Structure and dielectric behavior of Nd-doped BaTiO3 perovskites

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Materials Chemistry and Physics 109 (2008) 475–481

Structure and dielectric behavior of Nd-doped BaTiO3 perovskites Zhonghua Yao, Hanxing Liu ∗ , Yan Liu, Zhaohui Wu, Zongyang Shen, Yang Liu, Minghe Cao State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Ruo Shi Road, Wuhan 430070, PR China Received 8 July 2007; received in revised form 12 December 2007; accepted 15 December 2007

Abstract The structure and dielectric behavior of Ba1 − x Ndx TiO3 perovskites fabricated by conventional solid state mixed method were investigated. Phase transition behavior appeared from tetragonal to cubic structure with increasing Nd-doping level by XRD results. As the doping concentration increased, in the whole Raman spectra range except 848 cm−1 peak, the Raman intensities of doped BaTiO3 gradually decreased and the peaks became weaker and broader indicative of tetragonal to cubic phase transition. The appearance of the band at 848 cm−1 can be ascribed to the internal deformation/distortion of the TiO6 octahedra and the Nd3+ –barium vacancy pairs. Based on the XPS narrow-scan spectra of Ti 2p, binding energy of Ti 2p3/2 decreased with increasing Nd concentration showing the variation of the valence state of Ti ions, i.e. Ti4+ → Ti2+ . Nd3+ addition downshifted the temperature of the dielectric maximum and decreased the grain sizes. And the permittivity of doped BaTiO3 ceramics increased with decreasing grain sizes and a maximum value was attained at a size of 0.8–1 ␮m. The internal stress showed obvious effects on the permittivity of fine-grained ceramics, confirmed by doped BaTiO3 at x = 0.01. Compared with the relaxor ferroelectrics, these samples showed normal ferroelectrics and first-order type phase transition behavior. The TC − T0 (>0) deviation increased with Nd doping indicating the increasing effects of extrinsic non-ferroelectric phase on dielectric properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Ferroelectricity; Raman spectroscopy and scattering; Dielectric properties

1. Introduction Recent studies on doped barium titanate ceramics for their application as capacitor dielectrics, resistors, thermal sensors, etc., have been performed on the structural and phase transition behavior of the compounds and solid solutions, especially Asite substitution. Most of them exhibit normal phase transition behavior or diffuse phase transition behavior. The main A-site substitutions of ABO3 -type perovskites are classified into the following two categories: (1) iso-valent substitution, such as Ba1 − x Srx TiO3 and Ba1 − x Cax TiO3 , in which the phase transition behavior varies with doping concentration [1,2]. (2) off-valent substitution, such as (Ba,La)TiO3 ceramics [3] and (Pb,La)TiO3 with phase transition diffuse behavior. ∗

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0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.12.019

As we know, ferroelectric materials can be divided into two types, namely normal ferroelectrics and relaxor ferroelectrics. Compared with normal ferroelectric material, many studies [4,5] showed that the relaxor ferroelectric was highly diffuse and its temperature Tm shifted with frequency due to the dielectric dispersion. Because of the diffuseness of the dielectric anomaly and the anomalies in the temperature dependences of some other properties, relaxors are often called the “ferroelectrics with diffuse phase transition”, even though no transition into ferroelectric phase really occurs. Recently, many researchers have showed the doping of different ions to BaTiO3 -based ceramics, such as Sn4+ , Zr4+ , Mg2+ , Co3+ , Nb5+ , Mn4+ and rare-earth ions [6–10] to control grain size and to improve dielectric properties for electronic devices application. Many factors can be attributed to these two ferroelectric behaviors of BaTiO3 -based ceramics. Doping concentration is one of the most important among them. Shvartsman et al. [6] reported that Sn-doping concentration in BaTi1 − x Snx O3 ceramics could result in normal ferroelectric, relaxor behavior, even the intermediate state with the coexistence of these two behaviors.

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However, most of the doping ABO3 exhibit relaxor ferroelectric behavior [11]. From the above investigation, it is conferred that the addition of Nd3+ as a dopant can also modify the structure and dielectric properties of the BaTiO3 ceramics. In the present work, Ba2+ is displaced by Nd3+ at A-site (off-valent substitution) which can lead to A-site cation disorder and defect structure. The purposes of this investigation are to study the effect of Nd ions on the structure and dielectric behavior of modified barium titanate dielectrics. XRD, XPS, Raman scattering, FE-SEM and dielectric properties measurement are employed to illuminate intensively the structure and phase transition behavior of different Nd-doping level. 2. Experimental procedure Nd3+ -doped barium titanate were fabricated according to the stoichiometric composition Ba1 − x Ndx TiO3 (0 ≤ x ≤ 0.10) by conventional solid-state reaction method. Appropriate amounts of reagent-grade barium carbonate (BaCO3 , Merck, >99.0%), titanium oxide (TiO2 , SCRC, >99.0%) and neodymium oxide (Nd2 O3 , Merck, 99.9%), were mixed up to 48 h by ball milling in isopropyl alcohol. ZrO2 balls as a grinding media with appropriate size distribution were used to produce an optimized grain size. After milling, the milled powders were calcined in an open platinum crucible at 1100 ◦ C for 4 h, then submitted to ball milling again in isopropyl alcohol for 48 h. The ceramics were pressed into disks of 12 mm in diameter and 2 mm in thickness by uniaxial pressing at 200 MPa, burned out the binder at 650 ◦ C for 2 h and sintered at 1250–1400 ◦ C for 3 h. Crystalline phases of the sintered specimens were identified by a Philips ˚ vertical X-ray diffractometer (PW3050/60, MPSS) using Cu K␣ (l = 1.54056 A) radiation with 2θ in the range of 20–60◦ and step size of 0.02◦ with X’Pert HighScore Plus diffraction software for the calculation of cell parameters. The dielectric properties of the ceramics were measured at 1–500 kHz frequencies in the temperature range from −30 to 160 ◦ C using HP4284A Precision LCR Meter with a resistive homemade furnace controlled with a smart controller and a better than ±2 ◦ C temperature precision for temperature-dependent measurements. The silver electrodes were deposited onto the pellets for performing the dielectric measurements. The microstructures of etched ceramics were investigated by field-emission scanning electron microscope (FE-SEM, Hitachi, S-4800) at 10 kV. The bulk densities of the sintered pellets were measured by the Archimedes method. The Raman spectra were measured on pure and Nd3+ -doped BaTiO3 samples at 100–1000 cm−1 by InVia Raman Microscope made in Renishaw. The spectral excitation was provided by an Ar+ laser, using the 514.5 nm line and with proper power density on the sample surface. X-ray photoelectron spectroscopy was measured by multi-technique electron spectrometer (model: ESCALAB MK II, Mg K␣, 300 W; Ag3 d5/2 , FWHM = 0.8 eV). Narrow-scan XPS spectra of the Ti 2p peak of undoped and doped BaTiO3 ceramics were measured in this study. Spectra of all BaTiO3 based samples were referenced to the C 1s line of the residual carbon.

Fig. 1. XRD patterns of the sintered doped BaTiO3 ceramics.

Kutty and Murugaraj [12] confirmed by X-ray powder diffraction and electron diffraction studies that higher neodymium concentrations in BaTiO3 –Nd2/3 TiO3 ceramics could result in the appearance of Nd2 Ti2 O7 phase. With the increase of Nd3+ concentration, the diffraction peak 2θ angle increased gradually indicating the decrease in the volume of the unit cell for the lower ˚ substitution for Ba2+ ions (1.36 A) ˚ [13]. Nd3+ ions radii (0.98 A) ◦ Fig. 1(b) shows the changes of splitted 2θ = 45 peak with doping concentration. As the doping concentration increased, the well-resolved doublet till to x = 0.05 merged which exhibited tetragonal–cubic phase transition. The lattice parameters (a and c) of the BaTiO3 phase are determined by XRD results using HighScore plus software for the Ba1 − x Ndx TiO3 samples. The variation of a and c and the tetragonality factor c/a as a function of Nd-doping concentration, obtained from our XRD measurements recorded at room temperature, are shown in Fig. 2. For nominally undoped

3. Results and discussion 3.1. Phase transition by XRD studies Fig. 1(a) shows the XRD patterns of the sintered dopedBaTiO3 ceramics. It can be verified that, When x ≤ 0.10, in doped BaTiO3 perovskites the processing can favor the formation of the complete solid solution without secondary phase. Standard diffraction pattern (reference code: 00-005-0626) was employed to the indexing of our XRD results. Due to the solubility of Nd2 O3 , the higher doping concentration (x > 0.10) caused secondary phase or impurity phase (shown as arrow mark).

Fig. 2. The curve of lattice parameter and tetragonality factor c/a vs. Nd concentration in BaTiO3 -based ceramics.

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BaTiO3 (x = 0), the lattice parameters a = 0.3994(5) nm and c = 0.4032(7) nm are obtained in the present work. In contrast to c value, a value increased and the volume of unit cell decreased. The above results also showed an overall cubic structure for x ≥ 0.10 compositions. 3.2. Raman scattering analysis As an advanced nondestructive measuring technique, Raman spectroscopy has widely been used to study the structural properties of advanced functional materials, especially ABO3 -type perovskites materials. One of these interests is to detect any structural changes with the variation of crystal symmetry and to explain the possible reason to result in such structural transition. Nd-doped BaTiO3 perovskite is based on the original BaTiO3 and has similar phase structure at room temperature. Many theoretical explanations have been applied to the vibrational spectroscopy of single BaTiO3 [14,15]. The optical modes in cubic perovskite phase transform according to triply degenerate irreducible 3F1u + F2u representations. The 3F1u mode belongs to silent mode since it is neither Raman active nor infraredactive and so there is no Raman activity in ideal cubic symmetry [16]. Each triply degenerate F1u mode splits into A1 + E phonons in the tetragonal phase while F2u into E + B1 mode. Therefore, the -point Raman modes are 3A1 + B1 + 4E. The long-range electrostatic forces result in these modes further splitting into transverse and longitudinal optical modes (TO and LO). The room temperature-measured Raman spectra of undoped and doped BaTiO3 are shown in Fig. 3. As the doping concentration increased, in the whole Raman spectra range except 850 cm−1 peak, the Raman intensity of doped BaTiO3 gradually decreased and the peaks became weaker and broader indicative of some structural reordering taking place such as tetragonal to cubic phase transition. The coupling of A1 (TO1 ) mode at about 168 cm−1 with a broad A1 (TO2 ) mode at about 258 cm−1 should result in an anti-resonance effect at about 168 cm−1 frequency [17]. However, Raman spectra of doped BaTiO3 ceramics do not

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show any remarkable shift of the anti-resonance effect at about 168 cm−1 because of the similar masses of Nd and Ba ions. Furthermore, the coupling between the A1 (TO) and broad A1 (TO2 ) modes weakened as the intensity of A1 (TO2 ) mode decreased with the increase of doping concentration. The 305 cm−1 peak can be experimentally assigned to the E(LO) and E(TO) mode [18] related to the tetragonal–cubic phase transition [19]. The asymmetric and broad peak near 515 cm−1 are assigned as A1 (TO3 ) and B1 mode [20]. Owing to the features in tetragonal phase [21–23], the peak at about 717 cm−1 can be assigned as A1 (LO3 ) and E(LO3 ) mode. The two bands at 305 and 717 cm−1 disappeared with increase of doping level, which is also considered the signature of tetragonal–cubic phase transition. Seen from the Raman spectra, with the increase of Nd-doping level, a new peak at about 848 cm−1 appeared. Kchikech and Maglione [24] first reported the band at 840 cm−1 in La-doped BaTiO3 , with an intensity increasing linearly with La content. Mazon et al. [25] also confirmed the appearance of a new mode at about 850 cm−1 when the doping concentration of Nd3+ addition to Ba0.77 Ca0.23 TiO3 ceramics increased. The difference of cation ionic radii between Ba2+ , Ti4+ and Nd3+ can lead to the changes of the volumes of cell parameter in perovskites by XRD results indicative of the internal deformation/distortion of TiO6 octahedron. And that the internal deformation/distortion of TiO6 octahedron can be responsible for the band at 840 cm−1 [26]. On the other hand, previous work [27] confirmed that the excess of positive charge of donor ions was compensated by cation vacancy, as barium vacancy in Nd-doped BaTiO3 . The Nd3+ –barium vacancy pairs can also result in the appearance of the band at 848 cm−1 . Therefore, it is possible that the appearance of the band at 848 cm−1 can be ascribed to the internal deformation/distortion of the TiO6 octahedra and the Nd3+ –barium vacancy pairs. According to the above investigation, the ideal cubic symmetry should show no first-order Raman activities. The presence of some broader and weaker Raman modes in the cubic phase possibly can be ascribed to some disorder of Ti atoms along the cube diagonals because of partially Nd3+ substitution for Ba2+ [28]. Therefore, comparing with the tetragonal symmetry, the Raman intensities in cubic symmetry still exist and become weaker and broader. 3.3. XPS analysis

Fig. 3. The room temperature-measured Raman spectra of undoped and doped BaTiO3 .

Due to the off-valent substitution, the A-site cation disorder and defect properties can result in the changes of electronic band structure. As we know, for bulk intrinsic ferroelectric materials the fermi level is known to lie in the middle of the band gap. The shift in the binding energy peak gives the relative shift of the fermi level. To measure the effect of the dopants on the fermi level, high resolution measurement of the titanium 2p peak are taken by X-ray photoelectron spectroscopy shown in Fig. 4. Seen from the XPS spectra, the quantitative peaks showed that the Ti structure consisted of two peaks at the surface. One could observe a chemical shift of the Ti 2p (Ti 2p3/2 , Ti 2p1/2 ) level binding energy toward lower values with the increase of Nddoping level. The value of x varied from 0, 0.02 to 0.10, the

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Z. Yao et al. / Materials Chemistry and Physics 109 (2008) 475–481 Table 1 Curie–Weiss temperature (T0 ), Curie temperature, Curie constant (C), the difference (TC − T0 ), relative density and average grain size of ceramics as a function of Nd concentration Item Relative density (%)a Average grain size (␮m) C (×105 ◦ C) T0 (◦ C) TC (◦ C) TC − T0 (◦ C) a

x=0

x = 0.005

x = 0.01

x = 0.02

x = 0.05

x = 0.10

97.1

97.0

97.3

96.9

96.7

96.3

1.8

1.2

0.8

0.7

0.7

0.6

1.68 114.4 128 13.6

3.22 51.6 109 57.4

4.67 33.1 98 64.9

5.88 0.4 76 75.6

12.35 17.86 −88.8 −197.7 15 −25 103.8 175.7

The theoretical density is calculated by XRD results.

the above analysis, the shift of Ti 2p3/2 binding energy of doped BaTiO3 ceramics shows the variation of the valence state of Ti ions in present samples, i.e. Ti4+ → Ti2+ . Fig. 4. Narrow-scan XPS spectra of the Ti 2p peak of undoped and doped BaTiO3 ceramics.

binding energy of Ti 2p3/2 peak shifted from 458.0, 457.6 to 457.0 eV, respectively. Nasser et al. [29] confirmed that the peaks whose binding energies at 455.55 and 457.79 eV in the bulk region were attributed to TiO and Ti2 O3 (i.e., Ti2+ and T3+ ). Lu et al. [30] attributed the peaks of titanium in PbTiO3 , whose binding energies were 454.95 and 456.90 eV, to TiO (Ti2+ ). From

3.4. Dielectric characterization Fig. 5 shows dielectric properties of undoped and doped BaTiO3 ceramics as a function of temperature using the measurement frequencies of 1, 100 and 500 kHz. Curie temperature versus x value is shown in Table 1. Analyzing the above data, it was shown that the Nd3+ addition downshifted the temperature of the maximal dielectric constant from 128 ◦ C (x = 0) to

Fig. 5. Plots of dielectric constant and dielectric loss of undoped and doped BaTiO3 ceramics vs. temperature using the measurement frequencies of 1, 100 and 500 kHz, where (a) x = 0; (b) x = 0.005; (c) x = 0.01; (d) x = 0.02; (e) x = 0.05; (f) x = 0.10. Dielectric loss was measured at 500 kHz.

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Fig. 6. FE-SEM images of undoped and doped BaTiO3 ceramics, where (a) x = 0; (b) x = 0.005; (c) x = 0.01; (d) x = 0.02; (e) x = 0.05; (f) x = 0.10.

−25 ◦ C (x = 0.10). With increasing frequency, dielectric constant of each composition decreased. The value of the dielectric constant of each composition at higher frequencies markedly dropped. Such a marked drop of each composition in the value of the dielectric constant at higher frequencies can be explained in terms of interfacial polarization. The built-up of charges at the grain–grain boundary interface is responsible for large polarization therefore, the high dielectric constant at lower frequencies. It is well known that the permittivity of BaTiO3 ceramics is very sensitive to the grain size [31]. Coarse-grained ceramics of pure BaTiO3 show lower dielectric constant at room temperature than fine-grained. Generally, in heavily doped BaTiO3 ceramics grain size is usually suppressed and the permittivity of BaTiO3 ceramics attains the maximum when the grain size is 1 ␮m or less. Fig. 6 shows FE-SEM images of undoped and doped BaTiO3 ceramics. With the increasing doping level, the inhibition of grain growth was dependent on Nd content in agreement with previous results [32–37]. The relative densities and aver-

age grain sizes are listed in Table 1 showing very high relative density (≥96.3%) and fine grain sizes. The dielectric constant of doped BaTiO3 ceramics increased with decreasing grain size and a maximum value was attained at a size of 0.8–1 ␮m. According to Arlt et al. [31], this is a consequence of the decrease of the 90◦ domain width decreasing the grain size. Buessem et al. [38,39] and Bell et al. [40] also ascribed the increase of permittivity to the high internal stresses in fine-grained ceramics. So the permittivity of fine-grained ceramics should be related to ferroelectric domain and internal stress. In order to further study the effects of grain sizes on the permittivity, doped BaTiO3 at x = 0.01 (BN1 for short) is taken as an example. Fig. 7 shows the permittivity versus temperature at different grain sizes fabricated by variation of holding time. Generally, grain growth accompanies the decreased internal stress. It was shown in Fig. 7 that the permittivity depended strongly on grain size in the ferroelectric state (below Curie temperature) while it was independent on grain size in paraelectric state (over Curie temperature). The finer the

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Fig. 7. The permittivity vs. temperature using 1 kHz at different grain sizes.

grain size, the higher the permittivity in the ferroelectric state was. Therefore, in fine-grained BN1 ceramics the anomaly of the permittivity can be ascribed to the effects of internal stress. Furthermore, the broadening of the permittivity peak can be related to a non-homogeneous dopant distribution and to the existence of internal stresses because of the reduced number of ferroelectric domains in fine-grained ceramics. For normal ferroelectrics, it is characterized by a sharp dielectric constant peak showing no TC shift or no dispersion with frequency such as the composition of 0.99Pb1 − x La2/3x TiO3 + MnO2 with x = 0.05 [5] and (Pb1 − x Bax ) (Zr0.65 Ti0.35 )O3 ceramic at x = 0 [41]. Zhou et al. [42] confirmed that Bi3 − x Lax TiNbO9 at x ≤ 0.50 with broadening dielectric peak exhibited normal ferroelectrics while higher La-doping ceramics showed relaxational ferroelectrics. In the present work, all compositions at x ≥ 0 shows no dispersion or TC shift with frequency. The temperature at the peak permittivity determines the Curie temperature TC , over which the ferroelectric to paraelectric phase transition occurs, indicating that these compositional ceramics belong to normal ferroelectrics rather than relaxors. For normal ferroelectrics, the Curie–Weiss law 1/K = (T − T0 )/C is obeyed in the paraelectric phase only at higher temperatures than TC , where T0 is the Curie–Weiss temperature and C is the Curie constant. Fig. 8 shows plot of the reciprocal of dielectric constant at 500 kHz versus temperature and the fitting plot by Curie–Weiss law. Smolenskii [43] suggested the deviation of TC − T0 determined the order of phase transition, namely TC − T0 > 0 indicated the ferroelectric–paraelectric phase transition was of first-order type while TC − T0 = 0 with a broad dielectric constant peak was of a second-order transition. Curie–Weiss temperature (T0 ), Curie temperature (T), Curie constant (C) and the difference (TC − T0 ) as a function of Nd concentration are listed in Table 1. Based on the above results, the doped BaTiO3 ceramics obey the TC − T0 > 0, showing the first-order type phase transition behavior. The difference (TC − T0 ) is about 8–15 ◦ C in pure BaTiO3 from previous works [44,45]. With increasing Nd-doping level, the difference increased dramatically. The larger deviation values indicate that the dielectric properties are affected by extrinsic

Fig. 8. plot of the reciprocal of dielectric constant (1/K) at 500 kHz vs. temperature and the fitting plot by Curie–Weiss law.

effect, e.g. the existence of a non-ferroelectric grain boundary phase due to Nd doping [46–48].

4. Conclusion The structure and dielectric behavior of Ba1 − x Ndx TiO3 perovskites fabricated by conventional solid state mixed method were investigated in this paper. XRD analysis indicated that phase transition behavior appeared from tetragonal to cubic structure with increasing Nd-doping level. As the doping concentration increased, in the whole Raman spectra range except 848 cm−1 peak, the Raman intensities of doped BaTiO3 gradually decreased and the peaks became weaker and broader indicative of tetragonal to cubic phase transition. The appearance of the band at 848 cm−1 can be ascribed to the internal deformation/distortion of the TiO6 octahedra and the Nd3+ –barium vacancy pairs. Furthermore, due to some disorder of Ti atoms along the cube diagonals by partially Nd3+ substitution for Ba2+ , the Raman activities still existed in the cubic structural Nd–BaTiO3 ceramics. Based on the X-ray photoelectron spectroscopy spectra of Ti 2p, binding energy of Ti 2p3/2 decreased with the increase of Nd concentration showing the variation of the valence state of Ti ions, i.e. Ti4+ → Ti2+ . Nd3+ addition downshifted the temperature of the dielectric maximum from 128 ◦ C (x = 0) to −25 ◦ C (x = 0.10) and decreased the grain sizes. And the permittivity of doped BaTiO3 ceramics increased with decreasing grain size and a maximum value was attained at a size of 0.8–1 ␮m. The internal stress showed obvious effects on the permittivity of fine-grained ceramics, confirmed by doped BaTiO3 at x = 0.01. Compared with the relaxor ferroelectrics, there were no any shift of Curie temperature exhibiting normal ferroelectrics rather than relaxor ones and Curie–Weiss law was obeyed. The TC − T0 deviation increased with Nd doping. The TC − T0 deviation (>0) is indicative of first-order type phase transition and much larger TC − T0 deviation indicates the increasing effects of extrinsic non-ferroelectric phase on dielectric properties.

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Acknowledgement The authors would like to thank the Foundation for Innovative Research Team of Hubei Province of China, Grant No. is 2005ABC004.

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