Elsevier

Journal of Alloys and Compounds

Volume 640, 15 August 2015, Pages 355-361
Journal of Alloys and Compounds

XANES measurements probing the local order and electronic structure of Pb1−xBaxZr0.40Ti0.60O3 ferroelectric materials

https://doi.org/10.1016/j.jallcom.2015.04.015Get rights and content

Highlights

  • Structural characterization of Pb1−xBaxZr0.40Ti0.60O3 (PBZT) ferroelectric ceramic.

  • X-ray absorption and spectroscopy was used to probe the structure of PBZT.

  • O K-edge XANES spectra showed hybridization between the O 2p and Pb 6sp states.

  • Relaxor behavior are related to weakening of the O 2p and Pb 6sp hybridization.

Abstract

In this study, the electronic and local structures of Pb1−xBaxZr0.40Ti0.60O3 ferroelectric ceramic samples were characterized using X-ray absorption near-edge structure (XANES) measurements. The analysis of XANES spectra collected at the Ti K- and L-edges showed that the substitution of Pb by Ba leads to a decrease in the local distortion around the Ti atoms in the TiO6 octahedron. The analysis of O K-edge XANES spectra and density of states ab initio calculations showed that the hybridization between the O 2p and Pb 6sp states is related to the displacement of Ti atoms in the TiO6 octahedra. Based on these results, it is possible to determine that the degree of ferroelectricity in these samples and the manifestation of relaxor behavior are directly related to the weakening of O 2p and Pb 6sp hybridization.

Introduction

Lead zirconate titanate (PbZr1−yTiyO3, also referred to as PZT) ceramic systems have been extensively studied due to their unique properties, which permit a wide variety of applications such as in piezoelectric, pyroelectric and ferroelectric devices [1], [2], [3], [4]. The PZT system exhibits a cubic structure at higher temperatures and three different structures at room temperature depending on the composition: tetragonal, orthorhombic or rhombohedral. According to the proposed phase diagram [5], at the titanium-rich side of the phase diagram, all compositions are tetragonal with P4mm symmetry. On the other hand, depending on x and the temperature, two rhombohedral phases, R3m, often referred to as FR(HT), and R3c, referred to as FR(LT), are known to occur in Zr-rich PZT ceramics [6]. The region between the tetragonal and rhombohedral phases (y  0.50) is called the morphotropic phase boundary (MPB) and is characterized by the presence of these two phases and monoclinic symmetry with the Cm space group, which is a subgroup of the P4mm and R3m space groups [7]. More recently, Glazer et al. performed neutron diffraction measurements; their analysis allowed for the elaboration of a new phase diagram with mixtures of rhombohedral and monoclinic symmetries on the Zr-rich side, tetragonal and monoclinic on the Ti-rich side, and three phases coexisting in the MPB region [8]. For all values of y, the PZT system exhibits long-range ferroelectric order, micrometer domain and/or domain wall structures, and does not show any frequency dispersion (relaxational effect) in the audio frequency range [9].

However, pure PZT ceramic materials are rarely applied in electronic devices, and a doping process is used to enhance the properties of this class of material [5], [10]. For this purpose, La3+ cations have been used to replace the Pb2+ cations, forming a Pb1−xLaxZr1−yTiyO3 (PLZT) system. It is well known that this substitution induces a peculiar diffuse phase transition with frequency dispersion. It is widely believed that both the La3+ aliovalent ions and/or oxygen vacancies (necessary to preserve charge neutrality) break the translational symmetry of the lattice and represent a type of disorder responsible for the formation of polar nanodomains and, consequently, the relaxor feature [9].

Another doping method has also been used to enhance the properties of the PZT ceramic system: the substitution of Pb2+ by Ba2+ cations, forming the Pb1−xBaxZr1−yTiyO3 system. Since the publication of the phase diagram [11], several studies have been performed on this system due to its rich variety of interesting physical properties of both technological and fundamental importance [12], [13], [14], [15], [16]. In these papers, it has been reported that certain PBZT compositions present the characteristics of typical relaxor ferroelectrics [9], [16]. Moreover, relaxor behavior was observed in our previous study concerning the Pb1−xBaxZr0.40Ti0.60O3 system with x = 0.50 [17]. With a Zr/Ti ratio of 65/35, relaxor behavior is observed for 40 at.% of Ba, a higher amount compared to the amount of La in a PLZT system with the same Zr/Ti ratio (∼8 at.% of La) which exhibits this behavior [9], [18]. This difference between systems is not completely understood, although it is related to the fact that Ba substitution does not create vacancies because of its isovalent incorporation. Therefore, the appearance of relaxor behavior in the PBZT system could be related to defects in the structure caused by the difference between Pb and Ba ions [9].

The results of X-ray diffraction (XRD) in different relaxor materials with a perovskite structure have shown, in most cases, the existence of a cubic structure both above and below the temperature of maximum dielectric permittivity (Tm) [19]. From the standpoint of local order, the use of different techniques for structural characterization such as XAS and Raman spectroscopy has shown the existence of a certain degree of disorder both above and below Tm. This degree of disorder is not compatible with a local structure with cubic symmetry as detected by XRD measurements [20], [21], [22]. Furthermore, it has been shown that the analysis of XAS measurements of elements with lower energy edges or low atomic number can provide information concerning the local order and electronic structure of different materials, which can be important for elucidating aspects about ferroelectricity [23], [24], [25], [26], [27].

The purpose of this study was to verify the role of the substitution of Pb2+ by Ba2+ ions in the local structure of the Pb1−xBaxZr0.40Ti0.60O3 ferroelectric material. This Zr/Ti ratio was chosen because of the absence of studies in the literature describing the structure of a PBZT system with a Ti-rich composition. A correlation between the relaxor behavior observed in some Ba compositions and X-ray absorption near edge structure (XANES) measurements at the Ti K- and LII,III-edges and the O K-edge is presented.

Section snippets

Experimental procedures

PBZT samples, of the nominal composition Pb1−xBaxZr0.40Ti0.60O3 (denoted as PBZT100x) with x = 0.00 (PZT), 0.10, 0.20, 0.30, 0.35, 0.40 and 0.50 at.%, were prepared by the conventional mixed oxide method. The oxides PbO, BaO, ZrO2 and TiO2, weighed according to stoichiometry, were mixed by ball milling in isopropyl alcohol for 5 h. The slurry was dried and calcined in a covered alumina crucible at 850 °C for 4 h and mixed by ball milling again for 5 h. Ceramic bodies were then formed by uniaxial

Results and discussion

In order to investigate the local order around a Ti atom, Ti K-edge XANES measurements were performed for the PBZT samples; the spectra are presented in Fig. 1 and its inset. The pre-edge region of the K-edge XANES spectra of some transition metal oxides is characterized by a pronounced feature, several volts before the main rising edge [21], [30], [31]. In transition metal oxides that crystallize in centrosymmetric structures, this pre-edge feature is very small or absent, whereas in

Conclusions

In this study, we performed XANES measurements to probe the local order of PBZT samples and to correlate the results with the ferroelectric properties of these materials. Ti K-edge XANES measurements showed that Ba incorporation into the PZT structure leads to diminished local distortion of the Ti atoms in relation to O atoms. Moreover, we observed a reduction in Ti displacement from TiO6 centrosymmetric positions as the Ba concentration increased. Ti LIII-edge XANES measurements for PBZT

Acknowledgments

The authors are grateful to the funding agencies FAPESP and CAPES and to Dr. M.I.B. Bernardi for the ICP measurements. The presented research was partially carried out at the LNLS National Laboratory of Synchrotron Light, Brazil, and at the Canadian Light Source.

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