Microstructure, electrical properties,and electric field-induced phase transitionsin NaNbO3–LiTaO3 lead-free ceramics

Hailing Sun1, Qiaoji Zheng1, Yang Wan1, Qiang Li1, Yan Chen2, Xiao Wu2, Kin Wing Kwok2,Helen Wong Lai-Wa Chan2, and Dunmin Lin*,1

1 College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, P. R. China2Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Kowloon,Hong Kong, P. R. China

Received 27 August 2013, revised 16 December 2013, accepted 17 December 2013Published online 20 January 2014

Keywords ceramics, ferroelectric properties, niobates, phase transitions, tantalates

* Corresponding author: e-mail ddmd222@sicnu.edu.cn, Phone: þ86 28 84760802, Fax: þ86 28 84767868

Lead-free ceramics (1–x)NaNbO3–xLiTaO3 were synthesizedby an ordinary solid-state reaction method. Rietveld analysisreveals that the crystal structure changes from one orthorhom-bic phase with Pbcm space group to another orthorhombicphase with P2221 space group when increasing x. Themorphotropic phase boundary between two different ortho-rhombic symmetries is formed at 0.14< x< 0.18. ExcessLiTaO3 causes an abnormal grain growth behavior. The pureNaNbO3 presents four current peaks in the I–E loop, indicating

electric-field-induced transitions. A low LiTaO3 dopinginduces ferroelectricity in the initially antiferroelectric (AFE)NaNbO3. Enhanced ferroelectricity is obtained at x¼ 0.02–0.12. The addition of LiTaO3 makes the ceramics undergoa transition from an antiferroelectricity dominant to a ferro-electricity dominant phase state. The optimum d33, kp, Qm, andPr values of 46 pC/N, 18.3%, 1333, and 27.1mC cm�2 areobtained when x equals 0.12, and the ceramic exhibits goodtemperature stability because of its high TC.

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1 Introduction For environmental protection reasons,lead-free piezoelectric materials have attracted considerableattention in recent years. BaTiO3 (BT)- [1], Bi0.5Na0.5TiO3

(BNT)- [2], and K0.5Na0.5NbO3 (KNN)- [3] based ceramicsare the most intensively investigated lead-free materials andpossess high piezoelectric constant. However, these lead-free ceramics without acceptor doping (e.g., Cu, Mn, etc.)simultaneously exhibit low mechanical quality factorQm (e.g., KNN–BNT–LiSbO3 [3]: �33; Ba0.93Ca0.07-Ti0.95Zr0.05O3 [4]: �140; 0.79BNT–0.14Bi0.5K0.5TiO3–

0.07BT [5]: �110; BNT-(Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 [6]:�105; etc.). Moreover, due to the coexistence of tetragonaland orthorhombic phases near room temperature or lowdepolarization/Curie temperature [1–6], BT-, BNT-, andKNN-based lead-free ceramics exhibit poor temperaturestability.

In 1949, Matthias grew sodium niobate (NaNbO3)crystals and found them to be ferroelectric (FE) at roomtemperature [7], but then Vousden rejected the ferroelectric-

ity of NaNbO3 by reporting its nonpolar structure at roomtemperature [8]. Nowadays, it is well known that NaNbO3 isantiferroelectric (AFE) [9–12] with orthorhombic distortedperovskite structure at room temperature. During coolingfrom high temperature, it exhibits a large number ofsuccessive structural phase transitions from paraelectric toAFE and finally to a FE phase with a further temperaturedecrease [13]. Many NaNbO3-based lead-free systems(e.g., NaNbO3–CaTiO3 [12], NaNbO3–SrTiO3 [14],NaNbO3–BaTiO3 [15], NaNbO3–BiCrO3 [16], etc.) havebeen investigated, but these studies mostly focused on thedielectric properties and their remarkable relaxor character-istics since these materials do not exhibit considerablepiezoelectricity. It is well known that LiTaO3 is a FEwith ilmenite structure and possesses a high Curietemperature (�665 8C) [9]. To our knowledge, reports onLiTaO3-modified NaNbO3 material are extremely rare sinceSadel et al. [17] investigated the LiNbO3–LiTaO3–NaNbO3–

NaTaO3 quaternary system in 1980. However, their work

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did not report on the dielectric, piezoelectric, and FEproperties of the (1–x)NaNbO3–xLiTaO3 material. In thepresent work, a new lead-free piezoelectric solid solution ofAFE NaNbO3 with FE LiTaO3 was synthesized by anordinary ceramic technique and the effects of LiTaO3 dopingon the microstructure, electrical properties (dielectric, FE,and piezoelectric) and AFE–FE phase transition weresystematically studied. Our results show that LiTaO3-dopedNaNbO3 ceramics possess enhanced ferroelectricity, highQm and good temperature stability.

2 Experimental (1–x)NaNbO3–xLiTaO3 (NN–LT–x,x¼ 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18,0.20, 0.225, and 0.25) ceramics were prepared with anordinary ceramic technique using metal oxides andcarbonate powders: Na2CO3 (99.8%), Li2CO3 (98%),Nb2O5 (99.99%), and Ta2O5 (99.99%). The raw materialsin the stoichiometric ratio of NN–LT–x were mixedthoroughly in ethanol using zirconia balls for 10 h, driedand then calcined at 900 8C for 6 h. After the calcination, theresulting mixture was ball-milled in ethanol again for 8 h andmixed thoroughly with a PVA binder solution, and thenpressed into disk samples. The samples were sintered at1310 8C for 2 h in air. Silver electrodes were fired on bothsurfaces of the sintered ceramics at 810 8C for 10min. Theceramics were poled under a dc field of 4.5 kVmm�1 at80 8C for 30min in a silicone oil bath.

The crystalline structures of the sintered ceramics weredetermined using X-ray diffraction (XRD) analysis with CuKa radiation (SmartLab, Rigaku Co., Japan). The micro-structures were observed by scanning electron microscopy(SEM, FEI-Quanta 250, Hillsboro, Oregon, USA). Theaverage grain size was obtained by multiplying the averagelinear intercept length of at least 500 grains by 1.56 [18, 19].The relative permittivity er and loss tangent tan d weremeasured using an LCR meter (Agilent E4980A, SantaClara, CA). The polarization hysteresis (P–E) loops andswitching current–electric field (I–E) loops were measuredusing a FE measuring system (Premier II, RadiantTechnologies, Inc. Albuquerque, NM, USA), with atriangular waveform at the frequency of 10Hz. Theelectromechanical coupling factor kp and Qm were deter-mined by the resonance method according to the IEEEStandards 176 using an impedance analyzer (Agilent 4294A,Santa Clara, CA). The piezoelectric constant d33 wasmeasured using a piezo-d33 meter (ZJ-6A, Institute ofAcoustics, Chinese Academy of Sciences, Beijing, China).

3 Results and discussion The XRD patterns forNN–LT–x ceramics sintered at 1310 8C are shown in Fig. 1.Although the crystal structure of NaNbO3 (perovskitestructure) is very different from LiTaO3 (ilmenite structure),the XRD results suggest that LiTaO3 can successfully diffuseinto NaNbO3 lattices to form perovskite structure withorthorhombic symmetry within the solubility limit (�18%).For the ceramics with x� 0.2, a trace amount of LiTaO3

(second phase, ilmenite structure, JCPDS PDF card No. 87-

2461) is clearly detected. Rietveld analysis of these XRDpatterns reveals that the sample with x¼ 0.02 has anorthorhombic symmetry with Pbcm space group at roomtemperature, similar to pure NaNbO3 with the diffractionpeaks of orthorhombic NaNbO3 (PDF#73-0803) indicatedby vertical lines for comparison, and this structure canremain till x¼ 0.14. With a further increase in x (0.18�x� 0.25), the phase structure changes to another ortho-rhombic symmetry in the space group P2221. A two-phasecoexistence zone between two different orthorhombicsymmetries can be identified in the composition range of0.14< x< 0.18. Moreover, all the peaks around 468 shift tohigher angle with respect to the pure NaNbO3 with xincreasing as clearly shown in the enlarged diffraction angle.This is because Liþ is much smaller than Naþ (in the 12-foldcoordination sites), while there is no large difference in theB-site ionic radii between Ta5þ and Nb5þ in the sixfoldcoordination sites [20].

Based on the XRD analysis of NN–LT–x ceramicsshown in Fig. 1, the lattice parameters were calculated usingRietveld’s software MAUD [21]. For all the refinement, areliability factor Rwp of 9.2–13.7% (<15%) and a goodness-of-fit indicator S of 1.18–1.4 (<2) are obtained, suggesting agood agreement between the observed and calculatedpatterns. The pseudocubic lattice parameters ap, bp, and cpare calculated from the Rietveld refined orthorhombic latticeparameters, and the refinement results are shown in Fig. 2.Clearly, there is a strong dependence of lattice parameters onthe substitution level of LiTaO3. The observed ap, bp, and cpundergo a slight decrease with x increasing to 0.14, then bpand cp decrease greatly while ap increases suddenly atx¼ 0.14–0.16. At x> 0.18, the observed ap, bp, and cpdecrease with x increasing. The observed cp/ap ratio firstincreases close to 1 with x increasing to 0.14, then decreasesduring 0.14< x< 0.18, and finally increases slightly with x

Figure 1 XRD patterns of the NN–LT–x ceramics sintered at1310 8C.

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further increasing to 0.25. These characteristics areassociated with a change of space group from Pbcm toP2221 through an intermediate modulated orthorhombicphase. Similar to the well-known morphotropic phaseboundary (MPB) of PZT ceramics, this NN–LT–x systemshows aMPB-like characteristic transition zone between twodifferent orthorhombic symmetries in the composition rangeof 0.14< x< 0.18. Similar MPB-like behavior has beenreported in the NaNbO3–xCaTiO3 system [8]. The observedpseudocubic unit-cell volume Vp decreases monotonouslyfrom 59.70 to 58.18Å3 with x increasing from 0 to 0.25,which suggests the shrinkage of the cell after the addition ofLiTaO3 because of the much smaller radii of Liþ thanNaþ [20].

The SEM images of the surface microstructure of theNN–LT–x ceramics are shown in Fig. 3. It can be seen thatthe ceramic with x¼ 0.02 has a very loose structure and alarge number of pores can be found (Fig. 3a). After theaddition of 12mol% LiTaO3, the grains become significantlylarger, the ceramic is relatively dense and no pores can beobserved (Fig. 3b). However, for the ceramic with x¼ 0.18,many small grains are combined into the large grains(Fig. 3c). As x increases to 0.25, a clear bimodal grain-sizedistribution can be observed with large grains (diameters ofabout 60–100mm) uniformly distributed among the matrixgrains that are of much smaller diameters with average sizesof 7.2mm (Fig. 3d). The average grain size was determinedfrom the linear intercept method [18, 19]. Figure 4 shows thegrain size (average linear intercept length DL) distributionsof the ceramics with x¼ 0.02, 0.12, 0.18, and 0.25. FromFig. 4, the ceramic with x¼ 0.02 exhibits a range of averagelinear intercept length DL between 1 and 8.5mm and thegrain-size distribution is symmetric, suggesting that the grainsizes are relatively uniform. However, the observed DL

distribution becomes gradually wide and asymmetric with

increasing x, suggesting that an obvious abnormal graingrowth (AGG; bimodal grain-size distribution) occurs forthe ceramics with high LiTaO3 level. This abnormal grain-growth (AGG) behavior in the present ceramics has beenobserved in Li2O/PbO added PMN-35PT [22], Na2O-dopedK0.5Na0.5TiO3-LiTaO3 [23], TiO2-doped BaTiO3 [24], WC-Co alloy [25], and CaO2–SiO2/MgO added Al2O3 [26] andcan be explained according to interface reaction-controlled

Figure 2 Pseudocubic lattice parameters ap, bp, cp, and cp/ap ratio,and unit-cell volume Vp as functions of LiTaO3 content of theNN–LT–x ceramics sintered at 1310 8C.

Figure 3 SEM micrographs of the as-sintered surfaces of theNN–LT–x ceramics: (a) x¼ 0.02, mag. 2000�; (b) x¼ 0.12, mag.1000�; (c) x¼ 0.18, mag. 1000�; (d) x¼ 0.25, mag. 1000�;(e) x¼ 0.18, mag. 4000�; and (f) x¼ 0.25, mag. 4000�.

Figure 4 Grain-size distributions of the NN–LT–x ceramics withx¼ 0.02, 0.12, 0.18, and 0.25 sintered at 1310 8C.

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nucleation and growth [22, 23, 25]. It is known that therounded interfaces or boundaries of the grains lead to normalgrain growth, while the faceted interfaces or boundariesinduce AGG [22–26]. For the ceramic with x¼ 0.02, theinterfaces or boundaries of the grains are rounded and thusnormal grain growth is observed. However, as the dopinglevel of LiTaO3 increases, the interfaces or boundaries of thegrains become faceted and thus AGG occurs. The addition ofmore LiTaO3 decreases the critical driving force for rapidgrain growth–bimodal size distribution and thus causes theincrease in the number of abnormal grains in the ceramicswith high LiTaO3 level (x¼ 0.18 and 0.25). Moreover,corresponding to this AGG behavior, it can been clearly seenthat a crossover appears in the grain size (back to the 4000magnification SEM images in Fig. 3e and f), which mightprove that the formation of liquid phase results from theexcess LiTaO3.

The temperature dependence of er at 1 kHz for theNN–LT–x ceramics sintered at 1310 8C for 2 h is shown inFig. 5a, while the inset figure shows the variation of Curietemperature TC with x. Unlike BT-, BNT-, and KNN-basedceramics [1–3], only a dielectric peak associated with theFE to paraelectric transition is observed in the NN–LT–xceramics (Fig. 5a). From the inset in Fig. 5a, the pureNaNbO3 ceramic possesses a higher TC value of 443 8C thanthe reported TC value of 355 8C in NaNbO3 crystal [27]. As xincreases in the range 0.04< x< 0.25, the observed TCslightly decreases in comparison with x¼ 0, while it stillshows small fluctuations with variations between 351 8C

(at x¼ 0.04) and 281 8C (at x¼ 0.25). The decrease in the TCmay be attributed to the weakening of the orthorhombicity inthe Pbcm space group ceramics. The ceramic with x¼ 0.02–0.12 exhibits a sharp dielectric peak. However, as x furtherincreases, the peak becomes gradually broadened, suggest-ing that excess LiTaO3 leads to a diffusive phase transitionat TC, which may be attributed to the increase disordereddegree in cations and local compositional fluctuationinduced by the addition of LiTaO3. From Fig. 5a, it canbe noted that the ceramics with x¼ 0.14–0.25 exhibit ashoulder at �200 8C. The presence of these tiny shouldersmay be related to the space group change that is in agreementwith our XRD results. However, further studies are needed tobetter understand their origin. In order to understand thepermittivity–temperature dependence, the dielectric losstan d at 1 kHz versus temperature curves for NN–LT–xceramics are shown in Fig. 5b. It can be seen that there is ananomaly in loss values with temperature around 300 8C(inset in Fig. 5b). And the compositions of x¼ 0.02 and0.08 with an orthorhombic symmetry (majority Pbcm spacegroup) tend to have a more obvious anomaly peak,associated with the phase transition in Fig. 5a. Above TC,the high-temperature conductivity contributes to a rapidincrease in the loss values.

The P–E loops (at 10Hz) of the NN–LT–x ceramics areshown in Fig. 6a, while the composition dependences ofremanent polarization Pr and Pmax are shown in Fig. 6b. All

Figure 5 (a) Temperature dependence of er at 1 kHz for theNN–LT–x ceramics sintered at 1310 8C; (b) dielectric loss tan dat 1 kHz versus temperature curves for NN–LT–x ceramics.

Figure 6 (a) P–E loops for the NN–LT–x ceramics sintered at1310 8C; (b) variations of Pr and Pmax of the NN–x LT ceramicssintered at 1310 8C with x.

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the ceramics exhibit a well-saturated P–E loop under anelectric field of 9–10 kVmm�1 except the pure NaNbO3,which shows bananas-like P–E loop. Many studies [9–12]have claimed that the pure NaNbO3 is AFE at roomtemperature, but a double P–E hysteresis loop accompaniedby Pr¼ 0 (or close to zero), which represents a sign ofantiferroelectricity can hardly be found in the literature. Asdiscussed by Tan et al. [28], the double P–E hysteresis loops,marking the electric field-induced AFE to FE phasetransition, were only observed in high-quality NaNbO3

single crystals with an electric field applied perpendicularto the orthorhombic c-axis. In the present work, the pureNaNbO3 ceramic shows a flattened and slim loop under theelectric field of 10 kVmm�1; after the addition of 2.0mol%LiTaO3, a well-saturated and square-like P–E loop isobserved. This suggests that a small amount of LiTaO3 caninduce ferroelectricity in the initially AFE NaNbO3. As xincreases, the P–E loop becomes gradually narrow andslanted. The ceramics with x¼ 0.02–0.18 possess large Pr

and Pmax values (Pr¼ 28.1–16.0mC cm�2, Pmax¼ 31.5–28.3 kVmm�1, respectively). At x¼ 0.14, the differencebetween Pmax and Pr is at the minimum. As x furtherincreases, the disparity of Pmax and Pr also increases.

Our experiments failed to get double P–E hysteresisloops in the pure NaNbO3 ceramic, however, the followingswitching current curves (Fig. 7) suggest the existence ofelectric-field-induced transitions in pure NaNbO3 ceramics.The study of the switching current curve allows thepossibility of more precise evaluation of the coercive fieldby the current domain-switching peak and the detection of aneventual phase transition induced by the application of thefield [29, 30]. For the pure NaNbO3 (Fig. 7a), four currentpeaks emerge (marked with 1–4), indicating that the pureNaNbO3 ceramic experiences reversible electric-field-in-duced transitions from a possible AFE or a weakly polar to aFE state. After doing the amplitude dependence I–E loopexperiments for pure NN during the entire electrical history,we observe that peaks 1 and 3 do not appear until peaks 4 and2 appear, and a similar phenomenon has been reportedrecently [31]. When applying a positive electric field, thepeak 2, which presents the AFE–FE transition [31–34]becomes gradually obvious, indicating that high electric fieldamplitude can induce a more polar FE phase. During fieldreversal, the field correspondent to peak 3 can be regarded asthe field that recovers the effects produced at peak 2. At arelatively low electric field (3–6 kVmm�1), there are onlypeaks 2 and 4 and they are not very obvious, but at a higherelectric field (7–10 kVmm�1), peaks 3 and 1 appear. Thismay be attributed to the fact that the more polar the FE phaseinduced by a higher electric field at peaks 2 and 4, the moreback-switching into the original AFE state takes place atpeaks 3 and 1.

On the other hand, as shown in Fig. 7b and c, the currentpeaks become broader and the distinction between peaks 1–2and peaks 3–4 become less obvious with increasing x to 0.02and 0.12, which could be due to convolution of the FEswitching peaks and AFE–FE transition peaks. The ceramicof x¼ 0.18 presents a FE-like characteristic switchingcurrent peak (Fig. 7d), indicating that the FE phase starts todominate in competing with AFE phase. This is because,under electrical cycling regime conditions, the compositionwith x¼ 0.02 and 0.12 possess more AFE phase than that ofx¼ 0.18, meaning that more FE–AFE reversal needed totake place at peaks 3 and 1 to recover the composition-drivenAFE–FE transition effects produced at peaks 2 and 4 [31].Therefore, the samples with AFE dominantly show abroadened peak current peak than those of FE dominantlysamples in a broad electric field range. However, Fig. 7eshows clear FE switching current peaks, thus the AFE–FEtransition is virtually completed. This fact suggests thatthe ceramic undergoes a phase transition from an AFEdominantly state (orthorhombic, Pbcm space group) at0.02� x� 0.14 to a FE dominantly state (orthorhombic,P2221 space group) at 0.18� x� 0.25. This is in accordancewith the XRD results of a two-different orthorhombicsymmetry coexistence zone in the composition range of0.14< x< 0.18.

The compositional dependences of d33, kp, er, tan d andQm of the poled NN–LT–x ceramics sintered at 1310 8C for2 h are shown in Fig. 8. The value of d33, kp, er, tan d, andQm

Figure 7 Switching current–electric field (I–E) loops of theNN–LT–x ceramics with (a) x¼ 0 during the entire electricalhistory; (b) x¼ 0.02; (c) x¼ 0.12; (d) x¼ 0.18; and (e) x¼ 025.

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of pure NaNbO3 is 20 pCN�1, 11.7%, 157, 0.04, and 193,respectively. On increasing x, we can observe that d33 and kpincrease and then decrease, reaching the maximum values72 pCN�1 and 21.1% at x¼ 0.18, respectively, which couldresult from the MPB composition between two differentorthorhombic symmetries. The observed er exhibits a similardependence on x and gives the maximum value of 714 atx¼ 0.225. Unlike d33, the observed tan d shows an oppositedependence on x and retains a smaller value than 0.03 exceptfor the pure NaNbO3, which has a value of 0.041. It shouldbe noted that the NN–LT–x ceramics with x¼ 0.04–0.2possess the large Qm values of 632–1333, which are muchlarger than those of other lead-free perovskite materialswithout acceptor doping (e.g., Cu, Mn, etc.) [3–5].

Interestingly, it can be seen that for the ceramic withx¼ 0.12 (Fig. 8a), the extraordinary highQm value of 1333 isobtained. The Qm of the present ceramic is comparable tothat of commercial hard PZT-based ceramics [5]. In general,acceptor doping (e.g., Cu, Mn, etc.) can lead to high Qm inpiezoelectric ceramics. It has been known that KNN withoutacceptor doping possesses a low Qm (�215) [35]. However,CuO (acceptor) doping leads to a significant enhancementin the Qm (�1000 to 3000) of the KNN-ceramics, whichhas been frequently reported [35–40]. Recently, lead-freeperovskite hard ceramics have attracted considerableattention [35–40]. However, it should be noted that mostof the studies on lead-free hard ceramics with perovskitestructure are mostly focused on KNN-based ceramics withCu doping [35–40]. Therefore, the NN–LT–x ceramics maybe a new promising candidate for lead-free hard ceramicswith perovskite structure. It may be reasonably expected thatacceptor doping (e.g., Cu, Mn, etc.) could induce better hardpiezoelectricity in the present ceramics. Further study onacceptor-doped NaNbO3–LiTaO3 ceramics will be carriedout.

Poor temperature stability is observed in the BaTiO3/BNT/K0.5Na0.5NbO3-based ceramics because of the coexis-tence of tetragonal and orthorhombic phases near roomtemperature or low depolarization/Curie temperature [1–5].In order to examine the temperature stability of thepiezoelectricity of the NN–LT–x ceramics, annealingexperiments were carried out for the sample of the pureNN, NN–LT–0.12, and NN–LT–0.18 ceramics. Figure 9a–cpresents the thermal depolarization behavior of the pure NN,NN–LT–0.12, and NN–LT–0.18 ceramic, respectively. Thepoled samples were annealed at elevated temperatures for1 h and then their d33, kp, and Qm were re-evaluated. Theobserved d33, kp, and Qm for the NN–LT–0.12 ceramic(Fig. 9b) remain nearly unchanged with annealing tempera-ture increasing from 20 8C (d33¼ 42 pCN�1, kp¼ 15.9%,and Qm¼ 1333) to 240 8C (d33¼ 42 pC/N, kp¼ 15.7%, andQm¼ 1249). As temperature further increases above 250 8C(TC¼ 301 8C), the observed d33, kp, and Qm drop steeply.Unlike this, the pure NN and NN–LT–0.18 ceramics (Fig. 9aand c) display different characteristic. Although the TC of thepure NN (443 8C) and NN–LT–0.18 ceramic (335 8C) arehigher than that of NN–LT–0.12 (301 8C), the observed d33,kp, andQm for the pure NN, NN–LT–0.18 ceramic can hold a

Figure 8 Variations of d33, kp, Qm, er, and tan d with x for thepoled NN–LT–x ceramics sintered at 1310 8C.

Figure 9 Variations of d33, kp, and Qm with temperature for the(a) pure NN; (b) NN–LT–0.12; and (c) NN–LT–0.18 ceramics.

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slight change only at a low testing temperature. It can be seenthat the d33 of pure NaNbO3 (Fig. 9a) presents a slightincrease with increasing annealing temperature until about200 8C (far lower than the TC), also the data reported byReznitchenko et al. [41] shows similar increments and futurework is needed for better understanding. Finally, it falls closeto zero when the temperature further rises above 380 8C.For the NN–LT–0.18 ceramic (Fig. 9c), once the annealingtemperature further increases above 150 8C, the d33, kp, andQm visibly decrease and keep falling gradually. Around TC,there exists not a steep drop for the NN–LT–0.18 ceramiccompared with the NN–LT–0.12 ceramic. Together withthe temperature dependence of the permittivity (Fig. 5),we observe that the NN–LT–0.12 ceramic possesses bettertemperature stability, which should be attributed to the weaktemperature dependence of er for the ceramic with x¼ 0.12below 250 8C, while the pure NN and NN–LT–0.18ceramics exhibit relatively poor temperature stability causedby its broadened diffusive dielectric peak at TC, as well as thedielectric shoulders.

4 Conclusions New lead-free ceramics (1–x)NaNbO3–xLiTaO3 have been prepared by an ordinary solid-statereaction method. LiTaO3 diffuses into NaNbO3 lattices toform a perovskite solid solution with orthorhombicsymmetry. An AGG behavior takes place with high LiTaO3

level. The ceramics with x¼ 0.02–0.12 exhibit a sharp phasetransition peak at TC, while the FE–paraelectric phasetransition becomes diffusive at x> 0.12. Rietveld analysis ofXRD patterns for various compositions reveals that thecrystal structure changes from one orthorhombic phase inthe Pbcm space group to another orthorhombic phase inthe P2221 space group, indicating that LiTaO3 tends toincrease the orthorhombic distortion. The ferroelectricity ofthe ceramics is induced after the addition of LiTaO3 andenhanced ferroelectricity is obtained at x¼ 0.02–0.12. Thepure NaNbO3 presents four current peaks in the I–E loop,indicating its electric-field-induced transitions, and theaddition of LiTaO3 makes the ceramics undergo a transitionfrom an AFE dominantly to a FE dominantly phase state.The MPB between two different orthorhombic symmetry isformed in the composition range of 0.14< x< 0.18. Thesamples with x¼ 0.06–0.18 possess large remanent polari-zation (Pr¼ 16.0–28.1mC cm�2), strong piezoelectricity(d33¼ 41–72 pCN�1 and kp¼ 16.4–21.1%), high mechani-cal quality factor Qm (632–1333) and Curie temperature(TC¼ 288–335 8C). Our results show that the ceramic withx¼ 0.12 is a good candidate material for piezoelectrictransformers because of the extraordinarily high Qm

value of 1333, good temperature stability, and high Curietemperature of 301 8C.

Acknowledgements This work was supported by theEducation Department and Science and Technology Bureau ofSichuan Province (11ZA104, 2010JQ0046), State Key LaboratoryCultivation Base for Nonmetal Composites and FunctionalMaterials of Southwest University of Science and Technology

(10zxfk27), State Key Laboratory of Electronic Thin Films andIntegrated Devices of University of Electronic Science andTechnology of China (KFJJ201108), and The Hong KongPolytechnic University internal research account 1-ZV9B.

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