Ceramics International

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Ceramics International

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Preparation, structural and functional properties of PbTiO

ceramics

Khiat Abd elmadjid

, Felicia Gheorghiu

, Mokhtar Zerdali

, Mohammed Kadri

, Saad Hamzaoui

aLaboratoire de Microscopie Electronique & Sciences des Matériaux, Université des Sciences et de Technologie d'Oran (USTO), BP 1505, El M'Naouer, Oran, Algeria

bResearch Center in Industrial Technologies (CRTI), P.O. Box 64 Cheraga, 16014, Algiers, Algeria

cResearch Center on Advanced Materials and Technologies, Sciences Department, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I, nr. 11, 700506, Iasi, Romania

A R T I C L E I N F O

Keywords:

Ceramics Oxygen vacancies EDX spectra Dielectric properties Magnetic properties

A B S T R A C T

In the present study, oxygen deficient PbTiO3-δceramics were prepared by solid state-reaction method. The formation of the pure perovskite phase with tetragonal structure was confirmed for the 800 °C/2 h calcined sample by using X-ray diffraction analysis at room temperature. Energy dispersive X-ray spectroscopy analyses confirm the creation of oxygen vacancies in the system for charge compensations, as demonstrated by the percentage of O atoms of∼53%. The complex impedance data reveals important contributions of the oxygen vacancies to the total dielectric response that are homogeneous distributed within the sample. The room tem- perature magnetic properties show a weak ferromagnetic character in all the samples that might be attributed to the oxygen vacancies defects and to surface effects.

1. Introduction

Lead titanate oxide, with general formula PbTiO3(PT), is a well- known ferroelectric perovskite which presents a structural tetragonal- cubic phase transitions, with large anisotropic thermal expansion at a high Curie temperature of about 490 °C (763K) [1]. The tetragonal symmetry is obtained belowTCwhere PT belongs to the space group C4v¹–P4mm(with lattice parameters: a = 3.9023 Å and c = 4.1563 Å and a high tetragonality c/a = 1.06) while above TCthe cubic sym- metry describes the system. Due to the high polar perovskite structure, PT is very used in many practical applications, such as surface acoustic wave devices, infrared sensors, and high frequency transducers, ferro- electric random access (DRAM) memories, capacitorsetc. [2–4]. The main difficulty in obtaining pure PbTiO3 ceramics arises from the processing conditions because a high sintering temperature leads to a high tetragonal distortion which makes the sample to crack andfinally to breaking the ceramic. On the other hand, avoiding of the volatili- zation of the Pb element which leads to the formation of the pyrochlore phases becomes an important issue in processing the PbTiO3 in the ceramic form. Consequently, it is very important to obtain pure PbTiO3

ceramics without secondary phases and with a high density in order to control the functional properties. Until now, many researchers tried to overcome these problems by sintering PbTiO3as dense ceramics with the help of incorporation into the lattice of appropriate elements [5,6],

by Spark Plasma Sintering [7] or by using different methods of powder synthesis [8–11].

Recently, a new direction in thefield of perovskite oxides research was opened by inducing the formation of oxygen vacancies (VO) in order to produce the coexistence of both ferroelectric and ferromag- netic order at room temperature, and consequently, a new class of multiferroic materials [12,13]. It was already demonstrated that oxygen vacancies have important contributions on the oxides proper- ties. For example, Coey et al. reported that oxygen vacancies induce room temperature ferromagnetism in a few non-magnetic oxide systems [14,15]. It was also shown the increasing of the Curie temperature and enhancement of magnetic moment in EuO-based thinfilms due to the oxygen vacancies [13,16]. In the last years, due to the increasing re- search interest in searching new multiferroics, PbTiO3system become of high actuality and subjected to intensive studies (theoretical and experimental) in order to induce ferromagnetism at room temperature by the presence of oxygen vacancies [17–19], for demonstrating mul- tiferroic character in a non-magnetic system [20–22].

In this work, oxygen deficient PbTiO3-δceramics (denoted as PT) were prepared by solid state-reaction method. The role of the thermal treatment temperature influence on the PT phase formation and of the oxygen vacancies, the structural characterization and the functional properties of the optimized ceramic will be discussed in detail.

https://doi.org/10.1016/j.ceramint.2019.01.240

Received 13 November 2018; Received in revised form 3 January 2019; Accepted 29 January 2019

∗Corresponding author. Research Center on Advanced Materials and Technologies, Sciences Department, Alexandru Ioan Cuza University of Iasi, Blvd. Carol I, nr.

11, 700506 Iasi, Romania.

E-mail addresses:abdelmadjid.khait@univ-usto.dz,a.khiat@crti.dz,khiatmadjid@yahoo.com(K. Abd elmadjid),felicia.gheorghiu@uaic.ro(F. Gheorghiu).

Available online 30 January 2019

0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

T

2. Experimental details

PbTiO3-δ ceramics were prepared by solid state reaction method using commercial raw materials of high purity: PbO (Alfa Aesar, purity > 99.9%, average particle size of 3–5μm) and Ti3O5 (Alfa Aesar, purity > 99.9%, average particle size between 10 and 200μm).

The weighted powders were well-mixed in an agate mortar for 2 h by a wet homogeneous technique using ethyl alcohol. The resulted powders were then pressed into pellets and calcinated at several temperatures:

650 °C, 700 °C, 750 °C, 800 °C for 2 h in a high vacuum furnace. The final products are in according with the following chemical formula:

Ti3O5+3 PbO→3PbTiO_3-δ (1)

Whereδ= 1/3.

The perovskite phase formation was checked by XRD analysis using PANalytical Empyrean X-ray diffractometer with CuKα radiation (λ= 1.5406 Å), with scan step increments of 0.06° and counting time of 10s/step for 2θbetween 10° and 60°. The identification of the phase was performed using the HighScore database software. As com- plementary tools necessary in order to confirm the phase by ascribing the peaks to the various modes were employed Infrared (IR) and Raman experiments. The phase formation was checked by using infrared (IR) spectra, recorded in the range 500–1000 cm⁻¹on a Burker Vertex 70 FTIR Spectrometer using KBr pellet method. The Raman spectra were obtained with a LABRAM HR - 800 type micro-Raman Spectrometer that is working in a backscattering configuration, equipped with an He + ion (λ = 784.66 nm) laser as the excitation source. Raman spectra were collected at different temperatures between 305K and 705K in the spectral range of 200–1000 cm⁻¹. For the electrical mea- surements the ceramics calcined at 800 °C for 2 h was sintered at 1150 °C for 2 h. The microstructures of fractured surfaces of the this ceramics were examined by using a using scanning electron microscope (SEM) analysis performed with a JEOL JSPM 5200 type microscope coupled with energy-dispersive X-ray spectroscopy (EDX). The complex impedance measurements at room temperature in the frequency range 1–10⁶Hz, were performed by using a Solartron 1260A Impedance Analyzer on Ag-electroded ceramic pellet. The magnetic properties at room temperature were determined under magneticfields up to 14 kOe with a Vibrating Sample Magnetometer (VSM) MICROSENSE EV9.

3. Results and discussion

3.1. Phase and structural characterizations

The room temperature X-ray patterns of the investigated ceramics are presented in Fig. 1(a). The room temperature X-ray diffraction peaks reveal that the 650 °C calcinations temperature leads to an in- complete reaction with the presence of important amounts of secondary phases of PbTi3O7 besides the peaks corresponding to the used raw materials PbO and Ti3O5. It can be observed that the formation of the PbTiO3phase starts during the calcination at 700 °C at which the sec- ondary phases strongly decrease in amplitude simultaneously with in- creasing the calcination temperature. When calcination temperature increase to 800 °C, the perovskite PbTiO3was completely formed and the small amounts of PbTi3O7 still observed at 750 °C totally dis- appeared. According to the JCPDS file number 01-070-4258, it was identified a perovskite structure of tetragonalP4/mmsymmetry, with lattice parameters a = 3.900 Å and c = 4.114 Å (Fig. 1(b)) that are in good agreement with the literature data [1]. The increasing of calci- nation temperature induces the increase of bothaandclattice para- meters and also of the unit cell volume and therefore, results in a higher tetragonal distortion (c/aincreases), which most probably will affect the ceramic density. The other structural parameters obtained for the PbTiO3-δ ceramics calcined at different temperatures are listed in Table 1.

The IR (infrared) spectra of the prepared ceramics were collected at room temperature in the range 500 and 1000 cm⁻¹and are shown in Fig. 2. The measured spectra indicate that the PbTiO3-δhave crystal- lized in a single phase. The IR spectra show the bands at∼590 cm⁻¹ and indicate the presence of vibrations modes of the TieO bonds in TiO2. This vibration mode is clearly shifted towards higher wavelength after composites formation must probably due to the oxygen vacancy defects presence. The variations of their stretching frequencies de- monstrate a good interaction between metal and the other components from the system. The band recorded at∼760 cm⁻¹is indicating the presence of other metal oxygen vibration modes which implies an ad- ditional metal oxygen bond. As Park et al. [23] have investigated, two distinct types of oxygen vacancies-defects can be formed in the Fig. 1.(a) XRD patterns of PbTiO_3-δceramics produced at different tempera- tures; (b) Lattice and cell volume parameters vs. temperature.

Fig. 1. (continued)

Table 1

The structural results obtained for PbTiO_3-δ ceramics produced at different temperatures.

Samples Lattice Parameters Distorted ratio Cell volume Crystallite size

a(Å) c(Å) c/a V(ų) A(nm)

650°C 3.778 4.013 1.062 57.303 44.6

700°C 3.889 4.127 1.061 62.435 46.6

750°C 3.890 4.126 1.060 62.447 30.0

800°C 3.900 4.114 1.055 62.586 31.1

tetragonal PbTiO3 system: (i) O1 given by the vacancies that lie in TieOeTi chains along thecaxis and (ii) O2that appear in the TieOeTi chains along thea andz axis (a-zplane). Park et al. concluded that oxygen vacancies in TieOeTi chains along thecaxis are more favor- able than defects from the a-z plane. In general, the movements of the +2 of O1type oxygen defects with two nearest Ti⁺⁴and four Pb²⁺

cations are long-range order nature.

In order to complete the structural characterization, the room temperature Raman spectra (Fig. 3) of the PbTiO3-δceramics produced at different temperatures were measured in the frequency range 200–900 cm⁻¹. The sample calcined at 750 °C was not available for measurement because of its cracks which finally break the ceramic.

Similar to the BaTiO3[24], it was reported by Burns and Scott [25] a number of 12 optical phone modes in tetragonal PbTiO3that are: E (1TO), A1(lTO), E(1LO), A1(1LO), E(2TO), B1+E, A1(2TO), E(2LO) +A1(2LO), E(3TO), A1(3TO), E(3LO) respectively (TO and LO are re- ferring to the transversal and longitudinal modes) [26,27]. Shortly, the PbTiO3 optical modes can be classified in three types: A1-symmetry modes, eight E-symmetry modes (four degenerate pairs), and one B1- symmetry mode [26]. The A1(1TO) mode is given by the oscillations of the Ti-ions and oxygen ions relative to Pb ions. The displacements of the Ti-site ions relative to oxygen ions and Pb ions results in the A1(2TO) mode while the A1(3TO) mode is describing by Ti ions vibrations along the c-axis direction together with the oxygen ions that are lying be- tween Ti ions [26].

The analysis of Raman spectra for PbTiO_3-δceramics produced at

800 °C (temperature for which the PT was free of secondary phases) reveals clear peaks at∼200 and∼285 cm⁻¹that are assigned to the E (2TO) and B1+E, respectively. The broader peaks observed around

∼439, 500 and 710 cm⁻¹are assigned to E(2LO), E(3TO) and E(3LO).

The weaker peak shown in the Raman spectra is found at∼320 cm⁻¹ and is assigned to A1(2TO) soft mode that demonstrates movements of the vibrations modes of the TieO bonds. The changes in the peaks in- tensity may be attributed to the presence of the defect, like oxygen vacancies, in order to create the electrical neutrality of the system.

Energy dispersive X-ray spectroscopy (EDX) analysis presented in Fig. 4confirms the formation of pure perovskite PT phase in the sample calcined at 800 °C/2 h and the presence of Pb, Ti and O elements, only.

The atomic ratio of each element is given in the inset ofFig. 4showing that Pb:Ti:O is approximate to 1:1:2 stoichiometry. This indicates the creation of oxygen vacancies in the system for charge compensations, as demonstrated by the percentage of O atoms of∼53%.

From the structural characterizations, it can be concluded that the optimized pure phase PbTiO_3-δwere obtained by calcination at 800 °C/

2 h. The presence of both ferroelectric and magnetic order in this system will be confirmed by investigation of the functional properties.

The results of the electric and magnetic measurements will be presented and discussed in the next paragraphs.

For the electrical characterization, the PT sample calcined at 800 °C/2 h was sintered at 1150 °C/2 h.Fig. 5shows the SEM image of the fracture-cross section of such PbTiO_3-δceramic calcined at 800 °C/

2 h and then sintered at 1150 °C for 2 h. It is observed a heterogeneous microstructure with a bimodal grain size distribution, consisting of large and non-uniform (as shape and size) grains with average sizes between (10–50)μm surrounded by small grains of 1–5μm. The aspect of large grains is mostly elongated, while thefine grains are mostly rounded.

Fig. 2.The room temperature IR spectra of PbTiO_3-δceramics.

Fig. 3.The Raman spectra measured at room temperature of the PbTiO_3-δ ceramics produced at different temperature.

Fig. 4.EDX spectrum of the PbTiO_3-δsamples produced at 800 °C.

Fig. 5.Fracture-cross section of the PbTiO_3-δceramics calcined at 800 °C/2 h and then sintered at 1150 °C/2 h

3.2. Dielectric properties

Fig. 6shows the complex impedance data measured at room tem- perature for PbTiO3-δceramics sintered at 1150 °C/2 h. The complex impedance plot,i.e.the Z’‘(Z’) dependence showed inFig. 6(a) reveals an apparent single component, which demonstrate a good dielectric and conductivity homogeneity within the sample due to the fact that there is no notable difference between the properties of the grain boundary and grain bulk. This it means that the oxygen vacancies in our PbTiO3-δceramic are not segregated at the grain boundaries, but they are homogeneously distributed within the sample. The dielectric properties at room temperature, measured as a function of frequency in the range 10 Hz-1MHz, show a normal dielectric dispersion character with a monotonous decrease of the real part of permittivity (Fig. 6(b)).

At low frequency, the ceramic is characterized by high values of the real part of permittivity about∼1200 (which are associated with Maxwell- Wagner phenomena that are given by the oxygen vacancies presence) and then decreases up to 200 values for the high frequency range. The dielectric losses are below 1 value in the whole range of frequency and they decrease monotonous with frequency increase and reach a value of

∼0.06 at 1 MHz (Fig. 6(b)). It is known that an important contribution to the high losses in perovskite systems is attributed to the presence of oxygen vacancies [28,29] and this phenomenon is enhanced in the present PbTiO_3-δceramics and is indicated by the high losses in the low- frequency range. Both high values of the real part of permittivity to- gether with high losses values in the low frequency region are attrib- uted to a conductive behavior, which can be observed from the varia- tion of ac-conductivity as a function of frequency (Fig. 6(b)). The

frequency-dependent conductivity measured at room temperature show two frequency ranges, with linear dependences in the log-log plot, ac- cording to the Jonsker law [30]:σac(f)=σdc+Aωⁿ, whereσacis the total ac conductivity,σdcis the frequency independent dc conductivity, ω is the angular frequency, the coefficient A and the exponent n (0 < n < 1) are temperature and material dependent constants. The values of n constant as determined by linear fits are found to be n= 0.51 in low frequency region below 100 Hz and n = 0.78 at higher frequencies. A value ofn= 0 would indicate a dc conductivity, which is not the case for the present PbTiO_3-δat room temperature (but we do not exclude an increase of such dc component at higher temperatures), while values fornin the range of (0.5, 0.7) are typical to correlated ion doping andn= 1 describes the regime of nearly constant loss [31,32].

In our case, both frequency ranges may be described by a parametern describing correlated ions contributions. In the perovskites, the oxygen vacancies may be simple or double ionized and they may be detected by a temperature investigation of the dielectric properties in order to de- tect their activation energy. Such a study will be performed in the fu- ture.

In order to clarify which mechanism contribute to the dielectric response, imaginary and real part of dielectric modulus are represented in Fig. 6(d). It is known that a conductive relaxation mechanism is indicated by the presence of a peak in theM”(f)dependence and no peak in theε”(f)while a dielectric relaxation mechanism is indicated by maxima in both spectra. Fromε”(f)(Fig. 6(b)) andM”(f)(Fig. 6(d)) comparison, it can be observed that in the low frequency range, the data show a maximum only in theM”(f)spectra, which confirm the conductivity relaxation mechanism. In the 10⁴–10⁵Hz frequency range of M”(f) representation it can be observed a second maxima corre- sponding to the broad shoulder fromε”(f)spectra, which indicates the Fig. 6.Room temperature dielectric properties for PbTiO_3-δceramics sintered

at 1150 °C/2 h: (a) Complex impedance plot; (b) Frequency dependences of the real and imaginary part of permittivity (c) Frequency dependences of con- ductivity and tangent loss; (d) Real and imaginary part of dielectric modulus.

Fig. 6. (continued)

Fig. 6. (continued)

Fig. 6. (continued)

contribution from a second phenomenon that is a dielectric relaxation mechanism. In both processes the role of oxygen vacancies of PbTiO3-δ

is essential.

3.3. Room temperature magnetic characteristics

Fig. 7presents the room temperature magnetizationvs. magnetic field for the PbTiO_3-δsamples calcined at various temperatures. The investigated samples present a weak ferromagnetism with the value of saturation magnetization in the range of Ms∼0.01–0.33 emu/g. Mag- netism was reported in non-magnetic perovskites and in other non- magnetic oxides, both as a surface effect in ultrafine nanoparticles [20,22], but also as result of the presence of cationic or anionic va- cancies [33,34]. The observed weak ferromagnetic character typical to all the PbTiO3-δsamples, either ones having secondary phases (calcined at lower temperatures) or in the pure perovskite PbTiO3-δphase might be ascribed to the presence of the oxygen vacancies defects and to surface effects. No systematical trend of magnetization vs. calcination temperatures is noticed, because the increasing temperature induces three effects: (i) the conversion of secondary phases into the PbTiO3-δ

majority phase, (ii) the formation of higher amounts of oxygen va- cancies, and (iii) ceramic grain coarsening, thus reducing the surface- induced ferromagnetic phase. All these factors contribute in a compli- cated way to produce the room temperature ferromagnetic properties in such compound. In conclusion, a weak ferromagnetic state with the characteristics: Ms∼0.01emu/g and Hc = 120Oe in the pure perovskite phase calcined at 800 °C/2 h might be induced, which makes the PbTiO3-δphase a multiferroic system.

4. Conclusions

In conclusion, ceramics with composition PbTiO3-δwere prepared by traditional solid state method. The calcination temperature was optimized in order to produce pure PbTiO3-δperovskite phase with tetragonal symmetry and with oxygen vacancies defects. The XRD patterns have confirmed the room temperature tetragonal structure with pure perovskite phase for the 800 °C/2 h calcined sample. The EDX spectrum confirms the presence of very high percentage of O atoms of about∼53% besides the presence of only Pb and Ti elements. The real part of permittivity and dielectric losses presents high values in the low- frequency range which are due to the oxygen vacancies defects. The complex impedance plot reveals a good homogeneity of the oxygen vacancies in the sample demonstrated by the apparent single compo- nent from the Z” vs. Z’ representation. The magnetic hysteresis loop reveals that it was induced a non-linear M(H) that is attributed to a weak ferromagnetism that might be ascribed to the presence of the oxygen vacancies defects and to surface effects. The present dielectric

and magnetic data demonstrate that it might be induced a multiferroic character in PbTiO3-δ perovskite by controlling the processing para- meters, but further investigations are needed in order to increase the ferromagnetic character for possible multiferroic applications.

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Fig. 7.M(H) hysteresis loop measured at room temperature for the PbTiO_3-δ powders obtained from the ceramics produced at different temperature.