ferroelectric microdomain state caused by the random fields.

1. Introduction

Lead-free bismuth layer-structured ferroelectrics (BLSFs)have been widely expected to be a promising candidatefor ferroelectric random access memories (FeRAMs)which exhibit the advantage of nonvolatility, low powerconsumption, and high operation speed compared withconventional memories [1]. The general formula for thiskind of Aurivillius phase material can be expressed as(Bi2O2)

2+(Am−1BmO3m+1)2− where A denotes mono-, di- or

trivalent ions or a mixture of them, B represents Ti4+, Nb5+

and Ta5+, etc. The integer, m, represents the number of sheetsof corner-sharing BO6 octahedra forming the ABO3-typeperovskite-like blocks which are interleaved with (Bi2O2)

2+

slabs [2]. In this BLSF family, SrBi2Ta2O9 (SBT) andSrBi2Nb2O9 (SBN) thin films exhibit an excellent fatigue-free nature even with the Pt electrode, but their relativesmall remnant polarization (2Pr = ∼8 µC cm−2) as wellas high processing temperature (∼800 ◦C) are unfavourablefor practical applications [3]. Bismuth titanate Bi4Ti3O12

(BIT), another intensively studied BLSF, has large spontaneouspolarization, low switching field and high Curie temperature,but its high leakage electric current and poor fatigue resistancequality fail to satisfy industrial demands [4]. Recently,

3 Author to whom any correspondence should be addressed.

lanthanide substitution for BIT has attracted great attentionsince Park’s pioneering work, which reported a significantenhancement of 2Pr up to 24 µC cm−2 and a pronouncedimprovement of fatigue-free property in Bi3.25La0.75Ti3O12

films [5]. It has been reported as well that the randomlyoriented Bi3.15Nd0.85Ti3O12 thin films deposited by the sol–gel process show excellent ferroelectric response with 2Pr

and the coercive field (Ec) in the range of 82–86 µC cm−2

and 70–84 kV cm−1, respectively [6]. On the otherhand, preferentially-oriented epitaxial Bi3.15Nd0.85Ti3O12

films fabricated by pulsed laser ablation also possess a high2Pr of ∼40 µC cm−2 at a low Ec of ∼50 kV cm−1 [7]. Besides,negligible imprint shifts and excellent fatigue-free propertieswere also found in Bi3.15Nd0.85Ti3O12 films deposited by thechemical solution deposition method [8]. All these reportsindicate that neodymium modification plays a significant rolein the improvement of ferroelectric properties of BLSFs.

SrBi4Ti4O15 (SBTi), another Aurivillius family com-pound with four perovskite units, has been intensively inves-tigated due to its better fatigue-endurance property and highCurie temperature [9]. Irie et al have grown SBTi single crys-tal and reported 2Pr as high as 58 µC cm−2 along the a(b)-axisunder an electric field of 59 kV cm−1 [10]. However, SBTi stillhas several drawbacks such as small 2Pr (6.2–13 µC cm−2) andhigh Ec in the thin film forms. The fatigue-free performance

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Properties of Nd-substituted ferroelectric ceramics

deteriorates with the increase of the switching pulse width andthe decrease of operation frequency [9]. Zhu et al reportedthat the ferroelectric properties of SBTi were improved byLa-doping. 2Pr reaches a value of 24.2 µC cm−2 when theLa content is 0.25, while the coercive field decreases from80 to 60 kV cm−1 [11]. Considering the remarkable influenceof neodymium doping on BIT and lanthanum substitution forSBTi together with their structural similarity, it can be deducedthat neodymium-substitution should improve the ferroelectricproperties of SBTi. This paper reports that the ferroelectricproperties of SBTi ceramics are significantly improved byNd-doping, and the possible mechanism responsible for suchimprovement is discussed in detail.

2. Experimental

The SrBi4−xNdxTi4O15 (SBNT-x) (x = 0.00, 0.05, 0.10, 0.18,0.25, 0.50, 0.75, 1.00) ceramic samples were prepared by thesolid-state reaction method. Stoichiometric amounts of metaloxide SrCO3, Bi2O3, TiO2 and Nd2O3 with high purity wereused as starting materials. A 10 wt% excess bismuth oxide wasadded to compensate for the possible loss of bismuth duringthe high temperature process. The constituent oxides weremixed and ball-milled for 24 h, then precalcined at 800 ◦Cfor 8 h. The calcined powders were finely ground and thenuniaxially pressed into pellets. The obtained pellets are of12 mm diameter and 1.5 mm thickness, respectively. Thesepellets were finally sintered at 1180–1220 ◦C for 4 h, thennaturally cooled down in the furnace.

Phase identification of the sintered pellets was performedby x-ray diffraction using the M03XHF22 diffractometer withCu Kα radiation at an accelerating voltage of 40 kV anda current of 40 mA. These pellets were filed and polishedto a thickness of 0.2 mm and 0.5 mm and then coated withsilver electrodes for the measurements of ferroelectric anddielectric properties, respectively. Ferroelectric hysteresisloops were obtained by a standard ferroelectric analyser(Radiant Technologies, Precision LC). Dielectric permittivity(ε) dependence on temperature and frequency was investigatedby an HP-4192A low frequency impedance analyser.

3. Results and discussion

3.1. Crystal structure

Figure 1 shows the XRD patterns of the SBNT-x (x = 0–1.00)ceramic samples. The diffraction peaks are identified andindexed using the standard XRD data of SrBi4Ti4O15 [12]. Allthe ceramic samples are of random orientation and consist of asingle phase without a secondary pyrochlore or fluorite phase.The positions of the peaks (020) and (0018) shift slightlytoward larger Bragg angle, implying that the lattice parametersb and c decrease when the Nd doping content increases. Theslight decrease of this lattice spacing suggests that the Nd3+

ion has been solid soluted in SBTi because the comparativeionic radius for eight-fold coordination of Nd3+ is close to thatof Bi3+ (Nd3+: 1.109 Å, Bi3+: 1.170 Å) [13].

Figure 1. X-ray diffraction patterns of SBNT-x.

3.2. Ferroelectric properties

Figure 2 shows the P –E hysteresis loops of SBNT-x (x =0.00–1.00) under an applied electric field of 180 kV cm−1. Itcan be seen that all the hysteresis loops approach saturationunder such a driven field. The dependence of 2Pr and Ec on theneodymium content at an applied electric field of 180 kV cm−1

is given in figure 3. As the Nd content increases from x = 0.00to 0.18, 2Pr increases steadily and reaches a maximum valueof 25.8 µC cm−2, which is roughly 56% higher than that ofthe undoped SBTi, then decreases with further substitution.Ec remains almost unchanged at 80 kV cm−1 when x =0.00–0.18, then decreases monotonously. Therefore, thecomprehensive ferroelectric property is obviously improvedby Nd-doping with appropriate content.

In Pb(Zr,Ti)O3 and BLSFs, oxygen vacancies act asspace charge which assemble into extended structures nearthe domain boundaries and make these domains pinned andhard to switch, leading to a decrease of the switchablepolarizations. Theoretical calculations also suggest anincrease of polarization when the oxygen vacancies areeffectively suppressed [14]. On the other hand, theinvestigations on the neutron powder diffraction of La-dopedBIT have suggested that the substitution of La3+ for Bi3+ causesless structural distortion, which is responsible for a smallerspontaneous polarization and a lower Curie temperature [15].The Raman spectrum investigations on La-doped BIT andSBTi indicated that the La ions substitute for the Bi ions onlyat the A site of the perovskite slabs when x < 1.00 andx < 0.1, respectively. With further La-doping, La ions tend toget incorporated into the (Bi2O2)

2+ layers [15,16]. In BLSFs,(Bi2O2)

2+ slabs usually serve as space charge storage, and theself-regulating ability of (Bi2O2)

2+ slabs can prevent defectssuch as oxygen vacancies from assembling at domain walls,thus alleviating the domain pinning. The resistivity along thec-axis is two to three orders of magnitude higher than thatin the a–b plane due to the existence of insulating (Bi2O2)

2+

layers [17, 18]. In our case, due to the more stable chemical

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Figure 2. Ferroelectric hysteresis loops of SBNT-x under a drivenelectric field of 180 kV cm−1.

property of Nd3+, the substitution of Nd3+ for volatile Bi3+

brings about the restraint of Bi3+ vacancies and consequentlylowers the concentration of oxygen vacancies. Since SBTi hasa crystal structure similar to BIT and Nd is also a memberof the lanthanides, it is reasonable to consider that the Ndmodification in SBTi could have site selectivity and results inthe relief of structural distortion. When the doping content islower than 0.18, the Nd ions may only substitute for Bi ionsin the perovskite layer. Within this doping range, the effectof the restraint of oxygen vacancies is greater than that of therelief of lattice distortion, which causes the enhancement ofpolarization. With further doping, part of the Nd ions beginto be incorporated into the (Bi2O2)

2+ layers, which makesthe restraint of oxygen vacancies in the perovskite layer lessevident than before. On the other hand, the entry of Nd ionsinto the (Bi2O2)

2+ layers may destroy their original effects and

Figure 3. Dependence of 2Pr and Ec of SBNT-x on Nd content.

Table 1. Doping content of M (La, Nd) in BLSFs (m = 3, 4, 5)corresponding to the maximum 2Pr .

BLSFs M = La M = Nd

Bi4−xMxTi3O12 (m = 3) 0.75 0.60SrBi4−xMxTi4O15 (m = 4) 0.25 0.18Sr2Bi4−xMxTi5O18 (m = 5) — 0.10

cause greater relaxation of lattice distortion. These joint effectslead to the decrease of 2Pr when x > 0.18.

It was reported that 2Pr of La-doped BIT ceramicsincreases at first and reaches its maximum value at the criticaldoping content of x = 0.75, then decreases with furthersubstitution [19]. In our previous work, a similar tendency of2Pr was also found in BIT, SBTi and Sr2Bi4Ti5O18 ceramicsdoped by La or Nd [11, 20, 21]. Table 1 shows the differentdoping content of La and Nd corresponding to the maximumvalue of 2Pr in BLSFs with different TiO6 octahedral layers.It is found that the doping contents of both La and Nd decreasewith the increase of TiO6 octahedral blocks, which could beattributed to the inhomogeneous existence of A site ions inperovskite layers for BIT, SBTi and Sr2Bi4Ti5O18. In BIT,only two Bi3+ occupy the A site, while one Sr2+ and two Bi3+

occupy the A site in SBTi. As for Sr2Bi4Ti5O18, there are twoSr2+ and two Bi3+ at the A site. Because Sr2+ is more stable thanBi3+ and is more metal-like than the Bi3+ and lanthanide ions,its existence in the A site favours the suppression of the oxygenvacancies just as the doping lanthanide ions do. In a unit cell,the Sr2+ increases from 0 to 2 and the Bi3+ stays constant, sothe acceptability for lanthanide ions decreases from BIT toSr2Bi4Ti5O18. As shown in table 1, the doping content of Ndfor the maximum value of 2Pr is lower than that of La whendoped in the same BLSFs. This behaviour can be attributedto the difference in the effective ion charge between La3+ andNd3+, which causes a structural distortion to a different extent.As the atomic number increases in lanthanide, the effectiveion charge increases rapidly. The effective ion charge ofLa3+ and Nd3+ are 11.00 and 11.45, respectively [22], andthe radius of Nd3+ (1.109 Å) is smaller than that of La3+

(1.160 Å), so the Coulomb interaction between Nd3+ and O2− is

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Properties of Nd-substituted ferroelectric ceramics

Figure 4. Dependence of dielectric constant on temperature ofSBNT-x at a frequency of 50 kHz.

stronger than that between La3+ and O2−, which suggests thatNd-doping causes less relief of the lattice distortion than doesthe La-doping under identical doping conditions. BecauseNd-doping causes less negative influence on 2Pr, the 2Pr canreach the maximum value at a smaller doping content than thatof La-doping, as found in table 1.

As shown in figure 3, the coercive field (Ec) remainsalmost unchanged as Nd content ranges from x = 0.00 tox = 0.18 and then decreases with higher doping content.Chen et al attributed the unchanging of Ec in La-doped SBTi tothe anti-phase boundaries (APBs) [23,24]. The investigationsinto the APBs in BLT grains indicate that APBs can absorbthe stress in the crystal and serve as new sites for nucleationof new domains. In this way, the energy for domain reversal isreduced, and the number of reversible domains increases. Sowith increasing Nd content in SBTi, some pinned domains takepart in the reversal process and enhance Pr. Meanwhile, theEc that represents the highest reversal energy of the reversibledomain remains unchanged, as SBNT-0.18 does. With afurther increase of Nd content, no more pinned domains cantake part in the reversal process, which leads to the decreasingof Ec as the reversal energy decreases continuously.

3.3. Dielectric and relaxor properties

The real part of the dielectric permittivity (ε) of SBNT-xsamples as a function of the temperature is shown in figure 4.ε reaches a maximum value at the Curie temperature (Tc). Thepeak value of ε at Tc decreases with Nd doping, which is similarto that in La-doped SBTi. The dielectric peak broadens withNd-doping, indicating the diffusing of the ferro-paraelectricphase transition, which is attributed to the occurrence offerroelectric microdomains induced at high doping content.The Tc values of these microdomains are slightly different,which induce the dispersion of the phase transition. The Ndmodification in the SBTi brings about a decrease of Tc, asshown in figure 5. This implies that Nd doping leads to

Figure 5. Tc for SBNT and SBLT as a function of doping content.

Figure 6. Dependence of ε on the temperature of SBNT-0.75 nearTc with different frequencies.

the relaxation of the lattice distortion, thus decreasing thespontaneous polarization. Because the Nd3+ has no 6s electron,the covalent bonding force between O2− and Nd3+ is less thanthat between O2− and Bi3+, which has lone-pair 6s electrons.So the Nd-doping results in the relief of lattice distortion. TheTc of SBLT decreases more drastically than that of SBNT,indicating that the La-doping causes more drastic relief oflattice distortion than the Nd-doping does. The Tc of SBNT-0.18 is 475 ◦C, higher than that of SBT (300 ◦C) and BLT-0.75(420 ◦C), which confirms its better thermal stability.

The ε dependence on the frequency for SBNT-0.75 inthe vicinity of Tc is presented in figure 6. A typical relaxorbehaviour is observed with the peak position shifting towardshigh temperature and the height of the peak decreasing withthe increasing of frequencies. The relaxor characteristics

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disappear when the doping content is below 0.75, whichsuggests its close relationship with the doping content. Ramanstudies on the La-doped BIT and SBTi suggest that substitutionwith high La content leads to the ferro-paraelectric phasetransition which could also be induced by increasing thetemperature. So it is expected that doping will induce therandomly distributed paraelectric states. But there mustbe some small ferroelectric states (microdomains) inducedby the random strain fields as well as the local electricfields. However, the dipole–dipole interaction cannot becompletely prohibited, and it leads to the appearance ofmacrodomains [25]. The competition between these two kindsof domains results in the relaxor-like phase transition.

4. Conclusions

The SrBi4Ti4O15 ceramics are of single phase after the Ndmodification. 2Pr of SBNT-x increases at first, and thendecreases with the increase of Nd doping content. AsNd content is 0.18, 2Pr reaches its maximum value of25.8 µC cm−2, roughly 56% higher than that of pure SBTi.The variation of 2Pr is dominated by the restraint of the spacecharge and the relief of structural distortion. The coercive fieldremains almost unchanged from x = 0.00 to x = 0.18 and thendecreases monotonously with further Nd doping. Nd-dopingbrings about the decrease of the Curie temperature due tothe relief of the lattice distortion. The relaxor characteristicsappear in the samples when the Nd content exceeds 0.75, whichis attributed to the competition between macrodomains and themicrodomains. Appropriate Nd-doping is found to improvethe ferroelectric property of SBTi considerably.

Acknowledgments

The authors would like to acknowledge the financial supportby the National Natural Science Foundation of China (GrantNo 10274066) and the Natural Science Foundation ofEducation Bureau of Jiangsu Province, China (Grant NoGK0410181).

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