Ultrasonics Sonochemistry 33 (2016) 141–149

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Ultrasonics Sonochemistry

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Enhancing the formation of tetragonal phase in perovskite nanocrystalsusing an ultrasound assisted wet chemical method

http://dx.doi.org/10.1016/j.ultsonch.2016.05.0021350-4177/� 2016 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: ro_ashiri@yahoo.com (R. Ashiri).

Abdolmajid Moghtada a, Rouholah Ashiri b,⇑aDepartment of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Tehran, IranbDepartment of Materials Science and Engineering, Dezful Branch, Islamic Azad University, P.O. Box 313, Dezful, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 April 2016Received in revised form 2 May 2016Accepted 2 May 2016Available online 2 May 2016

Keywords:Titanate-based perovskitesNanoparticlesSonochemical methodTetragonal-phaseRaman spectrum

Synthesis of highly-pure tetragonal perovskite nanocrystals is the key challenge facing the developmentof new electronic devices. Our results have indicated that ultrasonication is able in enhancing the forma-tion of tetragonal phase in perovskite nanocrystals. In the current research, multicationic oxide per-ovskite (ATiO3; A: Ba, Sr, Ba0.6Sr0.4) nanopowders are synthesized successfully by a generalmethodology without a calcination step. The method is able to synthesize high-purity nanoscale ATiO3

(BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3) with tetragonal symmetry at a lower temperature and in a shorter timespan in contrast to the literature. To reach an in-depth understanding of the scientific basis of the pro-posed methodology, in-detail analysis was carried out via XRD, FTIR, FT-Raman, FE-SEM and HR-TEM.The effects of the sonication time and sonication (bath) temperature on the tetragonality of nanoscaleproducts were examined. Furthermore, Raman spectroscopy provides clear evidence for local tetragonalsymmetries, in particular when a band is observed at 310 cm�1.

� 2016 Elsevier B.V. All rights reserved.

1. Introduction

Ternary Perovskite materials include a wide group of com-pounds used in various electroceramic device applications suchas electronic, electro-optical and electromechanical devices. Per-ovskites have a cubic structure with general formula of ABO3,where A is usually an alkaline-earth or a large lanthanide, and Bis usually a transition metal or a smaller lanthanide [1]. Perovskitestructure has capability to host ions of different size. Focusing onparent compounds, in ABO3 series, only barium titanate (BaTiO3;BTO) is most studied and well explored compound in both bulkand thin film shapes. Although this compound shows a noteworthyrange of interesting properties, however till now, only few studieshave been reported on the sonochemical synthesis of perovskite-type materials. Sonochemistry uses the ultrasonic irradiation forinducing the formation of very fine powder products with high sur-face area in contrast to other synthesis methods [2]. It has beenemployed extensively for the synthesis of the nanostructuredmaterials due to its rapid reaction rate, controllable reaction condi-tions, simplicity and safety. Moreover, powder particles synthe-sized through this method normally have uniform shape withnarrow size distribution [1–4]. The crystal structure of BTO istypically observed by X-ray diffraction (XRD) and it appears to

change from a tetragonal ferroelectric phase to a cubic paraelectricphase, which is inappropriate and is not very sensitive for transi-tions involving oxygen/titanium displacements [5]. However,vibrational spectroscopy is sensitive to this type of transforma-tions, especially for Raman spectroscopy, which can detect locallattice distortions and crystallographic defects at the molecularlevel [6]. Chemical bonds vary widely in their sensitivity to scruti-nizing by infrared techniques. Thus, the capability of infrared spec-trophotometry (IR) is a function of the chemical bond, rather thanbeing applicable as a general probe. FT-IR analysis was carried outfor detecting the presence of the functional groups. Using this anal-ysis, the reaction mechanisms in the sonochemical process can bedetected. Raman and IR spectra are important in studying the fer-roelectric materials, since ferroelectricity and lattice dynamics areclosely related. Raman spectra give information on local symmetryand has been used by many researchers to study the relaxor behav-ior [7], the phase transitions [8], as well as in order to study otheraspects like stress, strain [9] and grain size effect [10] of theferroelectrics.

In the present work, we have tried to develop an innovativemethod in order to synthesize a variety of ceramic nanoparticleswith perovskite symmetry which has no by-products. This methodis able to prepare the powder products at a low temperature of333 K (60 �C) under the irradiation of the ultrasonic waves. Ourapproach provides a unified methodology for the synthesis of theperovskite materials which is a rapid one-step method with no

142 A. Moghtada, R. Ashiri / Ultrasonics Sonochemistry 33 (2016) 141–149

need for calcination the products. On the other side, an elaboratedexplanation regarding mechanism of synthesis and phase develop-ment in sonochemical synthesis is not available in the literature.Few publications have studied the path of synthesis and character-ized the phase formation, evolution and tetragonality of BTO pow-der. The goal of the present work is to study and characterize thestructure and tetragonality dependences of the synthesized ATiO3

nanopowders using X-ray diffraction (XRD), Raman and IR spec-troscopy which are able to identify the phase formation and evolu-tion in the obtained nanopowders.

2. Experimental

Synthesis temperature and purity of perovskite materials arekey challenges facing the scientific community. This workaddresses the challenges by developing a method for synthesizingthe perovskite nanopowders. Our new approach is based on anultrasound-assisted wet chemical processing method. To showthat the developed method can be used as a general strategy forsynthesizing carbonate-free perovskite nanocrystals, first BTO,strontium titanate (SrTiO3; STO) and barium strontium titanate(BaxSr1�xTiO3; BSTO) nanocrystals were synthesized. Then, inorder to study the tetragonality of the powder products, the secondstep of the study was focused on BTO as the most widely studiedperovskite material. Titanium chloride (>99%), strontium chloride(>99%), barium chloride (>99%) anhydrous sodium hydroxide(>99%) and ethanol (99.8%) were obtained from Merck. The flow-chart of the approach is shown in Fig. 1. The stoichiometricamounts of chloride salt and titanium chloride are dissolved indeionized water and ethanol, respectively. These solutions areadded into a glass vessel containing NaOH solution. The concentra-tion of the NaOH solution was required to guarantee a strong

Fig. 1. Flowchart of the sonochemical method used in this work for the synthesis ofthe titanate-based perovskite nanocrystals.

alkaline environment (pH = 14) during reaction. The glass vesselcontaining precursor solution is subjected to an ultrasonic bath(Soner 220H, 53 kHz, 500 W, New Taipei City, Taiwan). The advan-tages of the sonochemical method include green synthesis, nowaste product and ease of synthesis. The solution mixture wasplaced at the center of the ultrasonic bath and then was sonicatedat 25, 50 and 60 �C for 5, 10 and 20 min to see the effect of sonica-tion time and temperature. The sonication was conducted withoutcooling so that the temperature of the solution increased graduallyup to 60 �C during synthesis. Our previous results [1] have shownthat in order to synthesize BSTO (with general formula of BaxSr1�x-TiO3), first the corresponding molar ratios of the barium chloride(x) and strontium chloride (1 � x) should be mixed together, thentheir mixture should be dissolved in deionized water. The succes-sive steps of the synthesis are similar to those for BTO synthesis.After the reaction is finished and the mixture is cooled down toroom temperature, then the powder product is separated, washed,and dried in an oven. The crystal structure and average crystallitesize of the powder products are determined using an X-ray diffrac-tometer (Philips PW3710). Functional groups in the product aredetected using a FT-IR spectrophotometer (Hitachi 3140). FT-IRspectrum are recorded in the range of 400–4000 cm�1 and mea-sured on samples in KBr pellets. Raman spectrum was carriedout with a FT-Raman 960 (Thermo Nicolet model) using a5.5 mW laser with a wavelength of 636 nm. The morphologicalcharacteristics and microstructure of the nanoparticles wereobserved using field emission scanning electron microscopy (FE-SEM; Hitachi S4160, Tokyo, Japan) and high-resolution transmis-sion electron microscopy (HR-TEM; ZEISS, LIBRA200, Oberkochen,Germany).

3. Results and discussion

3.1. XRD characterization

It is known that advances in microelectronics and communica-tion industries have led to substantial miniaturization of manyelectronic devices, while the performance requirements haveincreased [11]. As a result, smaller and more uniform particle sizesof ternary perovskite materials with tetragonal phase are required[12]. For instance, BTO powder with a narrow particle size distribu-tion and high tetragonality is required for ceramic capacitors, self-controlled heaters, communication filters and non-volatile memo-ries. Unfortunately, most of the methods used for the production oftitanate-based (ATiO3; A: Ba, Sr, . . ..) powders, such as the conven-tional solid state reaction method, alkoxide-hydroxide route,solvothermal process, hydrothermal methods, etc. have not beenfully successful in preparing perovskite nanopowders with atetragonal phase. As a result, formation of tetragonal ATiO3

nanopowders remains as a challenging issue for scientific commu-nity. XRD method is a powerful nondestructive tool that can pro-vide information regarding crystal structure, plane of orientation,strain relaxation, etc. The aim of the present work is phase identi-fication, evolution and tetragonality of BTO powders. ATiO3-typeferroelectric (FE) compounds with perovskite structure often trans-form to a paraelectric (PE) phase when the temperature or pressurechange [13,14]. For instance, PbTiO3 tetragonal perovskite (P4mm)transforms to cubic perovskite (PmN3m) at a high temperature of763 K or at a high pressure of 11.2 GPa [15,16]. BaTiO3 also exhibitsa tetragonal (P4mm) to cubic (PmN3m) phase transition at a highpressure of about 2 GPa or at 393 K [17]. XRD patterns of the sono-chemically synthesized titanate-based (ATiO3) particles showstrong diffraction peaks as it can be seen in Fig. 2. X-ray diffractionpatterns are in good accordance with JCPDS No. 31-0174 (BTO),JCPDS No. 35-734 (SrTiO3; STO), JCPDS No. 34-0411 (Br0.6Sr0.4TiO3;

Fig. 2. XRD patterns of (a) BTO (b) STO (c) BSTO nanocrystals synthesized at 333 K (60 �C) using our generalized methodology.

A. Moghtada, R. Ashiri / Ultrasonics Sonochemistry 33 (2016) 141–149 143

BSTO), respectively. Therefore, it can be said that titanate-based(ATiO3) perovskite powders have been synthesized successfullythrough our general methodology. Patterns of the synthesizedproducts are consistent with other reports [18–21]. The titanate-based nanopowders were characterized by well-resolved peaks at22.15, 31.49, 38.79, 45.23, 56.09, 65.85� corresponding to the(1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 1), (2 2 0) planes. One difficultyin quantification of the tetragonality of BTO is the interpretation ofthe XRD measurement of (2 0 0) and (0 0 2) peaks. Theoretically,

100 pct tetragonal BTO has two separate peaks between 2h = 44and 47� Fig. 3. Complete cubic barium titanate shows just onepeak. A mixture of tetragonal and cubic barium titanate will showall intermediate forms between one and two peaks. The presenceof clear splitting of (2 0 0) peak around 2h = 45� in X-ray diffractionpatterns of BTO confirms its tetragonal symmetry at lower temper-ature. The extended scan of XRD around 2h = 45� is shown in Fig. 4.The XRD peak at 45.23� ((2 0 0) reflection), which is related to thec-axis of tetragonal phase of BaTiO3, shows much broader

Fig. 3. XRD curves of tetragonal and cubic barium titanate.

Fig. 4. XRD observation of the (2 0 0) peak splitting at 45.3� for BTO powders.

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full-width half-maximum (FWHM) than those of the (1 0 0) and(1 1 0) reflections, which are related to a-axis of tetragonal phaseof BaTiO3 [22]. Generally, it is known that the most stable structureof ABO3 is closely related to the tolerance factor as follows,

t ¼ rA þ rOffiffiffi

2p

ðrB þ rOÞð1Þ

where rA, rB and rO denote the ionic radii of A, B, and O ions, respec-tively. In general ferroelectric ABO3, the most stable structure istetragonal for t J 1, cubic for t � 1, and rhombohedral ororthorhombic for t[ 1 [23,24]. In our experiments, BTO with

t = 1.022 shows tetragonal structures has formed at a low tempera-ture of 333 K (60 �C).

In this work the effect of two synthesis parameters includingsonication time and sonication (bath) temperature on tetragonalityof nanoparticles has been investigated. Among the obtainednanoparticles BTO is selected in order to study the effect of sonica-tion time and synthesis temperature. Fig. 5 shows XRD patterns ofthe BTO powders synthesized at 333 K (60 �C) after different soni-cation times. It is obvious from XRD patterns that there is a mini-mum required sonication time to obtain a fully crystalline BTOpowders. It took 20 min to obtain crystalline BTO particles usingultrasonic irradiation. Samples sonicated for shorter times containboth crystalline and amorphous phases. Fig. 6 clearly shows that byincreasing in sonication time the amount of splitting of the men-tioned peak increases. The tetragonality (c/a), calculated throughthe indexes of XRD patterns of BTO nanopowders at different son-ication times is shown in Table 1. At sonication time lower than5 min, the powder did not exhibit tetragonal structure. The tetrag-onality of powders increases with sonication time increasing, andafter 20 min sonication, the tetragonality of sample was close toa fully tetragonal value of 1.022. Before 20 min sonication, thepowder products show a single peak of (2 0 0) which is a character-istic of a cubic crystal structure. When the powder products aresonicated for 20 min, a splitting including (2 0 0) peak reacted ata higher region and (0 0 2) shoulder at the lower region are seen,which this are attributed to the characteristics of a tetragonal crys-tal structure. However, if the size of the synthesized crystallitesdecreases, the splitted peaks of tetragonal phase may overlapbecause of the broadening of the diffraction peaks induced by sizeof the crystallites. Fig. 7 shows the XRD of BTO synthesized at dif-ferent sonication (bath) temperatures for 20 min sonication. Themost intense reflection appearing in XRD pattern of sample soni-cated for longer time indicates the formation of a BTO phase withtetragonal symmetry at sonication temperature of about 333 K(60 �C).The splitting of (2 0 0) peak is not visible in the case of low-est sonication (bath) temperature, but this peak is broader andasymmetric at higher sonication (bath) temperature. Hence, thesplitting of the peak is due to the existence of two (0 0 2) and(2 0 0) peaks of tetragonal phase as shown in Fig. 8. At the lowestsonication (bath) temperature 298 K (25 �C) a very broad lowintensity peak is seen which could be assigned to anatase phaselisted in the JCPDS No. 84-1286.The tetragonality (c/a) calculatedthrough the indexes of XRD patterns for BTO nanopowders at dif-ferent sonication (bath) temperatures is shown in Table 1. Theaverage crystallite size of ATiO3 perovskites calculated using Scher-rer’s formula as follows:

D ¼ 0:9kbhkl cos h

ð2Þ

Where D is the average crystallite size, k = 1.541 Å (X-ray wave-length), and bhkl is the width of the diffraction peak at half maxi-mum for the diffraction angle of 2h. The average crystallite size ofBTO, STO and BSTO was calculated from the corresponding XRD pat-terns. Lattice parameters, unit cell volumes and crystallite sizes ofthe perovskite phases estimated from the XRD patterns are summa-rized in Table 2. These results clearly indicate that ultrasonicationpromotes the formation of tetragonal BTO phase in the final powderproduct.

3.2. FT-IR spectrum

FT-IR spectrum of BTO synthesized nanopowders has beenshown in Fig. 9. FT-IR spectrum interprets the existence of absorp-tion bands at around 536, 1047, 1351, 1453, 1592 and 3400 cm�1.It is known that the broad band in the range 3400–3097 cm�1 isdue to the stretching vibration of the hydroxyl (OAH) group and

Fig. 5. XRD patterns of BTO powders synthesized at 333 K (60 �C) with different sonication times.

Fig. 6. XRD patterns magnification in 2h range of 40–50�.

Table 1Tetragonality evaluation with respect to sonication time and sonication (bath)temperature.

Sonication time and temperature Tetragonality (c/a)

5 min at 333 K (60 �C) –10 min at 333 K (60 �C) 0.4120 min at 333 K (60 �C) 1.02220 min at 298 K (25 �C) –20 min at 323 K (50 �C) 1.00220 min at 333 K (60 �C) 1.022

A. Moghtada, R. Ashiri / Ultrasonics Sonochemistry 33 (2016) 141–149 145

confirms the existence of water. Another band around 1592 cm�1

is due to the bending vibration of the HAOAH bonds. The absorp-tion bands at 1047, 1351 cm�1 can be considered as the alcoholicbending vibrations (CAOH functional groups) [25]. The appearanceof these absorption peaks is due to the washing stage of nanopar-ticles with alcohol in the final stage of synthesis. It is well knownthat the characteristic vibration bands corresponding to (MAO)metalAoxygen bonds are in the range of 400–800 cm�1. Verystrong bands related to TiAO and TiAOATi stretching vibrationsappeared at 400–600 cm�1 and 525–700 cm�1, respectively [26].

Meanwhile the absorption at 536 cm�1 is attributed to BaAO bond.Therefore, it can be said that the absorption at 536 cm�1 corre-sponding to the (formation of the tetragonal BTO phase) is inagreement with XRD diffraction pattern [27].

3.3. FT-Raman spectrum

Considering the structural results for cubic and tetragonalphases, in most of the cases the final product consists of a mixtureof cubic and tetragonal phases. If their crystallite size is smallenough to broaden the X-ray diffraction peaks, it is difficult to per-form the qualitative phase analysis using X-ray powder diffractiondata. Consequently, another approach is necessary to confirm thephase identification of BTO powder. Raman spectroscopy has beenemployed to determine the characteristic lattice vibration spectraof BTO powders. It is well known that symmetry-group analysisin cubic BTO exhibits no Raman active modes, while in tetragonalBTO (space group P4mm) there is eight optical Raman activemodes: 4 modes of E symmetry, 3 modes of A1 symmetry andone mode of B1 symmetry [27]. Each of these modes splits intotransverse (TO) and longitudinal (LO) optical components, due to

Fig. 7. XRD patterns of BTO after 20 min sonication at different sonication (bath) temperatures.

Fig. 8. XRD patterns magnification in 2h range of 40–50�.

Table 2Lattice parameters, unit cell volumes and crystallite sizes of BTO, STO, BSTO powderscalculated from X-ray diffraction patterns.

Synthesizedpowders

Dominantpeak position

Latticeparameter, a,(Å)

Averagecrystallitesize

Unit cellvolume (Å)3

BaTiO3 31.45� 3.9440 10.3 nm 62.75SrTiO3 32.37� 3.92 4.8 nm 60.24Ba0.6Sr0.4TiO3 32.00� 3.9650 14 nm 62.33

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long-range electrostatic forces associated with lattice ionicity [28].In our investigation, five Raman peaks have been recorded andassigned to more than one phonon mode of tetragonal BTO, as pre-sented in Fig. 10. According to literature data [29,30] Raman peaksare interpreted as follows: the sharp peak centered around270 cm�1 as a A1 (TO2) mode, a broad peak at 310 cm�1 as B1 + E(LO2) + E (TO3), an asymmetric sharp peak at 515 cm�1 as A1

(TO3) + E (TO4) and a broad weak peak at 742 cm�1 as a sum ofA1 (LO3) and E (LO4) modes. These are typical peaks in the Ramanspectrum for the tetragonal BTO phase. Among them, the peaksaround 310 and 742 cm�1 disappear at above the Curie tempera-ture, which is the stable region of the cubic phase. This means thatit is possible to differentiate between the cubic and tetragonalphases by the presence of the two peaks in Raman spectrum. Fromthe Raman spectrum, which is carried out on powder with crystal-lite size of 10.3 nm, it could be found that our sample contains thepeaks corresponding to the tetragonal phase. Among all peaks, the

peak located at 310 cm�1 is characteristic peak for the tetragonalBTO. This indicates that the results of the Raman spectroscopymeasurements are in accordance with those obtained by X-rayanalysis. The observed frequencies of Raman modes in BTOnanopowders as compared with those reported in previous works[31–33] are given in Table 3.

3.4. FE-SEM and HR-TEM analysis

The morphology and particle size of ATiO3 (BTO, STO, BSTO)nanopowders synthesized in this work was studied by a field emis-sion scanning electron microscopy (FE-SEM). FE-SEM micrographsof the ATiO3 are shown in Fig. 11. FE-SEM micrographs clearlyshow homogeneous morphology in shape and dimension, uniformcrystal size and agglomerated nature of the nanopowders.

The microstructure and size distribution of the powder prod-ucts were studied using HRTEM technique. TEM micrographs ofBTO and STO powder products synthesized in this work are shownin Fig. 12. Morphology of the powders is consistent with the FE-SEM results. Moreover, the crystallite size observed in TEM micro-graphs is in good agreement with the crystallite size calculatedfrom XRD patterns. As can be seen, the powders are uniform intheir size and shape. Our previous experiences in fabrication thenanoscale materials [34–45] critically indicate that a smart design-ing of the process and the use of the properly selected startingmaterials (or precursors) significantly influence the microstructureand performance of the final nanoscale product.

Fig. 9. FT-IR spectrum of BTO nanocrystals synthesized at 333 K (60 �C).

Fig. 10. Raman spectrum of tetragonal BTO powders synthesized at 333 K (60 �C.

Table 3Comparison of Raman mode frequencies (in cm�1) observed from BTO with otherliterature.

Mode Ref. [31] Ref. [32] Ref. [33] This work

A1(TO2) 267 278 270 270E(TO + LO) + B1 308 305 305 310A1(LO2) 473 470 471 430A1(TO3) + E (TO4) 512 520 516 515A1(LO3) + E (LO4) 740 727 719 742

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3.5. The role of the ultrasonication on the formation of tetragonalphase

Functionalities such as ferroelectricity, piezoelectricity andpiezoelectricity of the barium titanate are originated from domainstructure of dipole which forms by slight asymmetry of the crystalstructure [46–48]; this is affected by its processing. Therefore, the

synthesis methods resulting in formation of tetragonal phase are ofsignificant importance. Most of the synthesis methods such as sol-gel processing [48–50] and solid-state synthesis [18,51–52] led tothe formation of the cubic barium titanate as their final products.Our results have indicated that the synthesis method establishedhere results in the formation of tetragonal phase at a low temper-ature in contrast to the literature [18,48–52]. It seems that ultra-sonication is the reason behind this achievement. To discuss this,we should consider the ultrasonication effect and also the condi-tions required for the formation of the tetragonal phase simultane-ously. The previous results have shown [46,48,52] that theformation of tetragonal barium titanate needs more energy in con-trast to the cubic phase. Therefore, the formation of cubic phase ismore preferred from the kinetic point of view. During sonication,ultrasonic waves radiate through the precursor solution. Thiscauses alternating high and low pressure in the solution and alsoleads to the formation, growth, and implosive collapse of bubblesin the reaction mixture [2,53]. The collapse of bubbles with short

Fig. 11. FE-SEM micrographs of (a) BTO, (b) STO and (c) BSTO nanocrystals synthesized in this work.

Fig. 12. HR-TEM micrographs of (a) BTO and (b) STO nanocrystals synthesized in this work.

148 A. Moghtada, R. Ashiri / Ultrasonics Sonochemistry 33 (2016) 141–149

lifetime produces intense local heating. According to the hot spottheory [2], very high temperatures (>5000 K) and pressures ofroughly 1000 atm are obtained upon the collapse of a bubble.These critical conditions [54–55] provide the enough energy forthe formation of the tetragonal phase instead of the cubic one.Therefore, it is expected to obtain better tetragonality withincreasing the sonication time; a trend which is seen in our results(see Table 1 and Fig. 6).

4. Conclusions

This work reported a simple and cost-effective general method-ology for synthesizing ATiO3 (A: Ba, Sr, Ba0.6Sr0.4) nanocrystals. Themethodology was a novel ultrasound-assisted wet chemical syn-thesis which was operated at a low temperature of 333 K (60 �C).The results indicated that ATiO3 nanocrystals with tailored mor-phology and narrow size distribution with the particles size inthe range of 4–15 nm can be synthesized by using our developedmethod. X-ray diffraction pattern of BTO nanopowders suggestedthat in order to obtain tetragonal phase, a minimum temperatureof 333 K (60 �C) is required. Raman spectrum of BTO showed asharp peak at 310 cm�1, which is characteristic of the formationof the tetragonal phase. The tetragonality of BTO nanopowdersincreased with increasing sonication time and sonication (bath)temperature.

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