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Journal of the European Ceramic Society 36 (2016) 89–100

Contents lists available at www.sciencedirect.com

Journal of the European Ceramic Society

jo ur nal home p ag e: www. elsev ier .com/ locate / jeurceramsoc

nfluence of excesses of volatile elements on structure andomposition of solution derived lead-free (Bi0.50Na0.50)1xBaxTiO3 thinlms

. Pérez-Mezcuaa, M.L. Calzadaa, I. Bretosa, J. Ricotea, R. Jiméneza, L. Fuentes-Cobasb,. Escobar-Galindoa, D. Chateignerc, R. Sirerad,∗

Instituto de Ciencia de Materiales de Madrid (ICMM), C.S.I.C., 28049 Cantoblanco, Madrid, SpainCentro de Investigación en Materiales Avanzados, 31109 Chihuahua, MexicoCRISMAT—ENSICAEN and IUT-Caen, Université de Caen Basse-Normandie, 14050 Caen, FranceDepartamento de Química y Edafología, Facultad de Ciencias, Universidad de Navarra, 31008 Pamplona, Spain

r t i c l e i n f o

rticle history:eceived 30 June 2015eceived in revised form5 September 2015ccepted 16 September 2015vailable online 1 October 2015

eywords:

a b s t r a c t

The preparation of (Bi0.50Na0.50)1−xBaxTiO3 films requires a compositional/structural control, as theydetermine the functionality of these materials. We report a systematic compositional and structuralanalysis on (Bi0.50Na0.50)1−xBaxTiO3 films fabricated by chemical solution deposition. The effects of incor-porating Na(I) and Bi(III) excesses are analyzed through the comparison of the compositional depthprofiles of stoichiometric films (BNBT) and films containing excesses (BNBTxs). Heterogeneous compo-sitional profiles with larger bismuth content close to the substrate and thicker film-substrate interfacesare observed in BNBTxs, unlike stoichiometric films, which show atomic concentrations that correspond

hin filmead-freeerovskiteorphotropic phase boundary

hemical solution deposition

to the nominal composition of the precursor solution. Excesses induce structural differences in depth,observing a shift of the region of coexistence of rhombohedral and tetragonal phases (morphotropicphase boundary) toward higher x values and the formation of thick film-substrate interfaces. In contrast,stoichiometric films have homogeneous compositional and structural profiles with the MPB placed closeto that described for bulk ceramics.

© 2015 Elsevier Ltd. All rights reserved.

. Introduction

High performance piezoelectric ceramics, which are widely usedn micro- and nano-electromechanical systems (MEMS and NEMS),re mainly based on the lead zirconate titanate Pb(ZrxTi1−x)O3,PZT) [1,2]. The main concern over the use of this material is its envi-onmental impact, related to the pollutant lead content. Thus, theearch of lead-free alternatives is attracting much attention [3–5].

Among the lead-free materials, (Bi0.5Na0.5)TiO3 (BNT) [6] is anxample of a distorted perovskite where the charge differenceetween sodium and bismuth cations is large enough to providen energy ordering in the system. This results in a complex nano-omain structure, which confers it a particular electric response

7].

The main drawbacks of BNT, which limit its applications, arehe high conductivity and coercive field. These make difficult the

∗ Corresponding author.E-mail address: rsirera@unav.es (R. Sirera).

ttp://dx.doi.org/10.1016/j.jeurceramsoc.2015.09.023955-2219/© 2015 Elsevier Ltd. All rights reserved.

poling process and thus, modified BNT compositions are being used[8]. Among them, (Bi0.5Na0.5)1−xBaxTiO3 (BNBT) is considered anattractive system to be studied as it presents a morphotropic phaseboundary (MBP), similarly to PZT, where the electromechanicalproperties are maximized [9].

The MPB can be defined as the composition at which aphase transition occurs between two adjacent crystalline phasesthat have equal Gibbs free energy [10]. The optimization of theferro-piezoelectric properties at the MPB of BNBT was originallyexplained by the coexistence of tetragonal and rhombohedralphases [11]. However, the phase transformation seems to be pro-moted not only by the composition but also by the temperatureor even by an electric field; in this way the MPB in BNBT mate-rials could be modified during the poling process [12,13]. It hasbeen reported that the phase transition at the MPB occurs throughintermediate phases of monoclinic symmetry [14,15]. Furthermore,recently the structures observed at long-range order have been

found to be different from those detected at short-range order(nano-scale) in nanostructured BNBT ceramics [16], which shows

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he complexity of the crystallographic structure of these composi-ions.

Besides, it has been reported that for compositions with xetween 0.055 and 0.12 the BNBT solid solution exhibits a varietyf different crystalline structures. This has caused some contro-ersy, that has not been settled fully yet. For compositions between

∼ 0.06 and x ∼ 0.10, a cubic phase (Pm3̄m space group) is deter-ined by X-ray diffraction for unpoled BNBT [17,18]. This cubic

tructure is described as a multiphase system, which at short-rangerder is constituted by tetragonal nano-regions (P4bm space group)ith antiferroelectric or ferroelectric response, and ferroelectric

hombohedral regions (R3c space group). This structural model isn agreement with the relaxor-ferroelectric behavior reported inhe literature for these materials [19]. However, according to Jot al., a structural evolution from rhombohedral R3c/R3m to tetrag-nal P4mm occurs in the compositional range of 0.055 < x < 0.10020]. Free energies of these crystalline structures are very similar;hus, in BNBT bulk materials with close nominal compositions, dif-erent crystalline structures can be found. If the determination ofrystalline structures seems not to be clarified in bulk materials, itecomes a challenge in polycrystalline thin films.

The need to integrate this lead-free piezoelectric material inicroelectronic devices makes the fabrication of thin films essen-

ial. The high compositional control required to obtain films withompositions close to the MPB in these complex oxides makeshemical solution deposition (CSD) one of the most appropriateechniques to fabricate BNBT thin films [21]. For the process-ng it must be taken into account that the BNBT solid solutionontains two elements of high volatility, Na(I) and Bi(III). Theiross by volatilization during any of the thermal processes neededn CSD disrupts the compositional balance of the thin films. Inrinciple, high volatilization rates are expected due to the high sur-ace/volume ratio inherent in thin films. Traditionally excesses ofhese two volatile elements are incorporated in the precursor solu-ions to compensate the loss during the films annealing [22–25]. Itas been reported that the use of these excesses produces displace-ents of the MPB position for thin films [25] but the associated

ompositional and structural changes that lead to this shift areoorly understood.

Here, we show our recent results in solution derivedBi0.50Na0.50)1−xBaxTiO3 thin films, concerning the effects on theiromposition and crystalline structure produced by the use of Na(I)nd Bi(III) excesses in the precursor. At the same time, we exploren depth the diversity of crystalline phases present in these films

ith compositions around the MPB. The study is carried out byutherford Backscattering Spectroscopy and X-ray diffraction for aide range of x values (with x from 0.035 to 0.150), including those

eported as being in the MPB region for bulk ceramics. The structuref the (Bi0.50Na0.50)1−xBaxTiO3 thin films is studied in compari-on with their counterpart bulk ceramics prepared following andentical chemical route. The complexity of these analysis and thenterest of getting information about the structural profiles of thelms have required the use of Grazing-incidence X-ray diffractionGIXRD) with synchrotron radiation. New insights on the peculiar-ties of the crystalline characteristics of polycrystalline BNBT thinlms arise from the discussion of the results of this work.

. Experimental procedure

Details of the synthesis for (Bi0.50Na0.50)1−xBaxTiO3 precursorolutions can be found elsewhere [22,26]. It can be summarized as

ollows: (Bi0.50Na0.50)1−xBaxTiO3 precursor solutions were synthe-ized by a hybrid route. Solutions with the stoichiometric nominalompositions and different Ba(II) contents (x = 0.035, 0.055 and.100) were prepared; they are denoted as BNBT3.5, BNBT5.5

an Ceramic Society 36 (2016) 89–100

and BNBT10.0, respectively. Similarly, (Bi0.55Na0.55)1−xBaxTiO3.30nominal solutions containing a 10 mol% excess of Na(I) and a10 mol% excess of Bi(III), and with different concentrations of Ba(II)(x = 0.055, 0.100, 0.150), hereinafter BNBTxs5.5, BNBTxs10.0 andBNBTxs15.0, were also synthesized.

Diluted solutions (0.20 M) in dried ethanol were deposited ontoPt/TiO2/SiO2/(100)Si substrates (Radiant Technologies) by spincoating at 2000 rpm for 45 s and dried at 350 ◦C for 60 s, in a hotplate. The as-deposited amorphous films were crystallized by rapidthermal processing (RTP, JetStar 100T JIPELEC) in an oxygen atmo-sphere at 650 ◦C for 60 s (heating rate of 30 ◦C s−1). Deposition,drying and crystallization were repeated six times. Compositionsof the (Bi0.50Na0.50)1−xBaxTiO3 films were selected (i) for x val-ues lower than the MPB region; (ii) for x values inside the MPBregion, and (iii) for x values higher than the MPB region; that meansx = 0.035, 0.050 and 0.100 for BNBT films and x = 0.055, 0.100 and0.150 for BNBTxs films [19,26].

In addition to BNBT stoichiometric and BNBTxs thin films, con-taining 10 mol% excess of Na(I) and Bi(III), BNBT stoichiometricbulk ceramics were studied. For the preparation of bulk ceram-ics which are taken as a reference, the same solutions with thestoichiometric nominal compositions for x = 0.035, 0.055 and 0.100were used. Significant losses of volatiles elements are not expectedduring annealing of bulk ceramics, therefore Na(I) and Bi(III)excesses were not used here [17,18]. The solutions were driedat 120 ◦C for 12 h in air. The gels were heated at 350 ◦C inair for 12 h and subsequently annealed at 800 ◦C for 2 h (heat-ing rate 2 ◦C min−1). The powders were mixed with 2-propanol((CH3)2CHOH, Aldrich, 99.5%) used as a binder, and were pressedinto discs of 10 mm of diameter and 2 mm of thickness. The greenceramics were sintered in air at 1100 ◦C for 2 h (heating rate2 ◦C min−1). The bulk density of the resulting ceramics were deter-mined by the Archimedes method. The calculated values were5.62 g cm−3 (densification of 94.9%), 5.83 g cm−3 (densification of98.5%) and 5.72 g cm−3 (densification of 96.6%) for the BNBT3.5,BNBT5.5 and BNBT10.0 bulk ceramics, respectively.

Rutherford Backscattering Spectroscopy (RBS) experimentswere performed to analyze the compositional depth profile of thefilms. A 5 MV HVEE Tandetron accelerator was used with a 2 MeVHe+ beam. The data were acquired with a silicon surface barrierdetector located at a scattering angle of 170◦, with an energy reso-lution of 16 keV at an ion dose of 10 �C. The experimental spectrawere fitted with the software RBS [27].

Plan-view and cross-section micrographs of the crystallineoxide films were obtained by field-emission gun scanning electronmicroscopy (FEG-SEM, Nova Nanosem 230 FEI Company equip-ment, Hillsboro, OR).

The crystalline phases were initially studied using a SiemensD500 powder diffractometer with a Cu anode and a Bragg–Brentanogeometry. Diffraction patterns of the bulk ceramics were measuredin the 2� interval between 20◦ and 50◦, with a step of 0.05◦ pereach 3 s. More detailed patterns were recorded in the 2� intervalsbetween 37.0–42.0◦ and 46.0–47.5◦, using a step of 0.005◦ per each5 s. These patterns were analyzed with the V1–40 program. Peaks ofthe patterns were separated and fitted to pseudo-Voigt 2 functions.

The crystalline structure developed in the films was studiedusing a four-circle diffractometer equipped with a closed Euleriangoniometer (�, �), a Cu anode, a 120◦ curve linear position-sensitive detector (CPS120 from INEL SA) and a flat graphiteprimary monochromator. XRD patterns of 120◦ in 2� were recordedin a regular grid of 5◦ × 5◦ in � and �, with � from 0◦ to 50◦ and� from 0◦ to 355◦ (a total of 864 patterns). Rietveld refinement

calculations were carried out with the Materials Analysis UsingDiffraction Package (MAUD) [28]. The refinements were performedon the patterns resulting from the sum of the collected ones foreach angular position (�, �), in order to analyze a randomized

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D. Pérez-Mezcua et al. / Journal of the E

attern of the films. The random texture model has been selectedor carrying out the simulations of the experimental patterns, sincehanges of the intensity of the perovskite peaks with the angularosition have not been observed. Two layers were included for thetting: (i) a perovskite layer containing a R3c rhombohedral and/or4mm tetragonal structure and (ii) a Pt substrate layer with a cubictructure (Fm3̄m).

Grazing-incidence X-ray diffraction (GIXRD) was carried outt the beamline 11–3 of the Stanford Synchrotron Radiationightsource (Stanford, USA). A MAR345 two-dimensional (2D)osition-sensitive detector was used with a sample-detector dis-ance of 180 mm. The 2D diffraction patterns were recorded using

X-ray wavelength of � = 0.9744 Å and incidence angles of 0.05◦

nd 0.15◦ to study the structural changes at different depth of theNBT5.5 and BNBTxs10.0 films. Experimental 1D patterns werebtained by integration of the Debye rings processed by the soft-are Fit-2D and the phases fractions as well as the cell sizes were

efined with the software Fullprof. 2D intensity data have not beenorrected before the integration with Fit2D.

ig. 1. Compositional depth profiles of the BNBT thin films (stoichiometric samples), calcNBT10.0 thin films on Pt-coated silicon substrates. The experimental (dotted black linelms. (For interpretation of the references to color in this figure legend, the reader is refe

an Ceramic Society 36 (2016) 89–100 91

3. Results

In this work, three sets of samples were studied: (i) BNBT sto-ichiometric thin films, (ii) BNBTxs thin films, containing 10 mol%excess of Na(I) and Bi(III), and (iii) BNBT stoichiometric bulk ceram-ics (significant losses of volatile elements are not expected duringannealing of bulk ceramics, therefore Na(I) and Bi(III) excesses werenot used here) [17,18].

Figs. 1 and 2 show the average of atoms per cell across the film(a–c) and the corresponding experimental and fitted RBS curves(d–f) for the BNBT and BNBTxs thin films, respectively. Simulationsof the experimental RBS data allow us to gain a deep knowledgeof the compositional depth profile in the films. The calculated datafrom the RBS spectra are shown in Table 1. They reveal three differ-entiated zones/layers across the films: (i) a bismuth deficient layeron the surface, (ii) a bulk layer with a composition close to thatof the nominal composition of the precursor solution, considering

the composition of the (Bi0.55Na0.55)1−xBaxTiO3.30 (for BNBTxs) or(Bi0.50Na0.50)1−xBaxTiO3 (for BNBT) solid solution, and (iii) a bot-tom interface formed by the interaction between the film and thePt-coated substrate. Film thickness has also been calculated from

ulated from the RBS experimental curves, for the (a) BNBT3.5, (b) BNBT5.5 and (c)) and fitted (solid red line) RBS spectra for (d) BNBT3.5 (e) BNBT5.5 (f) BNBT10.0

rred to the web version of this article.)

92 D. Pérez-Mezcua et al. / Journal of the European Ceramic Society 36 (2016) 89–100

F Bi(III)B erimeB is figu

tfpfintascpwaoiwoatbo

c

ig. 2. Compositional depth profiles of BNBTxs thin films (samples with Na(I) and

NBTxs10.0 and (c) BNBTxs15.0 thin films on Pt-coated silicon substrates. The expNBTxs10.0 (f) BNBTxs15.0 films. (For interpretation of the references to color in th

he RBS data, considering the density of the perovskite obtainedrom the resolved unit cells by Rietveld refinement of the X-rayatterns shown in Table 2. For the BNBT3.5, BNBT5.5 and BNBT10.0lms, the average composition per atom (value normalized to theominal composition of the (Bi0.50Na0.50)1−xBaxTiO3 perovskite) inhe bulk layer, calculated from the simulation, is x ∼ 0.035, ∼0.055nd ∼0.100. They are practically identical to the nominal solidolution composition (Table 1). Most of the film thickness (∼90 %alculated value for the three films) presents the nominal com-osition, which proves the compositional homogeneity achievedithout the introduction of any excess in the precursor solution. In

ddition, these BNBT films exhibit a rapid increase in the intensityf the signal around 1400 keV, which corresponds to the scatter-ng cross section of the platinum (Fig. 1d–f), and can be associated

ith an abrupt film/substrate interface. The calculated thicknessf these BixPt interfaces is <6% for the three BNBT films, which isssociated with a low degree of interdiffusion between the film andhe Pt electrode. A thin bismuth deficient layer on the surface haseen also found which may account for the expected volatilization

f this element in the surface of the film during processing.

Unlike BNBT stoichiometric thin films, the experimental RBSurves of the BNBTxs5.5, BNBTxs10.0 and BNBTxs15.0 films show a

excesses), calculated from the RBS experimental curves, for the (a) BNBTxs5.5, (b)ntal (dotted black line) and fitted (solid red line) RBS spectra for (d) BNBTxs5.5 (e)re legend, the reader is referred to the web version of this article.)

gradual increasing of the intensity at around 1400 keV, (Fig. 2d–f). Inthese films, the calculations show that the BixPt interfaces are muchthicker (>10%, Table 1) than those of the BNBT films. A bismuth gra-dient seems to occur in the BNBTxs films, with a Bi-concentrationincrease from the surface to the substrate (Fig. 2). The bulk layer ofthese films has an average composition per atom with x values thatdiffer from the nominal ones, ∼0.055, ∼0.120 and ∼0.135, whenthe expected x values are 0.055, 0.100 and 0.150 (Table 1). More-over, the thickness of the bulk perovskite layer simulated in theseBNBTxs films has an average value of ∼80% of the total film thick-ness, far from the average value of ∼90% calculated for the BNBTfilms. Besides, a pure platinum layer, without bismuth contamina-tion does not give a reliable fit, indicating the massive interdiffusiongenerated in the BNBTxs films.

Fig. 3 shows the FEG-SEM micrographs of BNBT and BNBTxsthin films onto Pt/TiO2/SiO2/Si(100) (plan-view and cross sectionimages). Thicknesses of the BNBT3.5, BNBT5.5 and BNBT10.0 filmsobtained from these micrographs (Fig. 3a–c at the bottom) arecollected in Table 1 and show good agreement with those calcu-

lated from the RBS data. The BNBT5.5 film exhibits a low porosityon the surface and a dense microstructure across the film, withwell-defined grains of ∼110 nm. The BNBT3.5 and BNBT10.0 films

D. Pérez-Mezcua et al. / Journal of the European Ceramic Society 36 (2016) 89–100 93

Table 1RBS data for BNBT and BNBTxs thin films. The total thickness of the simulated layers (atom/cm2) up to 60% of Pt content is used to calculate the percent of the surface, bulkand interface thicknesses of each sample. The densities calculated from the crystallographic perovskite data obtained from the corresponding XRD patterns of the BNBT andBNBTxs films are considered to calculate the thicknesses from the RBS curves. Error in the composition calculated by RBS is 10%. Errors in the thicknesses calculated fromRBS and SEM are 5%. The RBS regions considered for the calculations are: (i) a bismuth deficient layer on the surface (ii) a bulk layer with a composition close to that of thenominal perovskite (in the nominal composition column) and (iii) a film-substrate interface layer with a composition close to the BixPt alloy.

Data from RBS analysis Thickness (nm) calculated from RBSand SEM

Sample Nominal composition Areas % of the total thickness RBS SEM Differ. (%)

BNBT3.5 (Bi, Na)0.965Ba0.035TiO3 Surface deficient on Bi 4 346 364 5Bulk:(Bi,Na)0.965Ba0.035TiO3 90Interlayer Bi0.82Pt 6

BNBT5.5 (Bi, Na)0.945Ba0.055TiO3 Surface deficient on Bi 5 330 341 3Bulk:(Bi,Na)0.945Ba0.055TiO3 92Interlayer Bi0.64Pt 3

BNBT10.0 (Bi, Na)0.900Ba0.100TiO3 Surface deficient on Bi 3 430 435 1Bulk:(Bi,Na)0.950Ba0.100Ti0.95O3 93Interlayer Bi0.48Pt 4

BNBTxs5.5 (Bi, Na)1.040Ba0.055TiO3.095 Surface deficient on Bi 5 482 559 14Bulk:(Bi,Na) 1.069Ba0.055Ti0.95O2.93 80Interlayer Bi0.64Pt 15

BNBTxs10.0 (Bi, Na)0.990Ba0.100TiO3.095 Surface deficient on Bi 4 484 553 13Bulk:(Bi,Na)1.005Ba0.12Ti0.98O2.92 85Interlayer Bi0.32Pt 11

BNBTxs15.0 (Bi, Na)0.925Ba0.150TiO3.095 Surface deficient on Bi 10 513 560 8Bulk:(Bi,Na)0.904Ba0.135Ti0.96O3 74Interlayer Bi0.44Pt 16

Table 2Data calculated using a Rietveld refinement of the experimental summed XRD patterns recorded for the BNBT and BNBTxs thin films, using a four-circle diffractometer.

Sample Phase Space group aR or aT(Å) cR or cT(Å) Cell volume (Å3) Mass per unit cell (g) Density (g/cm3) �2 Volume fraction (%)

BNBT3.5 Rhombohedral R3c 5.39 11.48 289 2.06 × 10−21 7.127 1.11 99.9BNBT5.5 Tetragonal P4mm 3.87 3.9 59 3.57 × 10−22 6.065 1.12 29

Rhombohedral R3c 5.52 11.67 308 2.14 × 10−21 6.956 71BNBT10.0 Tetragonal P4mm 3.87 3.89 59 3.63 × 10−22 6.192 1.176 99BNBTxs5.5 Rhombohedral R3c 5.44 11.69 300 2.16 × 10−21 7.195 1.19 99

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BNBTxs10.0 Tetragonal P4mm 3.87 3.9

Rhombohedral R3c 5.45 11.68

BNBTxs15.0 Tetragonal P4mm 3.87 3.89

how a higher porosity on the surface and across the sample, withlightly smaller grains of ∼70 nm (Fig. 3a–c at the top). However,he microstructures of the BNBTxs films are more heterogeneous,ith high porosity along the entire film thickness (Fig. 3d–f at the

op). Thicknesses calculated from the cross-section micrographsFig. 3d–f at the bottom) are collected in Table 1. In this case, mostrobably due to the appearance of a thicker interface with the sub-trate that may distort the thickness estimations from RBS data,he thickness values obtained by FEG-SEM differ significantly fromhose of the RBS data, around 10%. A larger grain size is observedor BNBTxs5.5 (∼100 nm) compared with those of the BNBTxs10.0nd BNBTxs15.0 films (∼90 nm and ∼80 nm, respectively).

Firstly, the structure of the BNBT bulk ceramics has first beentudied in order to assess the space groups of the phases devel-ped in the BNBT materials of this work. X-ray diffractograms ofhe BNBT3.5, BNBT5.5 and BNBT10.0 bulk ceramics (Fig. 4) clearlyllustrate the splitting of the reflections assigned to the tetrago-al or/and rhombohedral phases (Fig. 4b and c). The broadening ofhe peaks in thin films, as observed in the X-ray patterns of Fig. 5,aused by small grains and stresses induced by the substrate doesot allow us to distinguish this splitting.

The XRD pattern of BNBT3.5 bulk ceramic was obtained andnterpreted to be used as a reference in the study of the relatedhin films. The crystalline structure of the bulk ceramic of this com-osition is presently associated with the rhombohedral R3c space

roup [29] or to the monoclinic Cc group [30]. In the diffractionxperiments, the differences emerging from the mentioned modelsre subtle. For the resolution and counting statistics of our experi-ent, the R3c model works satisfactorily.

3.69 × 10 6.274 1.201 182.22 × 10−21 7.375 823.68 × 10−21 6.321 1.158 99.4

In the case of the tetragonal BNBT10.0 bulk ceramic, the doubletscentered at 2� ∼22.1◦ and 2� ∼46.8◦, ascribed to the (0 0 1)/(1 0 0)and (0 0 2)/(2 0 0) reflections, respectively, are characteristic of atetragonal P4mm space group (Fig. 4a and c, at the top).

The XRD analysis of these ceramics confirms that the MPBis around x = 0.055, where the coexistence of phases is observed(Fig. 4a–c, patterns in the middle).

The XRD patterns of the films in a powder diffractometer (Fig. 5)only exhibit pseudocubic-like signature of the parent perovskitephase. As expected for thin films, broad peaks are observed [31]. Thesymmetrical-centered pseudo-cubic phase detected in these pat-terns would not explain the piezo-ferroelectric behavior reportedfor these films [32–35]. Thus, a much more detailed structuralanalysis is needed to get deeper knowledge on the structuralcharacteristics of these films, the reason why we complementedour measurements using four-circle diffractometry and grazing-incidence X-ray diffraction (GIXRD) with synchrotron radiation.

Four-circle diffractometry (Figs. 6 and 7) offers the possibilityto collect a large number of patterns in as many sample orienta-tions as necessary to ensure suitable structural refinement. Thus,we used the sum diagram for Rietveld refinement. Such a sum dia-gram reduces considerably the textural effects associated with boththe film (only briefly considered here) and <1 1 1> Pt coated sili-con substrate. Fig. 6 shows the good refinement quality obtainedon such summed diagrams. The lattice parameters and the volume

fraction of each phase obtained from the Rietveld refinement of thepatterns are shown in Table 2. The goodness-of-fit (�2) always showvalues close to 1, which confirms the reliability of the refinements.

94 D. Pérez-Mezcua et al. / Journal of the European Ceramic Society 36 (2016) 89–100

F the B(

phrrrC[srtsofsp(piA

ig. 3. FEG-SEM plan views (at the top) and cross-section images (at the bottom) ofd) BNBTxs5.5 (e) BNBTxs10.0 (f) BNBTxs15.0.

For the calculations, R3c space group for the rhombohedralhase and P4mm for the tetragonal one have been chosen. Theyave been identified in the analysis of the XRD data of their cor-esponding bulk ceramics above (Fig. 4). Previous studies haveeported alternative space groups for BNBT materials: R3m for thehombohedral phase, P4bm for the tetragonal, and the monoclinicc symmetry as an averaged ‘best fit’ of the aforementioned phases7,12,15,16,29,30]. Our calculations show that the use of thesepace groups produce worse adjustment than the refinements car-ied out assuming the R3c space group for the rhombohedral andhe P4mm space group for the tetragonal phase. Reflections corre-ponding to the Pt are fitted to a cubic structure (Fm3̄m). In the casef BNBT5.5 and BNBTxs10.0 samples, a low reliability is obtainedor the refinement to a tetragonal (P4mm) or rhombohedral (R3c)tructure separately (Fig. 7). However, the adjustment using bothhases, what is distinctive of the MPB, give a better fit reliability

Fig. 6b and e, Table 2). The lattice parameters found for the twohases do not vary significantly, neither when the phases coexist

n the film nor when excesses are used in the precursor solutions.lso the relative content of the two phases in the films showing

NBT and BNBTxs films crystallized at 650 ◦C: (a) BNBT3.5 (b) BNBT5.5, (c) BNBT10.0

phase coexistence are not very different, showing always a largepercentage of the rhombohedral phase.

An important matter on resolving the crystalline structure ofBNT bulk materials is due to the reported low distortion withrespect to a cubic perovskite of both, tetragonal and rhombohe-dral phases, which coexist in the MPB of this compound [15,36].The incorporation of BT to the BNT solid solution increases signifi-cantly the distortion of the perovskite cell in [1 1 1] (rhombohedraldistortion) and [1 0 0] (tetragonal distortion). This is observed bya clear splitting of the reflections detected in the bulk ceram-ics of this work with a conventional source of X-ray (Fig. 4).Moreover, the structural study is much more complicated in thinfilms, because of the characteristic broadening of the diffractionpeaks collected by X-ray diffraction in polycrystalline thin films.Therefore, the use of more powerful tools of characterization isdemanded for the study of the crystalline structure of these

materials in thin film form. Grazing-incidence X-ray diffraction(GIXRD) using synchrotron radiation is used here because it allowsus to evaluate possible structural differences in the film depth. Infact, variations of the incidence angle permit the characterization

D. Pérez-Mezcua et al. / Journal of the European Ceramic Society 36 (2016) 89–100 95

F 10.0 be with

oatfim

Fp

ig. 4. XRD patterns of BNBT3.5 (at the bottom), BNBT5.5 (in the middle) and BNBTach 3 s (b) 2� values from 37.0◦ to 42.0◦ and (c) 2� values from 46.0◦ to 47.5◦ , both

f structural changes from the surface to the bulk of the films. Inddition, the high flux of photons of this source makes possible

o detect the presence of phases with a low-concentration in thelms. However, grazing incidence produces an additional instru-ental broadening of the diffraction peaks and thus, the resolution

ig. 5. XRD patterns of the (a) BNBT stoichiometric thin films and (b) BNBTxs thin filmserovskite phase.

ulk ceramics (at the top): (a) 2� values from 20.0◦ to 50.0◦ with a step of 0.05◦ pera step of 0.005◦ per each 5 s.

in the cell size is lower than that expected for a synchrotron sourcewhere a grazing incidence would not be used.

The 2D diffraction patterns obtained with incident angles ωof 0.05◦ and 0.15◦, are shown in Fig. 8 (BNBT5.5 film) and Fig. 9(BNBTxs10.0 film), together with their respective 1D patternsobtained by integration of the Debye–Sherrer rings. The Rietveld

, containing Na(I) and Bi(III) excesses. Pv indicates the reflections ascribed to the

96 D. Pérez-Mezcua et al. / Journal of the European Ceramic Society 36 (2016) 89–100

Fig. 6. Experimental (blue dotted line) and fitted (black solid) XRD patterns summed over sample orientations for BNBT (a–c) and BNBTxs (d–f) thin films. *Indicates peaksassociated with the sample holder. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Experimental (blue dotted line) and fitted (black solid) XRD patterns summed over sample orientations for BNBT5.5 (a–b) and BNBTxs10.0 (c–d) thin films. Ther mboha finemr

poai

efinements are performed in the experimental profiles using exclusively the rhossociated with the sample holder. The goodness-of-fit (�2) are included for each reeferred to the web version of this article.)

rofile refinements are also shown in the latter. The 2D patterns

nly show the presence of the perovskite structure from the filmsnd a cubic phase from the Pt substrate. Reflections correspond-ng to the Pt are fitted to a cubic structure (Fm3̄m) with a cell

edral or the tetragonal phases used in the adjustments of Fig. 6. *Indicates peaksent. (For interpretation of the references to color in this figure legend, the reader is

size of 3.90(7) Å (Fig. 8) and 3.89(5) Å (Fig. 9). The peak with the

highest intensity in all patterns is that recorded at 2� ∼20◦, whichcorresponds to the (1 1 0/1 0 1) perovskite reflections of the BNBTfilm. The lattice parameters and percentage of phases of these films,

D. Pérez-Mezcua et al. / Journal of the European Ceramic Society 36 (2016) 89–100 97

Fig. 8. Synchrotron radiation grazing-incidence scattering of the BNBT5.5 thin film. Experimental 2D diffraction patterns are measured with incident angles of (a) ω = 0.05◦

and (b) ω = 0.15◦ . (c) and (d) show the 1D patterns obtained by integration of the corresponding 2D patterns with their Rietveld refinements. Interval from 27.03◦ to 27.74◦ ,which presents a reflection associated with the sample holder, has been excluded for the fit of 1D pattern recorded at ω = 0.05◦ . R and T indicate rhombohedral and tetragonalstructures, which are ascribed to the perovskite phase. Pt indicates the (1 1 1) and (2 0 0) reflections of the Pt substrate at 25◦ and 29◦ .

Fig. 9. Synchrotron radiation grazing-incidence scattering of the BNBTxs10.0 thin film. Experimental 2D diffraction patterns are measured with an incident angle of (a)ω ion ofr e. Pt ia

oT

osm

= 0.05◦ and (b) ω = 0.15◦ . (c) and (d) show the 1D patterns obtained by integrathombohedral and tetragonal structures, which are ascribed to the perovskite phasssociated with a secondary pyrochlore phase (S) is observed in the pattern (c).

btained from the refinement of the GIXRD profiles, are shown inable 3.

For the BNBT5.5 film, secondary phases are not detected on any

f the diffraction patterns (Fig. 8), neither those showing moreuperficial information of the films (ω = 0.05◦) nor those containingore information from the bulk (ω = 0.15◦). The relative contents of

the corresponding 2D patterns with their Rietveld refinements. R and T indicatendicates the (1 1 1) and (2 0 0) reflections of the Pt substrate at 25◦ and 29◦ . A peak

rhombohedral and tetragonal phases are not significantly differentwhen the incident angle is changed (Table 3), and they are similarto the values obtained by using four-circle diffractometry (Table 2).

The differences observed for the lattice parameters are within theerror of the calculations, thus proving the structural homogene-ity across the film thickness for samples prepared from solutions

98 D. Pérez-Mezcua et al. / Journal of the European Ceramic Society 36 (2016) 89–100

Table 3Data calculated from the Rietveld refinements of the synchrotron radiation grazing-incidence patterns (� = 0.9744 Å) of Figs. 8 and 9, corresponding to the BNBT5.5 andBNBTxs10.0 thin films, respectively, and at two different incident angles, ω = 0.05◦ (surface of the film) and ω = 0.15◦ (bulk film).

BNBT5.5 BNBTxs10.0Incident angle (◦) 0.05 0.05

Crystal system Rhombohedral Tetragonal Rhombohedral TetragonalSpace group R3c P4mm R3c P4mm

aR(Å)/aT(Å) 5.39(8) 3.93(0) 5.64(0) 3.90(5)cR(Å)/cT(Å) 11.85(9) 3.94(3) 12.16(2) 3.91(5)

Volume fraction (%) 79.(0) 21.(0) 41.(8) 58.(2)

BNBT5.5 BNBTxs10.0Incident angle (◦) 0.15 0.15

Crystal system Rhombohedral Tetragonal Rhombohedral TetragonalSpace group R3c P4mm R3c P4mm

3.93.928

waiTtaccvdacfpbc

4

fispfitaiofagairgpttmdolIp[

aR(Å)/aT(Å) 5.43(3)

cR(Å)/cT(Å) 11.74(0)

Volume fraction (%) 72.(0)

ithout excesses. However, for the BNBTxs10.0 film, a small peakscribed to a secondary pyrochlore phase is observed at 2� ∼9◦

n the pattern measured at the lowest incident angle (ω = 0.05◦).his suggests the presence of a secondary phase at the surface ofhe film (Fig. 9c) as the relative intensity of this peak decreasess the incident angle increases (Fig. 9d). In addition, the relativeontents of the rhombohedral and tetragonal phases substantiallyhange with the incident angle. The results show that when theolume of material analyzed is more superficial, the rhombohe-ral phase represents a ∼42%, increasing to ∼89% when the X-raynalysis involves the total volume of the bulk of the film. This per-entage is even above the one obtained previously (Table 2). Apartrom these results that show the inhomogeneity of the films pre-ared from solutions with excesses, the cell parameters seem note significantly affected and only variations within the error of thealculations are observed.

. Discussion

The RBS curves of the BNBTxs films show that an interdif-usion process, specially ascribed to the bismuth, occurs. This isnferred from a discontinuity in the concentration values from oneimulated layer to another (Fig. 2). This fact is related to the incor-oration of bismuth excesses in the precursor solutions of suchlms. The increase of bismuth from the surface to the substrate inhe BNBTxs films (Table 1) can be due to the reactivity of bismuthnd platinum, already observed in other reported works concern-ng (Bi0.50Na0.50)1−xBaxTiO3 films prepared by CSD and films ofther Bi-containing compounds [26,37]. This is because bismuthorms the Bi2Pt eutectic and other BixPt alloys [37,38]. In fact,

small diffraction peak is detected at 2�∼38◦ in the XRD dia-rams recorded in a powder diffractometer (Fig. 5b), which could bescribed to a Pt–Bi intermetallic. This reflection cannot be observedn the XRD patterns recorded with a grazing-incidence configu-ation (Figs. 8 and 9), since X-ray penetration is not enough toive valuable information of the interface at the bottom. As XRDatterns recorded with a four-circle diffractometer (Fig. 6) collecthe sum of those obtained for each (�, �) angular position, theexture effects of the possible BixPt intermetallic are also mini-

ized; thus, reflections coming from the intermetallic BixPt are notetected in these XRD diagrams either. Typically, using excessesf volatile elements is considered a method to counterbalance the

osses produced during the heating process of thin films [39,40].n addition, these excesses hinder the formation of secondaryhases, which negatively affect the properties of the material41,42]. However, the crystallization temperature here employed

4(0) 5.67(4) 3.90(5)3(9) 12.01(5) 3.91(5).(0) 88.(9) 11.(1)

for the (Bi0.50Na0.50)1−xBaxTiO3 films seems not to cause a signif-icant volatilization of bismuth. Since this bismuth excess is notincorporated into the perovskite structure, it is able to react withthe Pt-coated substrate, producing an appreciable film-substrateinterface (Table 1).

For the BNBT stoichiometric films, the small fluctuations in thecomposition across the film thickness, as calculated by RBS, reveala homogenous compositional depth profile (Fig. 1). Furthermore,narrow BixPt film-substrate interfaces are formed, as inferred fromthe abrupt increase of the intensity at the energy associated withthe platinum cross section (∼1400 keV) (Fig. 1). Therefore, excessesof the volatile elements seem not to be required for the preparationof (Bi0.50Na0.50)1−xBaxTiO3 films with homogeneous compositionaldepth profiles.

Concerning the structural characteristics of the(Bi0.50Na0.50)1−xBaxTiO3 of this work, coexistence of tetrago-nal and rhombohedral phases, distinctive of the MPB region, isobserved for a x value of 0.055 in both bulk ceramics and stoichio-metric thin films (Figs. 4 and 6b), in accordance with data reportedin the literature [17,18,43]. The existence of the two structureswould facilitate the polarization rotation and subsequently itmight produce an improvement of the piezoelectric response[44,45].

The MPB in the BNBTxs films is detected for x values of ∼0.010.This shift of the MPB toward larger amount of BT observed in thisset of samples with excesses has been related [22–25] to the extrin-sic effect of a thin film conformation (e.g., strain). As obtainedfrom the RBS study, bismuth excesses (probably also sodium thatis not detected by RBS) are not volatilized from the film during theannealing process and, thus, the observed shift could be due to theincorporation of these bismuth excesses into the perovskite struc-ture. This implies that some titanium should be transferred fromthe BT phase to BNT phase in the solid solution, leaving free oxides,such as BaO. In addition, excesses of bismuth or sodium that are notincorporated into the perovskite could react among them and withfree BaO, giving rise to minor secondary phases, not easily detectedby X-ray diffraction. Note that such phases are observed only atthe surface of the BNBTxs10.0 film in the GIXRD patterns collectedwith synchrotron radiation, and only via a small reflection associ-ated with a pyrochlore phase (Fig. 9c). The content of this phaseis so low that laboratory measurements do not make possible thedetection of this crystalline phase (Figs. 5 b and 6 e).

The bismuth deficient layer on the surface of both the BNBT and

the BNBTxs films evidences that, although obtaining average valuesof this deficient layer, ∼4% and ∼7%, respectively, almost in the errorrange of the measurement, some loss of volatile elements proba-

urope

bHfia(Bwbtowa

cdpaht

((rmtsAfipuBfi

nftdmpSom

tdfia(src[r

5

(cTenpdB

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D. Pérez-Mezcua et al. / Journal of the E

ly occurs at the top surface of the films during annealing (Table 1).owever, the results also indicate that excesses of bismuth in theselms are not needed to compensate losses by volatilization duringnnealing; on the contrary, they generate thick bottom interfacesan average interface thickness of ∼14% of the total film thickness inNBTxs) and secondary phases. Therefore, the use of BNBT solutionsith the stoichiometric ratios of the metal cations makes possi-

le the synthesis of thin films with profile compositions close tohe nominal one. Additionally, an abrupt bottom interface is devel-ped in these films with an average percentage of thickness of ∼4%,hich is a clear indication of a low interdiffusion between the film

nd the substrate (Table 1).GIXRD analysis of the BNBT5.5 and BNBTxs10.0 films using syn-

hrotron radiation corroborates the results obtained by four-circleiffractometry. Peak asymmetries and splittings in the 1D GIXRDatterns (detected as doublets and triplets at 2� ∼20◦, 25◦, 30◦, 36◦

nd 42◦) demonstrate the formation and coexistence of a rhombo-edral and a tetragonal structure in both films, thus indicating thathese films are in the MPB (Figs. 8 and 9).

For the BNBTxs10.0 film, the intensities associated with the0 1 2)R and (0 0 1)/(1 0 0)T; (1 0 4)/(1 1 0)R and (1 0 1)/(1 1 0)T;0 0 6)/(2 0 2)R and (1 1 1)T; (0 2 4)R and (0 0 2/2 0 0)T peaks areeversed in the GIXRD pattern at ω = 0.05◦ comparing with thoseeasured at ω = 0.15◦ (Fig. 9). However, in the GIXRD patterns of

he BNBT5.5 film, the intensities seem not to change from the topurface to the bulk film (Fig. 8). This could be ascribed to a higher-site occupational disorder of the perovskite in the BNBTxs10.0lm due to excesses [46,47]. Moreover, the presence of secondaryhases on the surface (Fig. 9c) and the large variations of the vol-me fraction of each phase from the top to the substrate in theNBTxs10.0 film (Table 3) indicate structural heterogeneity for thislm.

Deviations in thicknesses obtained by the comparison of thick-esses measured from the FEG-SEM micrographs and calculated

rom the RBS curves in BNBTxs films may have several causes:he thicker interfaces and inhomogeneous compositional profilesetected by RBS, the inhomogeneous structural profiles deter-ined by GIXRD with synchrotron radiation and the higher

orosity. Actually, the high porosity of BNBTxs films observed in theEM micrographs (Fig. 3) could be formed during the volatilizationf the excesses of Na(I) and Bi(III), which would explain the densericrostructures of the BNBT films.In spite of the local complex structure of this system [15],

he crystalline structure at large-scale order has been successfulescribed here for these solution derived (Bi0.50Na0.50)1−xBaxTiO3lms. In general, the crystalline phases that best fit are ingreement with those reported for most of the works onBi0.50Na0.50)1−xBaxTiO3 bulk materials [7,20]. Particularly, thetructural results here presented for thin film samples in the MPBegion, are not so far from those which have described a near-ubic matrix with rhombohedral and/or tetragonal nano-regions48,49] and that can also explain the relaxor-ferroelectric behavioreported for these films [32].

. Summary

The composition and structure of solution derivedBi0.50Na0.50)1−xBaxTiO3 stoichiometric thin films and othersontaining Na(I) and Bi(III) excesses have been investigated.he incorporation of excesses of the Bi(III) and Na(I) volatilelements in the (Bi0.50Na0.50)1−xBaxTiO3 precursor solutions does

ot affect their possible losses during the film processing, a factresumed to be produced in solution derived thin film materialsuring the crystallization annealing. On the contrary, Na(I) andi(III) excesses in the (Bi0.50Na0.50)1−xBaxTiO3 films of this work

[

an Ceramic Society 36 (2016) 89–100 99

stimulate compositional and structural inhomogeneities. Thickfilm-substrate interfaces, a bismuth gradient in depth and remark-able differences (from the top to the bottom of the film) in thevolume fraction of the rhombohedral and tetragonal phases thatcoexist at the morphotropic phase boundary are observed in thesefilms. In both (Bi0.50Na0.50)1−xBaxTiO3 thin films prepared fromstoichiometric solutions and their counterpart bulk ceramics, theMPB is placed for values of x close to 0.055. However, the MPB isshifted toward larger x values when excesses are incorporated tothe films (BNBTxs). This fact proves that the excesses of the volatileelements in the precursor solutions produce a compositional shiftof the solid solution in the derived films, which can explain themovement of the MPB in the BNBTxs films.

The results of this work demonstrate that the chemical solutiondeposition (CSD) process here reported permits to obtain compo-sitionally and structurally homogeneous (Bi0.50Na0.50)1−xBaxTiO3thin films in the MPB region, potentially useful for applications.

Acknowledgements

This work was financed by Spanish Project MAT2013-40489-P.D. Pérez-Mezcua acknowledges the financial support of the FPUSpanish program (AP2012-0639). A portion of this research wascarried out at the Stanford Synchrotron Radiation Lightsource, anational user facility operated by Stanford University. D. Chateigneracknowledges the Conseil Régional de Basse Normandie for its par-tial financial of the four-circles X-ray diffractometer.

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