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Keisuke Ishihama, [Takao Shimizu](https://orcid.org/0000-0001-9508-7601), Kazuki Okamoto, Akinori Tateyama, Wakiko Yamaoka, Risako Tsurumaru, Shintaro Yoshimura, Yusuke Sato, Hiroshi Funakubo

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[Achieving High Piezoelectric Performance across a Wide Composition Range in Tetragonal (Bi,Na)TiO<sub>3</sub>–BaTiO<sub>3</sub> Films for Micro-electromechanical Systems](https://mdr.nims.go.jp/datasets/6d5daa86-addf-4f13-a3a6-9021056bf5da)

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Achieving High Piezoelectric Performance across a Wide Composition Range in Tetragonal (Bi,Na)TiO3–BaTiO3 Films for Micro-electromechanical SystemsAchieving High Piezoelectric Performance across a WideComposition Range in Tetragonal (Bi,Na)TiO3−BaTiO3 Films forMicro-electromechanical SystemsKeisuke Ishihama, Takao Shimizu, Kazuki Okamoto, Akinori Tateyama, Wakiko Yamaoka,Risako Tsurumaru, Shintaro Yoshimura, Yusuke Sato, and Hiroshi Funakubo*Cite This: ACS Appl. Mater. Interfaces 2024, 16, 1308−1316 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: Tetragonal (1−x)(Bi,Na)TiO3−xBaTiO3 films exhibit enhanced piezoelectric propertiesdue to domain switching over a wide composition range. These properties were observed over asignificantly wider composition range than the morphotropic phase boundary (MPB), which typically hasa limited composition range of 1−2%. The polarization axis was found to be along the in-plane directionfor the tetragonal composition range x = 0.06−1.0, attributed to the tensile thermal strain from thesubstrate during cooling after the film formation. A “two-step increase” in remanent polarization againstan applied maximum electric field was observed at the high-field region due to the domain switching, anda very high piezoelectric response (effective d33 value, denoted as d33,f) over 220 pm/V was achieved for awide composition range of x = 0.2−0.5 with high tetragonality, exceeding previously reported values forbulk ceramics. Moreover, a transverse piezoelectric coefficient, e31,f, of 19 C/m2 measured using acantilever structure was obtained for a composition range of at least 10 atom % (for both x = 0.2 and0.3). This value is the highest reported for Pb-free piezoelectric thin films and is comparable to the bestdata for Pb-based thin films. Reversible domain switching eliminates the need for conventional MPBcompositions, allowing an improvement in the piezoelectric properties over a wider composition range.This strategy could provide a guideline for the development of environmentally acceptable lead-free piezoelectric films withcomposition-insensitive piezoelectric performance to replace Pb-based materials with MPB composition, such as PZT.KEYWORDS: (Bi, Na)TiO3−BaTiO3, Pb-free, piezoelectric film, domain switching, out-of-morphotropic phase boundary composition,tetragonal structure■ INTRODUCTIONPiezoelectric materials play a vital role in various applicationsincluding sensors, actuators, resonators, and vibration energyharvesters, particularly in the rapidly developing field of micro-electromechanical systems (MEMS).1−4 MEMS that usepiezoelectric devices, also called piezoelectric MEMS, haveadvantages over their electrostatic and electromagneticcounterparts, such as a simple structure and small size.5,6 Todate, Pb-based piezoelectric materials, such as Pb(Zr,Ti)O3(PZT) and Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN−PT), havebeen preferred because of their excellent piezoelectricproperties.7,8 However, environmental issues and stricterregulations such as the Restriction of Hazardous Substances(RoHS) directive have necessitated the development ofalternative materials that do not contain toxic Pb.9−11 In thecontext of next-generation smart electronics, robotics, and theInternet of Things (IoT), advances in Pb-free piezoelectric thinfilms are essential for addressing the requirements of high-performance, energy-efficient devices while aligning withsustainable development practices.12−15In the past, various strategies have been employed toimprove the piezoelectric properties, including lattice con-tribution enhancement, domain reconstruction, morphotropicphase boundary (MPB) engineering, and defect control.9,16−19Among these, MPB engineering has achieved significantbreakthroughs and established itself as an important researchstrategy. In particular, PZT exhibits excellent piezoelectricproperties near the MPB composition, where the PbTiO3-richtetragonal and PbZrO3-rich rhombohedral exist in approx-imately equal proportions. Therefore, Pb-free materials withMPB compositions have been actively studied as alternatives toPZT. Typical Pb-free MPB materials, including (K,Na)NbO3-and (Bi,Na)TiO3-based solid solutions, have been successfullyused in bulk-ceramic applications.20−22Despite remarkable progress in Pb-free piezoelectricceramics, the development of film-based piezoelectric materialsfor MEMS applications has challenges. A primary obstacleReceived: September 8, 2023Revised: December 6, 2023Accepted: December 10, 2023Published: December 28, 2023Research Articlewww.acsami.org© 2023 The Authors. Published byAmerican Chemical Society1308https://doi.org/10.1021/acsami.3c13302ACS Appl. Mater. Interfaces 2024, 16, 1308−1316This article is licensed under CC-BY 4.0Downloaded via UNIV OF PENNSYLVANIA on December 13, 2024 at 06:06:05 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Keisuke+Ishihama"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takao+Shimizu"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kazuki+Okamoto"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Akinori+Tateyama"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Wakiko+Yamaoka"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Risako+Tsurumaru"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Risako+Tsurumaru"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Shintaro+Yoshimura"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yusuke+Sato"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hiroshi+Funakubo"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acsami.3c13302&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/aamick/16/1?ref=pdfhttps://pubs.acs.org/toc/aamick/16/1?ref=pdfhttps://pubs.acs.org/toc/aamick/16/1?ref=pdfhttps://pubs.acs.org/toc/aamick/16/1?ref=pdfwww.acsami.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acsami.3c13302?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://www.acsami.org?ref=pdfhttps://www.acsami.org?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/arises from substrate clamping, which substantially diminishesthe electromechanical response, owing to the constraint of in-plane deformation of the film.23−27 In addition, stress from thesubstrate alters the phase boundaries of piezoelectric materials,diverging from their original composition in bulk ceramics.26Therefore, an accurate composition adjustment is necessary tooptimize performance. Piezoelectric properties typically exhibitsignificant sensitivity to composition near the MPB, renderingthe control of these properties challenging in thin films.7,28Consequently, exploration of alternative strategies withoutMPB is crucial for developing piezoelectric materials that canovercome the limitations imposed by substrate clamping andstress-induced phase-boundary shifts, thereby enhancing thepotential of thin-film piezoelectric materials for MEMSapplications.Domain switching has recently emerged as a promisingapproach for enhancing the piezoelectric properties in Pb-freepiezoelectric thin films for MEMS applications. This strategyfocuses on microdomain structure formation, which is criticalfor a substantial piezoelectric response of ferroelectric thinfilms. The application of an electric field causes domainswitching, and upon removing the electric field, the domainstructure returns to its original state owing to the clampingeffect of the substrate. This leads to macroscopic filmdeformation and large piezoelectric properties. A highpiezoelectric coefficient of 310 pm/V has been reported fora tetragonal Pb(Zr,Ti)O3 film using this domain-switchingconcept.29,30 The factors controlling the piezoelectric responsedue to domain switching are the lattice anisotropy and changesin the volume fraction of the domain. For instance, themobility of similar elastic domains in magnetostrictivematerials such as Ni−Mn−Ga alloys has been reported to bedominated by the lattice anisotropy and volume fraction of theferroelastic domains.31,32 These results suggest that themobility of the domains can be controlled by using externalelectric fields to enhance their contribution to the piezoelectricresponse. That is, if a film with a large volume fraction of in-plane polarized domains has appropriate tetragonality, a c/aratio (where c and a are the lattice parameters along the polaraxis, or c-axis, and nonpolar axis, or a-axis, respectively) can beprepared, and large piezoelectric properties can be obtained bydomain switching even for tetragonal films that have an out-of-MPB composition. Nakajima et al. reported that the large c/aratio of PbTiO3 (c/a ≈ 1.06) was decreased by creating a solidsolution with PbZrO3 in ferroelectric PZT thin films, and alarge piezoelectric response was obtained at a tetragonalcomposition near Zr/(Zr+Ti) = 0.4 (c/a ≈ 1.02), withrelatively low tetragonality than that of PbTiO3.29We focused on a Pb-free material system, (1−x)(Bi,Na)-TiO3−xBaTiO3 (BNT-BT), which is a solid solution oftetragonal BaTiO3 and rhombohedral (Bi,Na)TiO3. Thematerials in the ceramics exhibited tetragonal symmetry overa wide composition range of x = 0.06−1.0.33,34 Our group hadpreviously reported that a tetragonal 0.7(Bi,Na)TiO3−0.3BaTiO3 (x = 0.3) film prepared on a Si substrate exhibitedconsiderably large transverse piezoelectric coefficients (e31,f =19 C/m2) owing to domain switching.35 Rao et al. reportedthat the tetragonality hardly changed over a wide compositionrange of approximately 30 atom % for x = 0.2−0.5.33 Theseresults suggest the possibility that tetragonal (Bi,Na)TiO3−BaTiO3 films exhibit a large piezoelectric response over a widecomposition range via domain switching. Taking the conceptof domain switching into account, the choice of a materialsystem showing a stable c/a ratio, such as (Bi,Na)TiO3−BaTiO3, is optimal for achieving high piezoelectric propertiesover a wide composition range, which is in contrast to thecontinuous change of the c/a ratio with the Zr/(Zr+Ti) ratioin the case of PZT. However, there are few studies on thesystematic composition dependence of the ferroelectric andpiezoelectric properties of the tetragonal composition side of(1-x)(Bi,Na)TiO3−xBaTiO3 films, except for the highcharacteristics MPB neighborhood composition, which wasobserved at x = 0.04−0.07 composition.36−38In this study, we investigated the composition dependenceof the piezoelectric performance of (1−x)(Bi,Na)TiO3−xBaTiO3 (x = 0.06, 0.2, 0.3, 0.5, and 1.0) films on a Sisubstrate in the tetragonal phase to develop Pb-free piezo-electric thin films with a large piezoelectric response. As aresult, we confirmed an out-of-plane piezoelectric response ofd33.f higher than 220 pm/V, which exceeded the reported valuefor bulk ceramics in the composition region of x = 0.2−0.5.39Furthermore, e31,f of 19 C/m2 was confirmed for samples withcantilever structures in the composition range of at least 10atom % for x = 0.2 and 0.3. This value is the highest reportedfor Pb-free piezoelectric thin films and is comparable to thereported data for Pb-based materials.15 Furthermore, impor-Figure 1. (a) Out-of-plane and (b) in-plane XRD profiles of prepared (Bi,Na)TiO3−BaTiO3 (x = 0.06−1.0) films on (100)c(La0.5Sr0.5)CoO3/(100)c LaNiO3/ Pt/Ti/SiOx/(100)Si substrates.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c13302ACS Appl. Mater. Interfaces 2024, 16, 1308−13161309https://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig1&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c13302?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-astantly, the films exhibited a large property over a compositionrange several times wider than that of MPB, which has alimited compositional range of 1−2%. This enables obtainingsteady properties for the deposited films despite theircomposition fluctuation. This is an advantage over filmsusing the MPB composition, which require precise composi-tion control, owing to the high composition sensitivity of thepiezoelectric property. The present innovative concept ofreversible domain switching allows for a departure from theconcept of using conventional MPB compositions and allowsfor improved piezoelectric properties over a wider compositionrange. These results will expand the scope of research forpiezoelectric materials, which has focused mainly on Pb-basedmaterials, especially near MPB composition, for the last 70years.■ RESULTS AND DISCUSSIONFigure 1(a,b) shows the out-of-plane and in-plane X-raydiffraction (XRD) patterns of the 2.0 μm thick (1−x)(Bi,Na)-TiO3−xBaTiO3 films with x = 0.06−1.0 on the Si substratewith buffer layers, respectively. Wider 2θ and 2θχ range scandata are presented in Figure S1(a,b). The out-of-plane XRDpatterns shown in Figures 1(a) and S1(a) in the SupportingInformation and h00 or 00l diffraction peaks from tetragonal(Bi,Na)TiO3−BaTiO3 were observed together with h00cdiffraction peaks from other underlying perovskite layers withpseudocubic cells, such as LaNiO3 and (La0.5Sr0.5)CoO3. Inaddition, the shift of the (200) diffraction peaks to lower angleswith an increasing x value without obvious different phaseindicates the increase in the out-of-plane lattice parameter, a-axis, and the formation of solid solution as the bulk references33,40. The in-plane GIXRD patterns shown in Figures 1(b)and S1(b) in the Supporting Information show the peaksderived from the perovskite structure. In addition, the presenceof both {101} and {100} peaks in these in-plane measurementsindicates that the in-plane direction is polycrystalline, therebyindicating a uniaxially oriented film.Figure S2(a,b) shows surface and cross-sectional SEMimages, respectively.As shown in Figure S2(a), the thin film is composed ofgrains with almost uniform size. The random shape of thegrains corresponds to the in-plane polycrystalline nature of thefilm. In addition, as shown in Figure S2(b), the film with adense and no clear columnar structure was detected.Figure 2(a,b) illustrates the composition dependence of theout-of-plane and in-plane lattice parameters obtained from out-of-plane XRD θ−2θ and in-plane GIXRD scans along with thetetragonality, defined as {(out-of-plane lattice parameter)/(in-plane lattice parameter) − 1} and presented in Figure 2(b).The squares and diamonds represent the out-of-plane and in-plane lattice parameters, respectively, calculated from the{200} peak position of the out-of-plane XRD θ−2θ scan andthe {002} peak positions on the in-plane GIXRD pattern,respectively. The previously reported c- and a-axes data for thesintered body, depicted using closed33 and open40 circles, andc/a ratios, depicted using triangles, are plotted in Figure 2(a,b),respectively.The results shown in Figure 2(a) reveal that all of the filmsprepared in this study have smaller c-axis values and larger a-axis values than the bulk lattice parameters. It is alreadyascertained that this orientation did not dramatically change bychanging the underlying (La,Sr)CoO3 to other bottomelectrodes, such as SrRuO3 (not shown here).As shown in Figure 2(b), the observed tetragonality wasconsiderably smaller than the reported values for ceramics.Relatively higher tetragonality was observed in the approximatecomposition range of x = 0.2−0.5 among the films prepared inthis study, indicated using a hatch in Figure 2, while thetetragonality of the film at x = 0.06 was nearly 0%. Consideringthe pseudocubic structure reported for (Bi,Na)TiO3−BaTiO3at x = 0.04−0.07 in bulk ceramics, the results presented hereinfor the films were almost consistent with the reported results,and thus, the (Bi,Na)TiO3−BaTiO3 films reported herein havea tetragonal structure when x = 0.2−1.0.According to our previous reports, tetragonal (Bi,Na)TiO3−BaTiO3 films (x = 0.06−1.0) deposited on SrTiO3 substratesare epitaxial films and were subjected to detailed XRDanalysis.41,42 These results show that the volume fraction ofthe (100) orientation and non-180° domain fraction of the(100)/(001)-oriented ferroelectric films are determined by thethermal strain from the substrate; the thermal expansioncoefficient of the SrTiO3 substrate is 10.9 × 10−6/K,43 which islarger than that of (Bi,Na)TiO3−BaTiO3 (approximately 6 ×10−6 /K).33 Thus, the (Bi,Na)TiO3−BaTiO3 film on theSrTiO3 substrate was confirmed to have a pure (001)orientation and c-domain structure with a polarization axisalong the out-of-plane direction, which can be attributed to thein-plane compressive strain experienced during the coolingprocess after deposition. Conversely, Shimizu et al. reportedthat (Bi,Na)TiO3−BaTiO3 films deposited on Si substrateshave a (100) orientation and a-domain structure with apolarization axis along the in-plane direction due to the in-plane tensile strain because the Si substrate has a thermalexpansion coefficient smaller than that of the films,35 that is,3.6 × 10−6/K.44 The in-plane lattice parameters were largerthan the out-of-plane parameters for all films prepared in thisstudy. These results suggest that the (Bi,Na)TiO3−BaTiO3films on the Si substrate in this study are considered to be a-domain oriented films for all tetragonal compositions.Figure 3 shows the measured electrical properties. Figure3(a,b) illustrates the polarization−electric (P−E) curvesFigure 2. Composition (x) dependencies of (a) out-of-plane(squares) and in-plane (diamonds) lattice parameters and (b) thec/a ratio (squares) for (1−x)(Bi,Na)TiO3−xBaTiO3 films with x =0.06−1.0. The closed33 and open40 circles and triangles represent thereported values for the powders.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c13302ACS Appl. Mater. Interfaces 2024, 16, 1308−13161310https://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig2&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c13302?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asmeasured at 10 kHz with various amplitudes of the triangularwave for (1−x)(Bi,Na)TiO3−xBaTiO3 films at (a) x = 0.06and (b) x = 0.2, respectively, where the amplitudes increasedsequentially. The measurement was conducted on a pristineelectrode without any applied electric field. For these twocompositions, the remanent polarization (Pr) was wellsaturated above a high electric field amplitude of 200 kV/cm; however, the manner of saturation was different,Figure 3. (a, b) P−E hysteresis curves for various amplitudes of the maximum electric field from the first cycle sweep-up and (c, d) the Pr value as afunction of the amplitude of the maximum electric field for (1-x)(Bi,Na)TiO3−xBaTiO3 films with (c) x = 0.06 and (d) x = 0.2. The data from thefirst and second cycles of sweep-up are shown in panels (c) and (d) as closed circles and squares, respectively.Figure 4. (a) Second-time sweep-up P−E curves measured at 10 kHz and the electric field amplitude of 250 kV/cm and (b) unipolar-drivenstrain−electric field (S−E) curves measured at 10 kHz and the amplitude of 150 kV/cm after the poling treatment by an amplitude of +250 kV/cmfor (1−x)(Bi,Na)TiO3−xBaTiO3 (x = 0.06−1.0) films. Composition (x) dependencies of (c) Pr and (d) piezoelectric properties, d33.f, for anamplitude of 150 kV/cm after applying a poling electric field of 150 kV/cm (closed circles) and 250 kV/cm (closed squares). The closed39 andopen45 circles and triangles are the reported values for the powders.ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c13302ACS Appl. Mater. Interfaces 2024, 16, 1308−13161311https://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig4&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c13302?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asparticularly at a low electric field amplitude of <150 kV/cm.Small loops were observed at small amplitudes compared withthose at large amplitudes for films with x = 0.2. Figure 3(c,d)shows the Pr value as a function of the amplitude for films withx = 0.06 and 0.2, respectively. These measurements wereperformed twice, from low to high electric fields, for eachcomposition film, and the results for the first and secondsweeps are indicated using black squares and red circles,respectively. The black-circle plots in Figure 3(c) indicate thatthe Pr value shows similar behavior in the first and secondcycles for the film with x = 0.06, an increase with increasingelectric field amplitude, and saturation above the coerciveelectric field, in agreement with the gradual change in the P−Eloop in Figure 3(a). This behavior is typical of ferroelectricmaterials. Conversely, as represented by the circles in Figure3(d), the Pr value of the film with x = 0.2 tends to saturateagainst the amplitude twice; the first saturation occurs at arelatively low value below approximately 150 kV/cm, and thesecond saturation occurs above 200 kV/cm through a rapidincrease at 160−200 kV/cm.30 The value after the secondsaturation is 1.5−2 times larger than that after the firstsaturation.No abrupt increase in the Pr value was observed during thesecond cycle. Consequently, the Pr value observed in thesecond cycle was larger than that in the first cycle, as shown inFigure 3(d), at low-field amplitudes such as 120 kV/cm. Themeasured amplitude dependencies of the Pr for all (1−x)(Bi,Na)TiO3−xBaTiO3 (x = 0.06−1.0) films in the first andsecond cycles are also plotted in Figure S2(a,b). Two-stepincrements in Pr are observed for films with high tetragonalityin Figure 2(b), that is, those with x = 0.2, 0.3, and 0.5.Figure 4(a) illustrates the P−E relationships measured at 10kHz and the electric field amplitude of 250 kV/cm from thesecond cycle of sweep-up for (1−x)(Bi,Na)TiO3−xBaTiO3 (x= 0.06−1.0) films. Clear hysteresis loops originating fromferroelectricity were obtained for all films. In addition, Figure4(b) shows the unipolar strain−electrical field (S−E) curvesmeasured at 10 kHz and an amplitude of +150 kV/cm after thepoling treatment with an amplitude of +250 kV/cm.The composition dependence of Pr and piezoelectricproperties, that is, the longitudinal effective piezoelectricresponse d33.f, at an applied electric field amplitude of 150kV/cm is presented in Figure 4(c,d), respectively. In this study,d33.f was defined as Smax/Emax, where Smax and Emax are themaximum strain and electric field, respectively. In Figure4(c,d), the circles represent Pr and d33.f, at a 150 kV/cmelectric field amplitude after poling at +150 kV/cm, while thesquares represent data after poling at +250 kV/cm, that is, thecircles and squares in Figure 4(c) correspond to the Pr valuesof the first and second cycles at a 150 kV/cm amplitude,respectively, presented in Figure S3.In Figure 4(c), the composition dependence of the Pr valueshows a continuous decrease as the x value deviates from theMPB composition, x ≈ 0.06, for both the first and secondcycles. The Pr of the films was smaller than those reported forceramics and c-axis oriented films on SrTiO3 substrates.39,41This is mainly due to the a-axis orientation of these films andsuppressed tetragonality in comparison with bulk ceramics, asshown in Figure 2(b).39 Furthermore, comparing the data forthe first and second cycles of sweep-up in Figure 4(c) revealedthat the films with x = 0.2, 0.3, and 0.5 exhibit a “two-stepincrease” in Pr value, as confirmed from the data presented inFigures 3 and S2. These films exhibit relatively hightetragonality values, as shown in Figure 2(b). In bulk ceramics,it has been reported that tetragonal and rhombohedral phasescoexist at x = 0.05−0.07.28 In the present study, the remanentpolarization value of the film with x = 0.06 is larger than that ofthe tetragonal films with x = 0.2−1.0 and the a-domain as amajority orientation, suggesting that the film with x = 0.06 ispossible to include a rhombohedral phase that has a [111]polar axis. The observed tetragonality (axial ratio) of almostunity also supports the existence of a rhombohedral phase inthe film with x = 0.06. In Figure 4(d), the compositiondependence of d33,f in the first cycle exhibits a trend similar tothat of the ceramics, where the values decreased with the xvalue departing from the MPB composition, x ≈ 0.06.Conversely, in the second cycle, films with compositions of x= 0.2−0.5, where a two-step increase in Pr was observed inFigure S2(b), showed high d33,f values beyond 220 pm/V.These values surpass the results for the film at x = 0.06, thecomposition closest to the MPB, and those previously reportedfor ceramics at x = 0.06 (d33,f ≈ 140 pm/V) near MPBcomposition.45 In short, large d33.f above 220 pm/V wasobtained over a wide composition range of 30 atom % fortetragonal BNT-BT films far from the MPB composition and itexceeds the previously reported value for bulk ceramics withMPB composition, as expected. Here, Figure S4(a−e) showsthe atomic force microscopy (AFM) images of the films with x= 0.06−1.0. The randomly arranged and uniformly shapedgrains were observed for all films. These results maycorrespond to the in-plane polycrystalline characteristics ofthe films. The average roughness (Ra) plotted againstcomposition is shown in Figure S5. As shown in Figures4(d) and S5, these results show no strong correlation betweenthe composition dependences of Ra and d33,f.Owing to the large thicknesses of the films, the misfit strainsare considered to be almost relaxed during film deposition,which is introduced by the difference in the lattice parametersbetween the film and the substrate.46,47 Shimizu et al. explainedthat the release of tensile strain accumulated during coolingafter deposition at the Curie temperature resulted in theformation of BNT-BT films with a-domain dominantstructures having both a- and c-axes along the in-planedirection deposited on a Si substrate with a low thermalexpansion coefficient.29,35 In contrast, compressive strain isgenerated upon cooling below TC after film deposition, mainlyowing to expansion in the c-axis domain structure along the in-plane direction. This strain is relieved by an increase in thedomain wall owing to an external force, such as the applicationof an electric field, and Pr, namely, the out-of-plane polarizationcomponent, increases as shown in Figure 3(d). This “two-stepincrease” of the Pr value appears to begin by applying anelectric field of about 150 kV/cm and saturate at about 180kV/cm. These results suggest that the domain wall remainedafter the removal of the electric field, and applying a smallelectric field is sufficient to activate wall motion. Therefore, thedomain could be reversibly moved under an applied electricfield, thereby enhancing the piezoelectric response.The in situ XRD was performed under an applied electricfield to ascertain the mechanism of the large piezoresponse.Figure 5 compares the XRD patterns of the (1−x)(Bi,Na)-TiO3−xBaTiO3 films before (black lines), under (blue lines),and after (red lines) the application of a + 150 kV/cm electricfield for films with (a) x = 0.06 and (b) x = 0.2. As shown inFigure 5(a,b), the XRD patterns before and after theapplication of the electric field are almost the same for bothACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c13302ACS Appl. Mater. Interfaces 2024, 16, 1308−13161312https://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c13302?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asfilms with x = 0.06 and 0.2, and thus, the crystal structure ofthe films are the same, with the in-plane polarized a-domain asthe main orientation. In Figure 5(a), only the {200} peakappears in the diffraction pattern, even under the application ofan electric field, and it hardly changes compared with thediffraction peaks before and after applying the electric field forthe film with a x = 0.06 film. However, additional peaks locatedat an angle lower than the original position were observedunder an electric field for the film with x = 0.2, as shown inFigure 5(b). Considering that the original peak position wasidentified as a {200} peak originating from the in-planepolarized domain, the novel peak can be identified as the {002}peak from the out-of-plane polarized domain. This suggests achange in the polarization direction when an electric field isapplied; that is, the a-domain switches to the c-domain byapplying an electric field. When the electric field was turnedoff, the XRD pattern was almost the same as that before theapplication of the electric field, suggesting that reversibledomain switching occurred due to the application of theelectric field. The piezoelectric response d33.f from the domainswitching can be estimated by using the peak-fitting method.The estimated volume fraction of the c-domain wasapproximately 33% under the electric field, as shown in FigureS6 and Table S1. On the basis of this change in the volumefraction, d33,f ≈ 152 pm/V was calculated using the followingequation35dc V a V c V a Va V c VE(1 ) (1 )(1 )1fc c c cc c33,0 0 0 00 0 0 0={ × + × } { × + × }× + ××(1)where E, Vc, c, and a represent the electric field, c-domainvolume fraction, c-axis lattice parameter, and a-axis latticeparameter, respectively. The subscript “0” denotes noapplication of an electric field. The angle of incidence of theX-rays was not 90°, causing the beam to spread over theelectrode of interest in an elliptical shape with a long diameterof 400 μm and a short diameter of 100 μm. Therefore, even atthe best beam position, the XRD pattern comprisedapproximately 42% diffraction from outside the electrode,resulting in an underestimation of the d33,f value. If theelectrode diameter completely covered the beam diameter, d33,fwas 260 pm/V, which almost agreed with the results shown inFigure 4(b,d). This result suggests that domain switching is theorigin of the large piezoelectric response of the films with x =0.2, as shown in Figure 4, similar to that of films with x = 0.3,as demonstrated in our previous study.35 This can be explainedby the similar tetragonality of the two films, as shown in Figure2(b).Finally, the transverse piezoelectric coefficients, e31,f, whichare widely used to characterize the piezoelectric properties ofthin films, were measured to compare the piezoelectricproperties of films prepared on other substrates and films ofother materials with x = 0.2. The e31,f were calculated from thecurvature of the cantilever, owing to the actuation of thepiezoelectric film on the beam.48,49Figure 6(a) displays the estimated e31,f versus the measuredvoltage, with closed and open squares representing the resultsfor the Si and SrTiO3 substrates for films with x = 0.2,respectively, while closed and open triangles represent theresults for the Si and SrTiO3 substrates for films with x = 0.3.The films on SrTiO3 show e31,f values of 4−5 C/m2 for bothcompositions, which are comparable to those of otherperovskite epitaxial films.15 In contrast, the films on the Sisubstrate exhibited a high e31,f value of 19 C/m2 for both filmcompositions.35,50 No clear “two-step increase” was detected inthe dependence of e31,f on an applied electric field. This may bedue to the piezoelectric signal at the first step being too smallFigure 5. XRD θ−2θ patterns before (after poling) (red line), under(blue line), and after (green line) the application of an electric field ofapproximately 200 kV/cm for (1−x)(Bi,Na)TiO3−xBaTiO3 filmswith (a) x = 0.06 and (b) x = 0.2.Figure 6. (a) e31,f values as a function of the measurement voltage(peak to peak) for (Bi,Na)TiO3−BaTiO3 films, with x = 0.2 and 0.3,deposited on the Si substrates. (b) e31, f values obtained in this studyalong with the reported data for other piezoelectric films.15ACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c13302ACS Appl. Mater. Interfaces 2024, 16, 1308−13161313https://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig5&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig6&ref=pdfhttps://pubs.acs.org/doi/10.1021/acsami.3c13302?fig=fig6&ref=pdfwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c13302?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asand/or the differences in the measurement frequency andelectrode size for the d33,f and e31,f measurements. However, asshown in Figures 4 and 6, both d33,f and e31,f show large values,suggesting that domain switching was already completed whenthe 20 V pulsed wave was applied.Figure 6(b) presents a comparison of the e31,f valuesobtained in this study with those of previous studies on Pb-based perovskite films and Pb-free materials.15 The e31,f valuesof the (1−x)(Bi,Na)TiO3−xBaTiO3 (x = 0.2 and 0.335) filmsare the highest among the Pb-free materials, surpassing thoseof most Pb-based films, except for the relaxor Pb(Mg,Nb)O3−PbTiO3 and Nb-doped Pb(Zr,Ti)O3 films. More importantly,an e31,f value of 19 C/m2 was obtained for a composition rangeof at least 10 atom % range (for both of x = 0.2 and 0.3), whichis much wider than a morphotropic phase boundary with alimited composition range of 1−2%.These findings imply that an improved piezoelectricresponse using domain switching can pave the way forpractical applications in various devices, including thosebased on MEMS technology, owing to the large piezoresponseover a wide composition range.■ CONCLUSIONSTetragonal (1−x)(Bi,Na)TiO3−xBaTiO3 films were depositedon Si substrates over a wide composition range (x = 0.06, 0.2,0.3, 0.5, and 1.0), and the polarization axis was principallyaligned in the in-plane direction owing to the tensile thermalstrain from the substrate. XRD measurements revealed a trendof composition dependency for the tetragonality, similar tothat for bulk ceramics, with a maximum value at approximatelyx = 0.2−0.5; however, its absolute value was smaller than thatreported for bulk ceramics. For the high tetragonalitycomposition region, we observed a “two-step increase” inremanent polarization due to domain rearrangement under ahigh-field amplitude and an exceptional piezoelectric response(d33,f > ∼200 pm/V), surpassing reported values of 30% forbulk ceramics in the composition range 0.2−0.5. In situ XRDanalysis confirmed domain switching from in-plane to out-of-plane polarization for x = 0.2. e31,f of 19 C/m2 was observed forfilms in the 10% composition range of x = 0.2−0.3 usingcantilever structures; this e31,f was almost the highest value inPb-free materials and comparable to that of Pb-based ones.These results demonstrate good piezoelectric properties over acompositional range several times broader than the limitedMPB range of 1−2%. The innovative concept of reversibledomain switching facilitates improved piezoelectric propertiesover an extended composition range, in a departure fromconventional MPB compositions. We believe that the achieve-ment of high environmental sustainability and compositioninsensitivity in lead-free piezoelectric materials will inspirefurther exploration of piezoelectric materials, which have beendominated by Pb-based materials near MPB composition forthe last 70 years.■ EXPERIMENTAL SECTIONFilm Preparation. Approximately 2.0 μm thick (1−x)(Bi,Na)-TiO3−xBaTiO3 films with x = 0.06−1.0 were deposited by pulsedlaser deposition (PLD) at 675 °C for about 2 h under varying the O2pressure (200 mTorr) using a KrF excimer laser (λ = 248 nm andpower of 170 mJ). The targets used for the deposition were preparedvia a solid-state reaction of Bi2O3, Na2CO3, BaCO3, and TiO2powders, with an excess of 20 mol % bismuth oxide and sodiumcarbonate to compensate for the high volatility of Bi and Na, similarto the process used for sintered ceramics and other film-depositionprocesses.(Bi,Na)TiO3−BaTiO3 films were deposited on (100)-oriented Sisingle-crystal substrates covered with a Pt electrode, Pt/TiO2/SiOx/(100)Si. To deposit {100}-out-of-plane-oriented textured films, aLaNiO3 buffer layer, which can achieve {100}-preferred-orientedtextured films independent of the kinds of substrate,51,52 was insertedbetween the (La0.5Sr0.5)CoO3 electrode layer and the (111)Pt/TiO2/SiOx/Si substrates. LaNiO3 films were prepared by RF sputtering at350 °C and subsequent heat treatment at 800 °C, showing the (100)corientation (the subscript c indicates pseudocubic cells). (La0.5Sr0.5)-CoO3 films were prepared by using PLD to ensure sufficientconductivity of the electrode.XRD Analysis. The crystal structures of the prepared films wereanalyzed using X-ray diffraction (XRD; X’Pert-MRD, Philips, andSmartLab, Rigaku, λ = 0.154 nm). The ω-2θ scans were carried out toestimate the lattice parameters by performing 2θ scans while changingthe incident angle (ω). The 2θ position of Si (lattice parameter: 5.43)was used as a reference (Coll. Code: 51,688). The film thickness wasestimated using wavelength-dispersive X-ray fluorescence (WD-XRF;Axios PW4400/40, PANalytical), and the results were compared tothose of a reference sample. The crystal structures of the films underan applied electric field were investigated using a microfocus X-raydiffraction (XRD) setup with a 2D detector (Bruker AXS D8DISCOVER) by focusing X-rays on the Pt-top electrodes. X-rays werefocused onto a Pt-top electrode with ϕ = 200 μm, to which an electricfield of 250 kV/cm amplitude was applied, and diffraction patternswere collected by a two-dimensional detector. A collimator with apinhole with a 100 μm diameter was used.Microstructure Analysis. The surface morphology and cross-sectional microstructure were observed by using a field emissionscanning electron microscope (FESEM; Hitachi, S-4800) and anatomic force microscope (AFM) (SPA400, SII).Electrical Characterization. Pt-top electrodes with ϕ = 200 μmwere deposited on (Bi,Na)TiO3−BaTiO3 films via evaporation tomeasure electric and piezoelectric properties. The ferroelectricity atroom temperature for the Pt/(Bi,Na)TiO3−BaTiO3/(La0.5Sr0.5)CoO3capacitor was measured by using a ferroelectric tester (TOYO, FCE-1A) at 10 kHz. The electric-field-induced strain was recorded usinglaser Doppler vibrometers (LDV, Polytec, NLV-2500-5) simulta-neously with the P−E measurements. e31,f was determined from the tipdisplacement of the cantilever using the LDV. The sample length andthickness of the cantilevers are 11.5 mm and 780 μm for the Sisubstrate and 11.9 mm and 500 μm for the SrTiO3 substrate,respectively. The tip displacement was produced by applying asinusoidal voltage with various amplitudes and a bias of −10 V, whichhad been polled with a 20 V pulse wave.■ ASSOCIATED CONTENTData Availability StatementThe data supporting the findings of this study are availablefrom the corresponding author upon reasonable request.*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsami.3c13302.Out-of-plane and in-plane XRD patterns, SEM images,surface morphology, and remanent polarization as afunction of measured maximum amplitude (PDF)■ AUTHOR INFORMATIONCorresponding AuthorHiroshi Funakubo − School of Materials and ChemicalTechnology, Tokyo Institute of Technology, Yokohama 226-8502, Japan; Material Research Center for Element Strategy,Tokyo Institute of Technology, Yokohama 226-8502, Japan;orcid.org/0000-0002-1106-200X;Email: funakubo.h.aa@m.titech.ac.jpACS Applied Materials & Interfaces www.acsami.org Research Articlehttps://doi.org/10.1021/acsami.3c13302ACS Appl. Mater. Interfaces 2024, 16, 1308−13161314https://pubs.acs.org/doi/10.1021/acsami.3c13302?goto=supporting-infohttps://pubs.acs.org/doi/suppl/10.1021/acsami.3c13302/suppl_file/am3c13302_si_001.pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hiroshi+Funakubo"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://orcid.org/0000-0002-1106-200Xhttps://orcid.org/0000-0002-1106-200Xmailto:funakubo.h.aa@m.titech.ac.jpwww.acsami.org?ref=pdfhttps://doi.org/10.1021/acsami.3c13302?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asAuthorsKeisuke Ishihama − School of Materials and ChemicalTechnology, Tokyo Institute of Technology, Yokohama 226-8502, Japan; orcid.org/0000-0003-0798-8384Takao Shimizu − School of Materials and ChemicalTechnology, Tokyo Institute of Technology, Yokohama 226-8502, Japan; Research Center for Functional Materials,National Institute for Materials Science, Tsukuba 305-0044,Japan; orcid.org/0000-0001-9508-7601Kazuki Okamoto − School of Materials and ChemicalTechnology, Tokyo Institute of Technology, Yokohama 226-8502, JapanAkinori Tateyama − School of Materials and ChemicalTechnology, Tokyo Institute of Technology, Yokohama 226-8502, JapanWakiko Yamaoka − Technical Center, TDK corporation,Ichikawa, Chiba 272-8558, JapanRisako Tsurumaru − Technical Center, TDK corporation,Ichikawa, Chiba 272-8558, JapanShintaro Yoshimura − Technical Center, TDK corporation,Ichikawa, Chiba 272-8558, JapanYusuke Sato − Technical Center, TDK corporation, Ichikawa,Chiba 272-8558, JapanComplete contact information is available at:https://pubs.acs.org/10.1021/acsami.3c13302Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was partly supported by the Element StrategyInitiative to Form a Core Research Center of the Ministry ofEducation, Culture, Sports, Science, Technology of Japan(MEXT) Grant Number JPMXP0112101001, MEXT Pro-gram: Data Creation and Utilization Type Material Researchand Development Grant No. JPMXP1122683430, JSPSKAKENHI Grant Numbers 23KJ0903 (KI) and 19K15288(TS), and MEXT KAKENHI Grant Number 20H05185 (TS).■ REFERENCES(1) Muralt, P.; Baborowski, J. Micromachined Ultrasonic Trans-ducers and Acoustic Sensors Based on Piezoelectric Thin Films. J.Electroceram. 2004, 12 (1−2), 101−108.(2) Eom, C. B.; Trolier-McKinstry, S. Thin-Film PiezoelectricMEMS. MRS Bull. 2012, 37 (11), 1007−1017.(3) Sano, R.; Inoue, J.; Kanda, K.; Fujita, T.; Maenaka, K.Fabrication of Multilayer Pb(Zr,Ti)O3 Thin Film by SputteringDeposition for MEMS Actuator Applications. Jpn. J. Appl. Phys. 2015,54 (10S), No. 10ND03.(4) Ito, M.; Okada, N.; Takabe, M.; Otonari, M.; Akai, D.; Sawada,K.; Ishida, M. High Sensitivity Ultrasonic Sensor for HydrophoneApplications, Using an Epitaxial Pb(Zr,Ti)O3 Film Grown onSrRuO3/Pt/γ-Al2O3/Si. Sens. Actuators, A 2008, 145−146, 278−282.(5) Funakubo, H.; Dekkers, M.; Sambri, A.; Gariglio, S.;Shklyarevskiy, I.; Rijnders, G. Epitaxial PZT Films for MEMSPrinting Applications. MRS Bull. 2012, 37 (11), 1030−1038.(6) Ruppel, C. C. W. Acoustic Wave Filter Technology−A Review.IEEE Trans. Ultrason. Ferroelectr., Freq. Control 2017, 64 (9), 1390−1400.(7) Jaffe, B.; Roth, R. S.; Marzullo, S. Properties of PiezoelectricCeramics in the Solid-Solution Series Lead Titanate-Lead Zirconate-Lead Oxide: Tin Oxide and Lead Titanate-Lead Hafnate. J. 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