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Physical Properties of Sol-Gel Lead Nickel Titanate Powder Pb(Ti1¹xNix)O3

Le Thi Mai Oanh1,2,+, Danh Bich Do1 and Nguyen Van Minh1,2

1Department of Physics, Hanoi National University of Education, 136 Xuan Thuy Road, Cau Giay District, Hanoi, 100000, Vietnam2Center for Nano Science and Technology, 136 Xuan Thuy Road, Cau Giay District, Hanoi, 100000, Vietnam

A series of sol-gel lead nickel titanate powder with composition of PbTi1¹xNixO3, where x = 0.00, 0.03, 0.06, 0.08, 0.10 and 0.12, werestudied using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and magnetization (M-H) curves. XRDpatterns show that PbTi1¹xNixO3 materials well crystallize in tetragonal phase. The tetragonal distorted ratio c/a of PbTi1¹xNixO3 was found todecrease with the increase of Ni content and to increase with increasing calcining temperature. The mean size of PbTi1¹xNixO3 crystal particlesgradually decreased with increasing Ni content. Some Raman modes shifted to lower wavenumbers when Ni content increases, that is assignedto the variation of crystal structure due to the incorporation of Ni into PbTiO3 crystal. In addition, the results of room temperature magnetizationmeasurements present an obvious improvement of ferromagnetism when Ni content increases in the range from 3 to 12mol%.[doi:10.2320/matertrans.MA201508]

(Received January 21, 2015; Accepted May 12, 2015; Published June 26, 2015)

Keywords: tetragonal, ferromagnetism, sol-gel, ferroelectric

1. Introduction

Perovskite ferroelectric PbTiO3 is of fundamental scientificinterest as well as varied technological application because ofits extraordinary electromechanical properties in novel devicedevelopment, such as nonvolatile memories, sensors, actua-tors, transducers, high-dielectric constant capacitors andtunable microwave circuits.1­3) Recent efforts in introducingferromagnetic order into this material by partially substitutingB sites with transition-metal elements were performed to seekout a facility route to prepare multiferroics materials anddevelop new realistic devices based on the coupling betweenferroelectric and ferromagnetic orders. Ren et al.4) showedthe transition of hysteresis loop from an anti-S-type M-Hcurve for PT sample into typical S-type in Fe3+-dopedPbTiO3 samples synthesized by hydrothermal process inwhich the author proposed that the mechanism leading to theferromagnetism in the nanocrystals is F-center exchange(FCE) similarly to that in the Fe doped SnO2 nanocrystals.5,6)

The largest ferromagnetic order was observed in 0.5% Fe-doped PbTiO3 sample with MS ³ 0.01®B/Fe and a coercivefield of ³100Oe. Verma et al.7) reported the largestferromagnetic order in 1.2% Fe3+-doped PbTiO3 samplesynthesized by chemical route with MS ³ 0.52®B/Fe andHC ³ 125Oe. Palkar et al.8) presented the conversion offerroelectric PbTiO3 to magnetoelectric material by partiallysubstituting Ti with Fe3+. The significant improvement offerromagnetic property as well as magnetoelectric coupling atroom temperature was also reported in ceramic Mn4+-dopedPbTiO3 series by Kumar et al.9) On the contrary, S. Stoupinet al.10) argued that multiferroicity at room temperatures isunattainable in PbTi1¹xMnxO3. In addition, many theoreticcalculations on doped and un-doped PbTiO3 were carried outto interpret the origin of the variation in ferroelectric as wellas ferromagnetic orders in this single phase magnetoelectricmaterial.11,12) However, the efforts to find out the appropriateelement doping were limited to only a few transition-metalions such as Fe3+, Mn4+. The studies have not yet been

extended base on the substituting of many other elements intoTi sites.

Here, we report for the first time the influence of Ni2+

substitution on crystal structure, vibration and magneticproperties of PbTiO3 (PTO) nanocrystals synthesized by sol-gel process with Ni concentration ranging from 3 to 12mol%(Ni3, Ni6, Ni8, Ni10 and Ni12).

2. Experimental Procedure

Ni-doped PbTiO3 nanoparticles, PbTi1¹xNixO3, weresynthesized by sol-gel process using ethylene glycol (EG)as a surfactant. The Ni concentrations are designed asthe molar ratio of Ni/(Ni + Ti) (in the form of Ni2+).Starting materials were titanium (IV) tetraisopropoxideTi[OCH(CH3)2]4, lead nitrate Pb(NO3)2 and nickel nitrateNi(NO3)2.6H2O. Titanium (IV) tetraisopropoxide was addedinto solution of acid citric and distilled water. This solutionwas stirred for 2 h at 80°C to achieve a colorless transparentsolution. Lead nitrate and nickel nitrate were dissolved inseparate cups of distilled water then poured slowly into abovesolution of Titanium. The obtained mixture solution wasstirred for 1 h at 80°C to get a uniform emerald green solcontaining the cations of Pb2+, Ti4+ and Ni2+. Ethyleneglycol was then added into this sol as a surfactant. Thissol was then vaporized gradually under low stirring andtemperature of 90­100°C until a clear gel was obtained.Gel was dried at 180°C for 5 h then calcinated for 3 h atvarious temperatures from 500 to 900°C to crystallize thespecimens.

The crystal structure and surface morphology of Ni dopedPbTiO3 particles were characterized by XRD (SIEMENSD5000) with Cu K¡ radiation and scanning electron micro-scopes SEM (Hitachi S-4800), respectively. Raman scatter-ing measurements were performed using a Jobin-YvonT64000 spectrometer operated with 514.5 nm line of Arion laser. The magnetization vs. magnetic field wasmeasured by a vibrating sample magnetometer (VSM-MicroSence EZ-9).

+Corresponding author, E-mail: [email protected]

Materials Transactions, Vol. 56, No. 9 (2015) pp. 1358 to 1361Special Issue on Nanostructured Functional Materials and Their Applications©2015 The Japan Institute of Metals and Materials

3. Results and Discussions

The influence of the nickel content on PbTiO3 crystalstructure can be observed in Fig. 1(a). This figure illustratesthe XRD patterns for the PbTi1¹xNixO3 powders heat treatedat 500°C with x ranging from 0 to 12mol%. By indexing,XRD patterns of these samples are consistent with tetragonalstructure phase of PbTiO3. Single phase and crystallinepowders are obtained after this treatment up to 12mol% of Nidopant. A gradual phase transformation from tetragonal tocubic with respect to the addition of Ni was evidenced by theleft shift of XRD peaks such as (100), (110), (200), and (201)and the right shift of some other peaks such as (001), (101),(002), and (102). Consequently, tetragonal distorted ratio c/aof PbTi1¹xNixO3 samples decreases obviously from 1.051 forPTO to 1.020 for Ni12 (squares dotted line in Fig. 1(b)) inwhich lattice parameter c sharply decreases from 4.11 to4.01Ǻ while lattice parameter a negligibly increases from3.91 to 3.93Ǻ. This suppression in lattice distortion was alsoobserved in many previous studies and was assigned to thereplacing of dopant ions with different ion radius such asFe3+ 4,7,8) and Mn4+ 9) into Ti4+ sites in the host PbTiO3

crystal lattice. It was claimed that the off-center displacementDB of ion Ti4+ and electric polarization P is proportional totetragonality in ferroelectric PbTiO3-based solid solution,8,13)

hence high c/a ratio is required for high electric polarizationperovskite material.

Figure 2(a) shows the XRD patterns of Ni8 samples whichare heat treated at different temperatures. In contrast toFig. 1(a), the gradual splitting of (101) and (110) doubletwith increasing calcining temperature is the obvious evidenceof the increase of tetragonality. The variation of latticeparameters (a and c) as well as tetragonal distorted ratio (c/a)as the function of calcining temperature is showed inFig. 2(b). Tetragonality of sample increases remarkably inthe early stage of temperature, from c/a = 1.029 for Ni8500°Csample to c/a = 1.045 for Ni8700°C sample. In the temper-ature range from 700 to 800°C, c/a ratio increases moreslowly, reaching 1.046 for Ni8800°C sample. In addition, XRDanalysis of PbTi1¹xNixO3 powders calcinated at 800°C for 3 hshows that tetragonality is well remained after substituting upto 12mol% of Ni dopant (the circle dotted line in Fig. 1(b)).c/a ratio of Ni12800°C sample is about of 1.044, much higherthan that of 1.019 of Ni12500°C sample. These results indicatethat tetragonality (c/a) and hence electric polarization can beeasily controlled by calcining temperature.

FESEM images of PTO, Ni6 and Ni10 samples heattreated at 500°C are displayed in Fig. 3. It is estimated thatthe particle size of PTO sample is within the range from 40 to60 nm, averaging at about 50 nm. The PTO powder exhibits

(a) (b)

Fig. 1 (a) XRD patterns of PbTi1¹xNixO3 powders heat treated at 500°C and (b) the dependence of tetragonal distorted ratio c/a ofPbTi1¹xNixO3­500°C and PbTi1¹xNixO3­800°C powders on Ni doping concentration.

(a) (b)

Fig. 2 (a) XRD patterns of Ni8 samples calcinated at different temperatures and (b) the influence of lattice parameters and tetragonaldistorted ratio c/a in Ni8 samples as a function of calcining temperatures.

Physical Properties of Sol-Gel Lead Nickel Titanate Powder Pb(Ti1¹xNix)O3 1359

some rod-shape grains beside a relatively regular roundshape. The average grain size of PbTi1¹xNixO3 powdersdecreases gradually with Ni content which is estimated to beabout 40 nm and 35 nm respectively for PNT6 and PNT10sample. The suppression of particle growth process due to thereplacement of Ni can be assigned to the difference in radii ofNi2+ and Ti4+.

The replacement of Ni into Ti sites in PTO crystal latticehas a huge influence on the vibration property of PTOpowders, which is exhibited obviously in Raman spectra.Figure 4(a) shows Raman spectra of PbTi1¹xNixO3 powderscalcinated at 500°C. In the region of 100 ª 900 cm¹1, allsamples exhibit 9 Raman peaks as assigned in Fig. 4(a). Thevariation of Raman peak position as a function of Ni contentin Fig. 4(b) shows that A1 peaks significantly shift to thelower wavenumber when Ni content increases. For example,A1(2TO) and A1(3TO) peaks respectively shift from 330and 487 cm¹1 to 321 and 478 cm¹1 when Ni concentrationincreases from 0 to 12mol%. Meanwhile, E and B1 peaksalmost do not change their position with the increase of Nicontent up to 12mol%. This can be explained from the sharpdecrease of lattice parameters c with increasing Ni contentin PbTi1¹xNixO3­500°C series as discussed above. A1 modesconsist of the displacements of Ti ion along the c axis relativeto O ions and Pb ions while E modes consist the displace-ments of Ti ion along a or b axis and B1 mode consists the

displacement of O ions in ab plane along c axis.14) Thesignificant decrease of c parameter only strong affects to thevibration of Ti ion along c axis hence resulting the left-shiftof A1 mode which is similar to the left-shift occurred undertetragonal-cubic transition with increasing temperature.14) Inaddition, the decrease of frequency of Ti4+-related Ramanmodes also can be originated from heavier atom weight ofNi2+ cations with respect to Ti4+ cations, implying thereplacement of Ni2+ ions into Ti4+ sites.

Figure 5 displays M-H curves of high tetragonalityPbTi1¹xNixO3 samples calcined at 900°C. M-H curve ofPTO sample in the insert figure shows intrinsic diamagnetismof PbTiO3 material which comes from 3d° electron config-uration of Ti4+ ion.4) The existence of a weak ferromagneticorder in pure phase PbTiO3 can be assigned to the oxygenvacancy which causes a very small saturation magnetizationabout of 0.0006 emu/g.11,12) All PbTi1¹xNixO3 samplesexhibit the S-type M-H curves of ferromagnetism with thegradually increase of both coercive force HC and saturationmagnetization MS with increasing Ni content. For Ni12sample, the values of saturation magnetization MS andcoercive field HC reach to 0.0056 emu/g and 705Oe,respectively. Bound magnetic polaron model, first proposedby Torrance,15) can be employed to explain the ferromagnet-ism in PbTi1¹xNixO3 materials. In such model, an electronassociated with an oxygen vacancy will be confined in a

Fig. 3 FESEM images of PTO, Ni6 and Ni10 samples heat treated at 500°C.

(a)

(b)

Fig. 4 (a) Raman spectra of PbTi1¹xNixO3 powders calcinated at 500°C and (b) the dependence of Raman peak position on Ni content.

L. T. M. Oanh, D. Bich Do and N. Van Minh1360

hydrogenic orbital which is called donor electron. Theseelectrons tend to form bound magnetic polarons, coupling the3d moments of the ions within their orbits.16) The effectivecoupling between two Ni2+ magnetic ions in the samehydrogenic orbital is ferromagnetic. As the density ofimpurity ions increases, the number of Ni2+ ions in the sameelectron orbital increases leading to the increases of totalmagnetic moment. Otherwise, the electric neutralizationcondition also leads to the increase of oxygen vacanciesdensity with increasing Ni2+ content which results to theoverlapping between the different hydrogenic orbitals.16)

Thus, total magnetization will still increases until Ni2+

content is large enough to make the appearance ofantiferromagnetic super-exchange interactions, which reducethe average magnetic moment per doping ion due to theantiferromagnetic order ↑↓↑.

4. Conclusions

In summary, the influences of Ni2+ substitution on crystalstructure, vibration and magnetic properties were observed inPbTi1¹xNixO nanocrystals prepared by sol-gel method withNi concentration ranging from 0 to 12mol%. Tetragonaldistorted ratio c/a decreased with increasing Ni content,

demonstrating the substitution of Ni into PbTiO3 crystallattice, and increased with increasing calcining temperature,benefiting for ferroelectric polarization. The substitution ofNi into PbTiO3 crystal lattice was also proved by the left-shiftof Raman peaks related to Ti vibration along c axis. Room-temperature ferromagnetism of PbTi1¹xNixO3 nanocrystalscalcined at 900°C improved in the range of Ni dopingconcentration from 0 to 12mol%. The observation of roomtemperature ferromagnetism in high tetragonality PbTi1¹x-NixO series may provide an approach to widely explore moresingle-phase multiferroics.

Acknowledgement

This research is funded by Vietnam National Foundationfor Science and Technology Development (NAFOSTED)under grant number 103.02-2014.21.

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Fig. 5 The M-H curve of PTO, Ni3, Ni6, Ni8 and Ni12 powders calcinedat 900°C (the insert figure displays the intrinsic diamagnetism nature ofpure PbTiO3 material).

Physical Properties of Sol-Gel Lead Nickel Titanate Powder Pb(Ti1¹xNix)O3 1361