A study of optical and magnetoelectric properties of BaTi1−xNixO3

JOURNAL OF SCIENCE OF HNUE Mathematical and Physical Sci., 2012, Vol. 57, No. 7, pp. 44-50 This paper is available online at A STUDY OF OPTICAL ANDMAGNETOELECTRIC PROPERTIES OF BaTi1−xNixO3 Nguyen Van Khanh Faculty of Physics, Hanoi National University of Education Abstract. We present the results of X-ray diffraction (XRD), Raman scattering, absorption, impedance spectroscopy and magnetization studies of Ni2+-doped BaTi1−xNixO3. Some Ba atoms were replaced by Ni with x atoms in the range of 0.0 - 0.4. XRD analysis of these samples indicates a change in the cell parameters with doping content and there appears an abnormal point at x = 0.10. Our main focus is the Raman peak vs. Ni content, which have separated peaks in the range of 450 cm−1 - 600 cm−1. The shift in Raman and change in absorption spectra are evidence that would relate to the structure change or disorder and impurity in the samples. From the above results, a critical substituent content at which the structural phase change takes place is shown. We also present the impedance spectroscopy and magnetization vs. magnetic field to elucidate the role of dopant in the BaTi1−xNixO3 samples. Keywords: BaTi1−xNixO3, Raman scattering, absorption, impedance spectroscopy, magnetization. 1. Introduction Multiferroic materials are of great scientific and technological interest due to their magnetoelectric properties, originating from the coupling between ferroelectric and ferromagnetic order parameters [10]. The interplay between ferroelectricity and magnetism allows a magnetic control of ferroelectric properties and an electric control of magnetic properties and could yield new device concepts, such as ferroelectric and multiferroic tunnel junctions [13]. Barium titanate (BaTiO3) is a ferroelectric material widely studied because of its many potential useful properties. The phase of BaTiO3 at room temperature is a Received October 15, 2011. Accepted February 23, 2012. Physics Subject Classification: 62 44 07 01. Contact Nguyen Van Khanh, e-mail address: khanhnvsp@gmail.com 44 A study of optical and magnetoelectric properties of BaTi1−xNixO3 tetragonal phase, which then transforms into cubic phase at 1300C. There are numerous proposals regarding its application in electronic and electro-optic devices apart from its high permittivity phenomena, associated with the basic ferroelectric behavior. In particular, the transition metal (TM) ion doped BaTiO3 system has been of great interest [7]. There are some reports on the effect of the doping concentration on ferromagnetism of the BaTiO3 system. Rajamani et al. [9] prepared ferromagnetic Ba(Ti1−xFex)O3 thin films and found that the saturation magnetization (MS) gradually increased with a change in Fe content. On the other hand,Maier and Cohn [3] reported that the change in magnetization with the doping concentration showed two different trends for Ba(Ti1−xFex)O3 thin films grown on MgO and SrTiO3 substrates. W. Weidong et al., [12] investigated the structure of Co-BaTiO3 nano-composite films and found that the interaction between the BaTiO3 matrix and Co nanoparticles results in a Raman shift and an enhanced Raman scattering intensity but they were not concerned with ferromagnetic property. Although the dependence of magnetization on the doping concentration was measured, the mechanism responsible has not yet been put forward because the practical TM valence in ferromagnetic transition metal doped BaTiO3 has not been investigated experimentally or theoretically up to now, and this is essential for discussion of optical and magnetic behavior. In the present study, Ni2+-doped BaTiO3 ceramics using various concentrations were fabricated by solid-state reaction using the sol gel method. We mainly discuss the influence of the doping concentration on the structure, optical properties and ferromagnetism of Ni2+-doped BaTiO3 samples. 2. Content 2.1. Experiment Nanopowders of BaTi1−xNixO3 were prepared by the polymeric precursor method (PPM) which is based on the chelation of the metal cations by citric acid in a solution of water and ethylene glycol. The precursor solution was prepared from a titanium citrate formed by dissolving titanium isopropoxide in an aqueous solution of citric acid heated to about 700C. A stoichiometric amount of Ba(NO3)2 was added to the titanium citrate solution, which was stirred slowly until the reactional mixture became clear. Ni(NO3)2.6H2O was then added slowly. To completely dissolve the Ba(NO3)2 and Ni(NO3)2.6H2O, ammonium hydroxide was added a drop at a time until the pH reached 7 - 8. The complete dissolution of the salts resulted in a transparent solution. After the solution containing Ba and Ni cations was homogenized, ethylene glycol was added to 45 Nguyen Van Khanh promote a polyesterification reaction. The solution became more viscous as the heat rose to 900C, albeit without any visible phase separation. The mass ratio of the citric acid/ethylene glycol was set at 60:40. This resin was then placed in a furnace and heated to 4500C for 4 hours, causing it to form a powder. This powder was heated at 3000C for 10 hours to obtain the disordered phase of the BaTi1−xNixO3. The crystalline phase was obtained by heating the powder at 12000C for 8 hours. Structural characterization was performed by means of X-ray diffraction using a D5005 diffractometer with Cu Kα radiation. The absorption spectra were recorded using a Jasco 670 UV-vis spectrometer and the Raman measurements were performed using the 514.5 nm line of an Ar ion laser in a back scattering geometry using Jobin Yvon T 64000 triple spectrometer equipped with a cryogenic charge-coupled device (CCD) array detector. Impedance spectroscopy data was obtained using Le Croy equipment. 2.2. Results and discussion Samples of BaTiO3 doped with Ni 2+ ions of different concentrations and sintered at 12000C were characterized by X-ray diffraction. The XRD patterns of BaTiO3 powders doped with Ni2+ ions of various concentrations are presented in Figure 1. Peaks shown in the patterns are sharp and well defined, indicating that all samples are well crystallized. The patterns were compared to the reference standard cards for tetragonal and cubic BaTiO3 [6]. An analysis of the XRD patterns showed that all samples were in the tetragonal phase. It is well known that the dopant ionic radius is a main parameter determining the substitutions in the BaTiO3 crystal lattice. Thus, Ni 2+ ions (0.69 A˚) are mainly placed at Ti4+ sites (0.68 A˚) and not at Ba2+ sites (1.35 A˚) due to size incompatibility. Increasing dopant concentration causes an overall shrinkage of the BaTiO3 unit cell, changing lattice parameters and resulting in a change of the crystal structure (see Figure 2). We assume that the 10% content is the limit of solubility of Ni in the BaTiO3. Using the scherrer formula: D = 0.89λ β cos θ (2.1) and the particle size is about 130 nm. Only the tetragonal phase is found in the BaTiO3 specimens. After the addition of Ni, some new peaks are raised. However, the tetragonal phase is the major phase that is found in the Ni2+-doped BaTiO3 specimens. The values of c/a ratio are shown as a function of Ni content. The c/a ratio values drop from 1.011 to a lower value of 1.005 as the Ni content is raised above 0.10. This indicates that the Ni2+ ion is capable of 46 A study of optical and magnetoelectric properties of BaTi1−xNixO3 dissolving into BaTiO3 crystals and the solubility of Ni 2+ ion in BaTiO3 is 0.10 [5]. This is in agreement with XRD patterns in Figure 1 and data of cell parameter in Figure 2. Figure 1. XRD patterns Figure 2. Cell parameter vs. of BaTi1−xNixO3 ceramics Ni content It was known that the solubility of a dopant in BaTiO3 depends not only on the charge but also on the radius of the dopant [8]. Though the charge of the Ni2+ ion is the same as that of Ba2+, the radius of the Ni2+ ion (0.069 nm) is close to that of the Ti4+ ion (0.0605 nm). Therefore, it is possible for the Ni2+ ion to substitute for the Ti2+ ion, as in the reaction shown below: Ni BaTiO3−−−−−−−→ Ni′′Ti + V••O (2.2) V ••O is the oxygen deficiency. Since the charge of the Ni 2+ ion is lower than that of the Ti4+ ion, the Ni ion acts as the acceptor to BaTiO3. Furthermore, an oxygen vacancy is needed to compensate for the substitution of the Ti4+ ion with an Ni2+ ion. As the Ni content approaches the solid solution limit, the concentration of oxygen vacancy also reaches its highest value. The solubility of Ni in BaTiO3 is as high as 0.10, from Eq. 2.2, and the concentration of oxygen vacancy is the same as that of Ni solute. This suggests that an amount of oxygen vacancy is formed due to the solution of Ni into BaTiO3. The addition of the acceptor is thus an effective method to produce a relatively large amount of oxygen vacancy. The Raman spectra for BaTiO3 ceramic powder samples are presented in Figure 3. BaTiO3 has five atoms and fifteen degrees of freedom per unit cell. In cubic phase it has Oh symmetry and 15 degrees of freedom divided into the optical representations 3F1u + F2u, while another F1u symmetry mode corresponds to the acoustical branch. At room temperature BaTiO3 is tetragonal and has C4v symmetry. The frequency covered 47 Nguyen Van Khanh ranged from 100 cm−1 to 800 cm−1. Based on the crystallography, Raman-active modes for tetragonal BaTiO3 (P4mm) are 4E (TO + LO) + 3A1 (TO + LO) + B1 (TO + LO) while no Raman-active mode is predicted for the cubic phase (Pm3m). The three E (TO) modes with frequencies of approximately 190, 280 and 516 cm−1 are labeled in Figure 3. As discussed above, the 190 cm−1 and the 516 cm−1 modes come from the F1u cubic phase modes and the 303 cm−1 mode comes from the splitting of the cubic silent F2u mode. The 140, 303, 640 cm−1, and the somewhat broader 720 cm−1 modes constitute the E(LO) modes. The TO [LO splitting is fairly small and cannot be identified]. The A1(TO) mode at 280 cm−1 is also shown in Figure 3. The intensity of the peak at around 303 cm−1 was assigned to the overlap of E (3TO) + E (2LO) + B1. Many researchers have found that the Raman mode around 303 cm−1 is characteristic of the tetragonal BaTiO3. The Raman peak near 303 cm−1 shows a large decrease in intensity for increasing the Ni. Specifally, the band near 516 cm−1 was separated into two peaks in the case of Ni2+-doped BaTiO3. This evidence supports the comment in the XRD analysis above. Figure 3. Raman spectra Figure 4. Absorption spectra of BaTi1−xNixO3 ceramics of BaTi1−xNixO3 ceramics Table 1. Optical phonon frequencies (ω) and their mode symmetry assignments in tetragonal BaTiO3 [11] ω (cm−1) Symmetry ω (cm−1) Symmetry 170 A1(TO) 475 A1(LO) 185 E(TO + LO) 486 E(TO) 270 A1(LO) 518 E(TO) 305 A1(LO) 520 A1(TO) 305 E(TO + LO) 715 E(TO) 463 B1 720 A1(LO) 48 A study of optical and magnetoelectric properties of BaTi1−xNixO3 Figure 5 shows the absorption spectra of BaTi1−xNixO3 ceramics with various Ni content. Besides the absorption edge of around 450 nm in the spectra of Ni2+-doped BaTiO3 samples, there appear some others at around 900 nm and 1350 nm. These absorption peaks are until now not explained and an investigation is in progress. It has already been shown that intensity dependent absorption is consistent with the presence of secondary centers [2]. These are intermediate-level charge trapping impurity sites that are usually highly ionized at room temperature, but can be populated by photoinduced charge carriers from the deep centres. The photoinduced transfer of charge from a deep to a secondary centre leads to a change in the absorption of the crystal if different photoionisation cross sections exist for these centres. Direct intra band charge transfer is also possible in principle but it is not usually included in band transport models. Figure. 5 Magnetization vs. magnetic field of BaTi1−xNixO3 ceramics Ni2+-doped BaTiO3 also produced signatures of ferromagnetism, as is shown in Figure 2. The coercivities were again in the order a few hundred Gauss at 300K for all Ni concentrations. XRD data showed an evidence of secondary phase formation related to the presence of Ni clusters that would influence the magnetic properties of these samples. The mechanism for the observed ferromagnetism is still not clear and may be due to bound magnetic polarons [1] or carrier-induced magnetism inherent in the Zener mechanism [4]. 3. Conclusion Barium titanate powder was obtained via the sol-gel route. The structure of BaTiO3 nanocrystallites was determined. The influence of dopant concentration on the crystal structure of BaTiO3 was established. An addition of 0.10 Ni is essential to the formation of a foreign phase. The critical content of 0.10 corresponds to the solid solution limit of Ni in BaTiO3. This was also confirmed by Raman spectroscopy. The Ni 2+ ion acts as 49 Nguyen Van Khanh the acceptor and induces the increase of oxygen vacancy concentration. The large amount of oxygen vacancy helps the formation of h-BaTiO3. 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