Effects of lanthanide amphoteric incorporation on structural, electrical, and photoluminescence properties of BaTi0.925(Yb0.5Nb0.5)0.075O3 ceramic

Accepted Manuscript Effects of lanthanide amphoteric incorporation on structural, electrical, and photoluminescence properties of BaTi0.925(Yb0.5Nb0.5)0.075O3 ceramic C. Chalfouh, A. Lahmar, S. Zghal, R. Hannachi, N. Abdelmoula, H. Khemakhem PII:

S0925-8388(17)31164-7

DOI:

10.1016/j.jallcom.2017.03.351

Reference:

JALCOM 41389

To appear in:

Journal of Alloys and Compounds

Received Date: 2 February 2017 Revised Date:

30 March 2017

Accepted Date: 31 March 2017

Please cite this article as: C. Chalfouh, A. Lahmar, S. Zghal, R. Hannachi, N. Abdelmoula, H. Khemakhem, Effects of lanthanide amphoteric incorporation on structural, electrical, and photoluminescence properties of BaTi0.925(Yb0.5Nb0.5)0.075O3 ceramic, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.03.351. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

AC C Wavelenght (nm)

EP 600

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3 3 P0 - F2

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3 3 P0 - H6

700 3 3 P0 - F4

3 3 P1 - F3 3 3 P1 - F4

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500 3 3 P0 - H 5

3 3 P1 - H 5

PL intensity (u.a)

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800

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Effects of lanthanide amphoteric incorporation on structural, electrical, and photoluminescence properties of BaTi0.925(Yb0,5Nb0,5)0.075O3 ceramic. C. Chalfouha*, A. Lahmarb, S. Zghala, R. Hannachic, N. Abdelmoulaa, H. Khemakhema a

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Laboratory of Materials Multifunctional and Applications (LaMMA, LR16ES18), Faculty of Sciences of Sfax (FSS), University of Sfax, Street Soukra km 3.5 BP 1171, 3000 Sfax, Tunisia. b

Laboratory of Physics of Condensed Matter (LPMC), University of Picardie Jules Verne, Scientific Pole, 33 rue Saint-Leu, 80039 Amiens Cedex 1, France.

c

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University of Sousse, LabEM-LR11ES34 Energy- Materials Laboratory nano-materials research group for telecommunications and sensors, Superior Institute of Informatics and Communication Techniques, 4011, Hammam Sousse, Tunisia.

The

influence

of

the

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Abstract lanthanide

incorporation

in

lead-free

ferroelectric

(BaTi0.925)(Yb0.5Nb0.5)0.075O3 (BTYN) perovskite on its structural, dielectric, ferroelectric behavior, piezoelectric and photoluminescence properties was investigated. X-ray diffraction study permitted the identification of an isostructural phase with P4mm-tetragonal symmetry.

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Praseodymium was found to occupy only Ba- sites instead of the distribution over both sites as in the case of Nd and Eu elements. The improvement of dielectric and ferroelectric properties obtained from Nd- and Eu-phase is discussed in terms of amphoteric behavior

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effect of these elements. Incorporation of Ln3+ was found to improve piezoelectric properties

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from 3pC/N for BTYN to 18, 40 and 26pC/N for BTYN:Eu, BTYN:Nd and BTYN:Pr respectively.The photoluminescence (PL) properties of Pr-phase gives rise to the expected electronic transitions associate to Pr3+ ions. In contrast, PL carried out on Eu- and Nd-phases showed dominate strong red emission centered at 690 nm attributed to defect structure and non-stoichiometry.

The

simultaneous

existence

of

ferroelectric

piezoelectric

and

luminescence properties in the titled system could be promising for electro-optical applications. *Corresponding author: e-mail: [email protected] 1

ACCEPTED MANUSCRIPT 1. Introduction Ceramic materials with an ABO3 perovskite structure have received growing attention in recent research due to their promising applications in a large number of technological applications, including the production of electromechanical transducers, sensors, and

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capacitors [1]. In addition, they have been reported to offer attractive potential for the development of a wide range of devices covering different application such in medical ultrasonic imaging, non-destructive testing, and marine seismic exploration [2, 3]. The

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technical utility of these materials stems from the various possibilities they offer for cation substitution or mixing in the A or B sites, which provide valuable opportunities for the

dominant

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optimization and tailoring material to meet application requirements. but up to present, the materials are lead based, represented by Pb(Zr,Ti)O3 (PZT) systems,

which

exhibit excellent piezoelectric property near the morphotropic phase boundary (MPB). However, the use of lead-based ceramic may cause serious environmental problems because

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of the toxicity of lead oxide, which have led to an urgent need for the development of high performance, lead-free piezoelectric materials [4,5]. Recently, the finding of a new BaTiO3based piezoelectric system, namely, BaTi1-x(Nb0.5Yb0.5)xO3 (BTYN), has aroused significant

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interest because of its higher piezoelectric coefficient (d31 of 94 pC/N) at an optimal

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composition of x = 0.075 near MPB, with high transition temperature ( TC ) and low dielectric permittivity [6]. These properties can also be modified by the addition of dopants in either A or B site [7]. The questions that still remain largely unexplored are which properties must be enhanced and which properties should be combined. Noteworthy that perovskite structure is considered ideally matrix for luminescence investigations. Moreover, rare earths ions such (Pr3+, Er3+, or Eu3+) doped ferroelectric matrix, were found to induce photoluminescence (PL) with keeping good ferroelectric properties [8, 9]. Furthermore, and to some extent, the beneficial addition of small amounts of lanthanide on

2

ACCEPTED MANUSCRIPT electrical properties of Na0.5Bi0.5TiO3 (NBT) based materials, was reported in several research works and was found to depend on the selected lanthanide element [10, 11]. However, the vast majority of these materials have low luminescent intensity or weak piezoelectric properties, which constrains their practical applications in integrated optical and electrical

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devices. However, to shape desirable materials with interesting properties that can meet the current technology needs remains a challenge. In this context, our interest in the present work is focused on the doping BTYN phase by a small amount of rare earth to stay near the

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morphotropic phase boundary to keep good ferroelectric properties, as well as be able to use the lanthanide element as luminescence activator and to fabricate a multifunctional material.

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However, the electrical and luminescence properties of Ln-doped BTYN ceramics have not been reported yet. Ln-doped BTYN ceramics exhibit strong and sharp green–red emission excited by blue and red light while maintaining piezoelectric properties. In this study, we reported in detail our results on the synthesis, electrical, photoluminescence, and piezoelectric

2. Experimental

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properties of Ln-doped BTYN.

Ba1-xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3(Ln = Eu3+ , Nd3+ and Pr3+) ceramics with x= 0 and 0.0025

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were prepared by a conventional solid state reaction technique. The starting materials are highly-pure (Aldrich 99.9%) powders of BaCO3, Nd2O3, Eu2O3, Nb2O5, Pr6O11, Yb2O3 and TiO2. Raw materials of these precursors were weighed in the equimolar proportions and

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calcined at 1150 K (15 h). After grinding, the pressed pellets were sintered at 1400 K (4 h). The phase analysis of the ceramic was characterized by X-ray diffraction (Bruker D4, CuKα, emission, λ= 1.5418 Å) at room temperature. The microstructure of the samples was observed by scanning electronic microscopy (FESEM; JEOL-JSM-7600F). The densities of the specimens, ranging between 93% and 95%. For the dielectric measurements, the sample electrodes were obtained by painting both sides of the pellets with low firing temperature silver paint. The dielectric permittivity of the sample was measured as a function of both temperature (100–600 K) and frequency (103–106 Hz) using a liquid nitrogen cryostat and an HP4284 impedance analyzer interfaced with a PC.

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ACCEPTED MANUSCRIPT Raman scattering data was collected in the frequency range 50–1000 cm-1 using a Raman spectrometer (HoribaHR800, Jobin Yvon). The ferroelectric polarization was investigated at 1Hz using a TF Analyzer 1000 aixACCT in the temperature range of 213–473 K at 5 K cooling rate. The piezoelectric at room temperature was evaluated on measuring the d33

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parameter. PL measurements were performed on a cornestrone 260 monochromator (from Oriel instruments) coupled to an R955 photomultiplier (Hamamatsu, Japan). A multi-line

nm was used as excitation source. 3. Results and Discussion

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3.1. Microstructure analysis and X-ray diffraction study

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Lasos Argon laser with output power of 40 mW and a selected wavelength of 488 nm or 532

Fig. 1 represents the microstructure evolution of the prepared samples. As it can be observed from the micrographs, the samples seem to exhibit high density and low porosity. The average particle size values obtained from SEM micrographs were 2µm, 1.7µm, 1.3µm et 1µm for Ln-

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free BTYN, BTYN: Eu, BTYN: Nd, and BTYN: Pr respectively. The effect of the addition of lanthanide on grains size is clearly seen. However, the grains size decreased gradually with increasing the ionic radius. This results are in agreement with the calculus of crystallite size

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starting from RDX by principal pick (2θ =31°C) (Fig. 2). It seems that lanthanide with large ionic radius could be considered as grain growth inhibitor as it is more efficient to suppress

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the grain growth than those of small ionic radius [12]. Such behavior may lead to an overall change in the dielectric, ferroelectric, and optical properties of these materials (see below). Room temperature structural refinement of the powder XRD patterns of

Ba1-xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3 (Ln = Pr, Nd and Eu) was carried out assuming a tetragonal symmetry (P4mm) for all samples. Fig.3 presents the X-ray diffraction patterns matching using Fullprof software, where a satisfactory agreement was obtained from profile adjustment in P4mm-tetragonal symmetry. The refinement produced satisfactory agreement

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ACCEPTED MANUSCRIPT factors and lattice parameters are summarized in Table 1. In the Table 2 we gathered the atomic positions with the atomic occupation factor for all studied compositions. It is worth mentioning that if the substitution of Ba2+ by Eu, Nd or Pr occurred, and taking in the account the relatively less ionic radius of Eu3+ [(1.130 Å), CN (coordination

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number) = 12] , Nd3+ [1.276 Å CN= 12] and Pr3+ [ 1.286 Å CN= 12] comparing to that of Ba2+(1.610 Å, CN = 12) [13], we should observe a decrease in cell volumes. The analysis of the obtained lattice parameters revealed that only the incorporation of small amounts of Pr3+

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in BTYN host lattice seems to reduce the volume cell. In contract, the unit cell volume increase for Nd and Eu. It is documented that in BaTiO3, the introduction of Lanthanide

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element induced the creation of A-site vacancy. In fact, Ln3+ ions will behave as a donor, according to the equation (in Kroger–Vink notation):

BaTiO 3 → 2Ln • + V " + 3O x Ln 2O3  o Ba Ba

(Eq.1)

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As we can deduce from the equation, for every two Ln3+ ions positioned in the A-site, one cationic vacancy VBa is necessary for charge neutrality in the perovskite structure. Here, Ln-doped BaTi0.925(Nb0.5Yb0 .5)0.075O3 ceramics have been prepared with doping

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concentration ranging from 0 and 0.0025 according to the following equation (Eq.2):

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Excluding the mother phase (x=0), all the other compositions might be nonstoichiometric, exhibiting A-site-deficient perovskite-type according to the following structural formula (Eq.3):

Ba1− x Ln 2 x x Ti0.925 (Yb0.5 Nb0.5 )0.075 O3 3 3

Where
denotes A-site vacancy in the perovskite structure. The inhomogeneous distribution of the lanthanide thorough the specimens might govern the nature of defect chemistry and induced such difference in the behavior. In fact, Ln3+ could be

5

ACCEPTED MANUSCRIPT located either in Ba-sites, Ti-sites or both of them. Recall that ionic radii of lanthanide element are smaller than that one of Ba2+. In this case, a slight decrease in the lattice parameters could be expected. In contrast, an increase of lattice parameter might be induced in the case of Ti-sites occupation, because lanthanide element has a large ionic radii

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comparing to Ti (r(Ti4+) = 0.74 Å [13]). Another possibility, when Ln3+ substitute Ba2+ and Ti4+, the differences in radii and valences will lead to variation in the vacancy concentration in the host lattice in order to compensate the charge.

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Based on our structural refinement (Table 2), we observed that Pr3+ is located in the A-sites with the creation of vacancies responsible of the decrease of volume (Eq.2 and Eq.3).

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However, in the case of BTYN: Eu and BTYN: Nd, Eu3+ and Nd3+ are distributed over both sites without creating vacancies. The presence of Nd3+ (respectively Eu3+) in the Ti-sites induced more distortion that could explain the increase of the cell volume. 3.2. Dielectric, ferroelectric and piezoelectric studies

εr'

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The evolution of the real part

of the complex permittivity of Ba1-xLn2x/3

Ti0.925(Yb0.5Nb0.5)0.075O3 ceramics was determined as a function of both temperature and

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frequency for all investigated compositions (Figure 4). The obtained results showed that the values of TC are independent on frequency for all specimens, which is a typical behavior of a

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classical-ferroelectric material [14]. Even though, the temperature TC decreases with decreasing the ionic radii of the rare earth element. Such decrease may be associated with the existence of A-sites vacancies able to break the cooperative vibration of the Ti-O chains and decrease the region of correlation, thus leading to the reduction of TC [15]. Another possibility could be the shrinking of Ln-O bond length when decreasing Lanthanide size, that my change orbital around Ti(Yb, Nb)-O distance (lanthanide contraction).

6

ACCEPTED MANUSCRIPT Furthermore,

εr'

seems to be depending on the nature of rare earth element. The

εm

maximum of permittivity

decreases with the introduction of Praseodymium. However,

unexpectedly when decreasing the size of rare earth element, the value of the dielectric

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permittivity increase considerably (see Fig.4a). This behavior is intriguing as normally the reduction of rare earth element in a selected crystal family should reduce the polarizability, hence the permittivity [16]. In addition, no clear dependence of the maximum of permittivity

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could be observed in the mother phase (Fig. 4(b)). However, with the incorporation of Lnelement a clear frequency dependence of permittivity could be spotted which seems to be also

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governed by the nature the lanthanide element as it is shown in Figs. 4(c) and 4(d). As we can observe, for Pr-sample the dependence is almost similar to that observed for BTYN. In contrast, such dependence is more pronounced for the rest compounds. This difference in the behaviors is in good agreement with the XRD results and consolidates the hypothesis of the

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introduction of Nd3+ (or Eu3+) in the Ti-sites instead to incorporate of Pr3+ in only Ba- sites. In order to confirm the ferroelectric character of our prepared compounds, P-E hysteresis loops were recorded at different temperatures under 1 Hz test frequency. The

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polarizations versus the electric field at the selected temperature for all specimens are given in Fig. 5. We note that at temperature below Tmax, nonlinear P–E loops are observed that

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confirms the ferroelectric behavior of this material. The non- zero value of the polarization, Pr, personify in the plots by a slim P–E loop, implies that some residual domains are persisting even above TC , asserting that this transition is of the second order nature [11]. In addition the two parameters Ec and Pr decrease with increasing temperature, especially above

TC for all compositions. It is worth noting that the effect of Ln substitution of the ferroelectric properties is clearly seen. The remnant polarization and coercive field of BTYN are Pr=2.01µC/cm2, Ec= 1.42 kV/cm respectively. A close look to the obtained room temperature

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ACCEPTED MANUSCRIPT P-E hysteresis, we observe that the introduction of Pr- reduced both remnant polarization to 0.90 µC/cm2 and conceive field to 0.98kV/cm. In the contrast, the introduction of Nd-reduced Ec to 0.94 kV/cm but exhibited a strong enhancement of remnant polarization in the vicinity of that of BTYN (Pr = 2.083 µC/cm2).This value decreased a little bite with Eu3+

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incorporation to 1.49 µC/cm2. In the case of Pr-phase, the weak remnant polarization value and the small coercive field could be related to the increase of domain pinning caused by residual vacancies [17, 18]. As mentioned above, these prepared ceramics are A-site "

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deficiency, hence, under applied electric field these vacancies ( VBa ), along with some oxygen

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vacancies, may hop to lower free energy sites such as domain walls and interfaces with electrodes. This generally weakens the defect mobility and contributes to domain pinning that should decrease the remnant polarization and coercive field values. The sharpness of the P-E hysteresis loop indicates the better homogeneity and crystalline of the prepared system [19]. In the case of Nd- and Eu-phase, the improvement of remnant polarization (with less coercive

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field compared to the mother phase) gets more conviction that Nd- and Eu- elements distributed over Ba- and T- sites, and corroborate our DRX and dielectric investigations.

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The piezoelectric response was performed by the main parameter d33 at room temperature for all investigated specimens (Fig. 6). As it can be seen from the plot, d33 value

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gradually increases to the incorporation of Ln3+ in BTYN phase. The maximum value of d33 is obtained in BTYN: Nd compound. It’s about 40 pC/N. 3.3. Raman spectroscopy investigation Fig.7 (a) shows the room temperature depolarized Raman spectra of all ceramics. The

spectra are similar to that of BaTiO3 in tetragonal phase [20]. No clear influence on the nature of rare earth element could be spotted from these plots.

The Raman spectrum of the

tetragonal Ba1-xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3 showed the commonly-reported A1(TO1), A1

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ACCEPTED MANUSCRIPT (TO2), B1+E (TO+LO), A1 (TO3), and A1(LO3)+E(LO3) optical modes, peaking at approximately 180, 240, 305, 510,and 714 cm-1, respectively. The presence of the E+B1 mode at 305cm-1 is considered a sign of the long range ferroelectric tetragonal phase. In addition, from the Raman spectra, a new peak at about 820

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cm-1 appeared. Mazon et al. [21] reported that the difference of ionic radii between Ba2+ and Ln3+ can lead to the changes of the volumes of cell parameter in perovskites, and the caused internal deformation/distortion of TiO6 could be responsible for the band at 820 cm-1 in the

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Raman spectra.

Furthermore, the temperature dependence of the Raman spectra presented in Fig. 7((b)

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and (c)) shows the persistence of 305 and 714 cm-1 mode beyond the transition temperature. This could be attributed to a short-range structural disorder in the paraelectric phase. Finally, we note that broad Raman bands are observed and persisted in the paraelectric phase. Such disorder destroyed the perfect local cubic symmetry, thus allowing Raman activity in the

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above of the transition temperature in our BTYN solid solution. We attributed this behavior to the presence of a local structural disorder in the paraelectric phase created by the inhomogeneous distribution after the incorporation of the lanthanide ions.

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3.4. Optical properties: photoluminescence (PL) and UV visible spectroscopy 3.4.1. Photoluminescence spectroscopy (PL)

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PL is a powerful method to study the structural organization levels at medium range.

The properties of emission spectra are changed in general by the impurities and defect structure. In addition, to exhibit room temperature PL, a system must have at least two types of differently charged clusters creating a polarization within the structure and/or some localized states existing in the band gap that directly affects the degree of order–disorder. Introduction of defect is a mechanism to create disorder in a system. This defect may be a vacancy.

When

the

Ln

atoms

occupied

the

Ba

sites

into

the

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ACCEPTED MANUSCRIPT BaTi0.925(Yb0.5Nb0.5)0.075O3 perovskite, defect is introduced in the structure in the form of Asite vacancy. Noting that in this work, PL emission spectra of non-doped (x = 0) was examined at room temperature under irradiate with of 532 nm and 488 nm and no emissions were observed for the title composition. However, for the doped compositions we

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distinguished two different behaviors, depending on the nature of rare earth element. Firstly, the PL spectra of BaTi0.925(Nb0.5Yb0.5)0.075O3:Pr3+ is shown in Fig. 8. The spectrum consists of f-f emission transitions from the 3H4 excited state to the 3P0 level of the Pr3+ ion [22]. The

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emission spectra show two weaker green peaks centered at 530 nm and 542 nm due to Pr3+ ions transition (3P1→3H5), (3P0→3H5). Additional two intense peaks and three other weaker

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corresponding to the red emissions located at 615, 645, 677, 696 and 734 nm, respectively attributed to (3P0→3H6), (3P0→3F2), (3P1→3F3), (3P1→3F4) and (3P0→3F4) transitions respectively [23]. This behavior can be generated by the increase of defect levels below the conduction band as we have reported above in structural and electrical investigations.

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Regarding the Eu- and Nd- phases, the behavior obtained PL is completely different. As can be seen from Figure 9, an intense, dominate and asymmetric broad band centered at 690 nm accompanied with a small band one around 636 nm. It's worthy of mentioning that the

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intense PL band arises in the region of the known Europium ion

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D0→7F4 transition, but it is

so far from any known neodymium ion transition. The PL spectrum seems not depending of

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Eu3+ or Nd3+, suggesting that the origin of such strong red emission is not related to the rare earth element. It is worth to notice; however, that a similar broad band centered at 930 nm is widely reported in several glass matrixes such silicate [24, 25] and was attributed to the charged intrinsic defects in the network, with deeper energy levels such as oxygen deficient centers or broken bonds. Indeed, the substitution of Ba- sites by Lanthanide element, may causes an electronic compensation through the formation of barium vacancies (VBax, VBa’ or VBa”). In this case, it is 10

ACCEPTED MANUSCRIPT very probable that the Lanthanide atoms are coordinated to six oxygen (O) atoms (distorted [LnO6] clusters), while the Ba atoms are bonded to twelve oxygen atoms ([BaO12] clusters). The incorporation of Eu3+ and Nd3+ could induce a stronger distortion of the Ti-sites This could be due again to the higher concentration of defects [26]. In fact, oxygen vacancy is

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an efficient trap for photo excited electrons and normally acts as a center of radiative recombination [24]. Another possibility herein, that Nd3+ (respectively Eu3+) works as traps of holes. In order to better clarify the origin of the strong red emissions observed only for

3.4.2. Optical band gap energy and Urbach energy

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Nd3+, Eu3+ and Pr3+ we proposed a model based on the diffuse reflectance spectra.

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It is well established that the optical properties, especially the intensity of PL, could be influenced by the band gap energy [27]. The optical band gap energy (Eg) of ABO3 powders was estimated by the Kubelkae-Munk method [28, 29, and 30]. Such method is generally used for analyzing the diffuse reflectance spectra obtained from weakly absorbing samples. In this

F ( R) =

(1 − R )

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case the Kubelka–Munk equation at any wavelength becomes. 2

2R

(Eq.4)

function.

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where R is the absolute reflectance of the sampled, F(R) is the so-called Kubelka–Munk

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F(R) values were converted to the linear absorption coefficient (α) by the following relation [31, 32] α=

F ( R) t

(Eq.5)

where t is the thickness of pure and doped BTYN pellets which approximately equals 1 mm. The optical band gap of samples can be then determined by the following relation [33]:

α hν = A( hν − E g )

n (Eq.6)

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ACCEPTED MANUSCRIPT where hν is the photon energy, A is an energy independent constant, Eg is the optical band gap and n is a constant associated with the different types of electronic transitions (n = 1/2 for direct allowed, n = 2 for indirect allowed, n = 3/2 for direct forbidden and n = 3 for indirect forbidden). In this process, the photon absorption can promote an indirect electronic transition

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from minimum energy states in the valence band (VB) direction to a maximum energy state in the conduction band (CB). The Eg values of BTYN doped and undoped samples were calculated using n = 2 in Equation (6).

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1/2 The optical band gap of the samples was determined from the plots of ( F ( R ) hν / t )

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as a function of photon energy hν (Fig. 10) by extrapolating the linear portions of these plots to the x-axis (photon energy). The Eg values are found to be decreased when BTYN was doped with Eu, Nd and Pr respectively. (3.08eV, 2.963eV, 2.823eV, and 2.600eV). We can note that, the Eg decreases from undoped BTYN to doped samples. So, the obtained Eg values for the doped ceramics can be associated with the structural disorder introduced into the

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lattice due to the creation of A-site vacancy [34]. In fact, A-site vacancies introduce defects in the band gap of BTYN decreasing the Eg value which raises the intermediary levels within the band gap region. The width of defect formed in the band gap is associated with Urbach energy

hν

)

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α = α 0 ex p (

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Eu [35, 36, 37], and could determine from the following equation:

Eu

(Eq.7)

where α is the absorption coefficient and α 0 is a constant. The Urbach energy Eu was calculated by plotting ln( α ) versus photon energy (hv) Figure 11 . The values of Eu were found to be 0.099 eV, 0.162 eV, 0.228 eV and 0.270 eV for BTYN, BTYN:Eu, BTYN:Nd and BTYN:Pr respectively. The relationship between the band gap and Urbach energy for different samples is shown in table 3. Therefore, according to this calculates, while increasing the lanthanide size, the band gap decreases and the Urbach energy increases. Clearly, the 12

ACCEPTED MANUSCRIPT observation of PL could be attributed to the localized electronic level in the band gap with small Eg value (increase of defect levels between the valence band and the conduction band with small Eg values). This is generally responsible for the green and red PL emission at room

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temperature.

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ACCEPTED MANUSCRIPT 4. Conclusion In this study, BTYN: Ln (Ln= Pr, Nd and Eu) materials were prepared by conventional solid

state

method.

The

structural,

ferroelectric,

dielectric,

piezoelectric,

and

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photoluminescence properties were investigated. Praseodymium was found to behave differently to the other rare earth element with a preferred location in Ba- sites simultaneously with the creation of A- sites vacancies. In contrast, Nd- and Eu- elements seem to distribute over both Ba- and Ti- sites accompanying with charge compensation. This difference in the

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behavior was found to affect the physical properties. However, the electrical characterization

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showed that the incorporation of Eu or Nd in BTYN leads to the improvement of dielectric permittivity, down shift of transition temperature near to RT as well as better ferroelectric properties compared with the mother phase. For praseodymium phase, the dielectric properties were found comparable to BTYN phase, albeit a deterioration of ferroelectric properties is observed. The incorporation of Ln3+ was found to improve piezoelectric

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properties from 3pC/N for BTYN to 18, 40 and 26pC/N for BTYN:Eu, BTYN:Nd and BTYN:Pr respectively. The PL property was found to dominate by defect centers for Eu- and

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Nd- phases instead of luminescent activator for Pr3+ ions. The simultaneous presence of luminescence and ferroelectric properties in the present

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system is very promising for electro-optical devices and multifunctional material application.

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ACCEPTED MANUSCRIPT References [1] V. O. Sherman, A. K. Tagantsev, N. Setter, D. Iddles, and T. Price Ferroelectric-dielectric tunable composites. J. Appl. Phys. 99 (2006) 074104 [2] Zuo-Guang Ye Hand Book of Advanced Dielectric, Piezoelectric and Ferroelectric Materials. Woodhead, England (2008).

Sci. 44 (2009) 5049.

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[3] P.K. Panda Review: environmental friendly lead-free piezoelectric materials, J. Mater.

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Electroceram. 19(1) (2007) 113–26.

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[5] Shrout TR, Zhang SJ Lead-free piezoelectric ceramics: alternatives for PZT. J

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[6] Z. Abdelkafi, G. Khasskhoussi, N. Abdelmoula, H. Khemakhem Ferroelectric, piezoelectric and Raman spectroscopy studies of BaTi0.925(Nb0.5Yb0.5)0.075O3ceramic. J. ceram. Int. 41(2015) 14839–14844.

[7] C. Chalfouh, S. Zaghal, A. Lahmar, Z. Sassi, N. Abdelmoula, H. Khemakhem, Effect of Pr3+ doping on structural, electrical, and optical properties of BaTi0.925(Yb0.5Nb0.5)0.075O3 ceramics. J. All. Comp. 686 (2016)153-159.

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ACCEPTED MANUSCRIPT Figure captions Fig. 1. SEM micrographs of Ba1-xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3 compounds: (a) Mother phase; (b) Pr-phase; (c) Nd-phase; (d) Eu-phase. Fig. 2. Crystallite size and micro distortion as a function of Ln content

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Fig. 3. Measured and calculated room temperature X-ray diffraction patterns for Ba1xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3 compounds: (a) Mother phase; (b) Nd-phase; (c) Eu-phase; (c) Pr-phase. The vertical lines show calculated positions of Bragg reflexions and the lower curve is the residual diagram.

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Fig. 4. (a) Temperature dependance of the dielectric permittivity of BTYN: Ln ceramics at 1 kHz; (b),(c) and (d) Temperature dependence of the permittivity at different frequencies of Prphase, Nd-Phase and Eu-phase, respectively.

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Fig. 5. Thermal variation of P–E hysteresis loops of Ba1-xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3 ceramics at 1Hz. Fig. 6. Piezoelectric coefficient (d33) as a function of Ln content for the samples sintered at 1400°C.

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Fig. 7. (a) Room temperature Raman spectra of BTYN: Ln compounds. (b) and (c) Temperature dependence of Raman spectra of BTYN and BTYN: Eu respectively over the wavenumber range 50 –1000 cm-1. Fig. 8. Photoluminescence (PL) spectra of the Ba0.997 Pr0.0016Ti0.925(Yb0.5Nb0.5)0.075O3.

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Fig. 9. Photoluminescence (PL) spectra of the Ba0.997 Ln0.0016Ti0.925(Yb0.5Nb0.5)0.075O3(Ln= Nd et Eu).

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Fig. 10. Kubelka–Munk transformed reflectance spectra of Ba1-xLn2x/3Ti0.925(Yb0.5Nb0. 5)0.075O3 at room temperature. Fig. 11. Plot of ln versus hv for the determination of Urbach energy ( Eu ) for Ba1xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3 ceramics

with different rare-earth.

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ACCEPTED MANUSCRIPT Table captions Table. 1. Lattice parameters data of Ba1-xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3 compounds obtained by fullprof refinement.

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Table 2. Atomic positions and occupation factor of the pure and doped BTYN

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Table. 3. Band gap energy (Eg) and Urbach energy (Eu) of BTYN:Ln ceramics(Ln= Nd Eu Pr).

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ACCEPTED MANUSCRIPT Table 1: Crystallographic data of Ba1-xLn2x/3Ti0.925(Yb0.5Nb0.5)0.075O3 compounds obtained by solid-state reaction a= b(Å)

c(Å)

V (Å3)

Rwp

Rexp

χ2

4.0172

4.0363

65.1364

14.9

9.32

2.54

BTYN:Pr

4.0225

4.0235

65.1014

9.75

7.07

1.90

BTYN:Nd

4.0263

4.0270

65.2814

10.07

7.59

2.00

BTYN:Eu

4.0237

4.0250

65.1657

9.98

7.02

2.02

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Table 2: Atomic positions and occupation factor of the pure and doped BTYN x

y

z

Occupation factor

O1

0.50000

0.50000

0.03868

1.28415

O2

0.50000

0.00000

0.53336

1.33204

Ba

0.00000

0.00000

0.09455

Yb

0.50000

0.50000

0.55974

Ti

0.50000

0.50000

0.55974

Nb

0.50000

0.50000

0.55974

BTYN:Pr

x

y

z

O1

0.50000

0.50000

0.13460

1.06626

O2

0.50000

0.00000

0.52010

1.12256

Ba

0.00000

0.00000

0.05437

0.27723

Pr

0.00000

0.00000

0.05437

0.18433

Yb

0.50000

0.50000

0.59574

0.03824

Ti

0.50000

0.50000

0.59574

0.50793

Nb

0.50000

0.50000

0.59574

0.02106

x

y

z

Occupation factor

0.50000

0.50000

0.09992

1.28415

0.50000

0.00000

0.47310

1.33204

0.00000

0.00000

0.01798

0.29824

0.00000

0.00000

0.01798

0.18479

Yb

0.50000

0.50000

0.55479

0.03802

Ti

0.50000

0.50000

0.55479

0.50927

Nb

0.50000

0.50000

0.55479

0.02052

Nd

0.50000

0.50000

0.55479

0.01212

O2 Ba Nd

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BTYN:Nd

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BTYN

0.60276

0.05766

0.50902

0.03166

Occupation factor

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y

z

Occupation factor

O1

0.50000

0.50000

0.10293

1.28415

O2

0.50000

0.00000

0.47397

1.33204

Ba

0.00000

0.00000

0.01374

0.29824

Eu

0.00000

0.00000

0.01374

0.18479

Yb

0.50000

0.50000

0.54847

0.03802

Ti

0.50000

0.50000

0.54847

Nb

0.50000

0.50000

0.54847

Eu

0.50000

0.50000

0.54847

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BTYN:Eu

0.50927

0.02052

0.01212

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Eu (eV)

Mother-phase

3.08

0.099

Eu-Phase

2.963

0.162

Nd-Phase

2.823

0.228

Pr- Phase

2.600

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0.270

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(b)

Mother phase

(d)

Eu-Phase

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Nd-Phase

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Pr - Phase

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We explored the effect of Eu3+, Nd3+ and Pr3+ substitution on physical properties of BaTi0.925(Yb0.5Nb0.5)0.075O3 ceramics

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P–E hysteresis loop showed a decrease in remnant polarization and coercive field.

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BTYN: Pr sample exhibits simultaneous existence of ferroelectric and luminescence

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properties at room temperature.

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