Relationship between ferroelectric properties and local structure of Pb_1−xBa_xZr_0.40Ti_0.60O₃ ceramic materials studied by X-ray absorption and Raman spectroscopies

Highlights

• Structural characterization of Pb_1−xBa_xZr_0.40Ti_0.60O₃ (PBZT) ferroelectric ceramic.

• X-ray absorption and Raman spectroscopies were used to probe the structure of PBZT.

• Dissymmetry of Zr and Pb sites was observed in samples long-range cubic symmetry.

• Local disorder in all PBZT samples through the observation of Raman active modes.

Abstract

This paper reports on the structural characterization of Pb_1−xBa_xZr_0.40Ti_0.60O₃ (PBZT) ferroelectric ceramic compositions prepared by the conventional solid state reaction method. X-ray absorption spectroscopy (XAS) and Raman spectroscopy were used in the probing of the local structure of PBZT samples that exhibit a normal or relaxor ferroelectric behavior. They showed a considerable local disorder around Zr and Pb atoms in the samples of tetragonal or cubic long-range order symmetry. The intensity of the E(TO₃) mode in the Raman spectra of PBZT relaxor samples remains constant at temperatures lower than T_m, which has proven the stabilization of the correlation process between nanodomains.

Introduction

Lead zirconate titanate (PbZr_1−yTi_yO₃), also referred to as PZT ceramic system, has been extensively studied due to its unique properties that enable a wide variety of applications in piezoelectric, pyroelectric and ferroelectric devices [1], [2]. The PZT system exhibits a cubic structure at higher temperatures and three different structures at room temperature, depending on the composition, which can be tetragonal, orthorhombic or rhombohedral. According to the proposed phase diagram, all compositions are tetragonal and exhibit a P4mm symmetry on the titanium-rich side [2]. On the other hand, depending on y value and the temperature, two rhombohedral phases, R3m, often referred to as FR(HT), and R3c, referred to as FR(LT), occur in Zr-rich PZT ceramics [3]. The region between tetragonal and rhombohedral phases (y~0.50) is called morphotropic phase boundary (MPB) and characterized by the presence of those two phases and monoclinic symmetry with Cm space group, which is a subgroup of P4mm and R3m space groups [4]. For all values of y, the PZT system exhibits a long-range ferroelectric order, micrometer domain and/or domain wall structures and shows no frequency dispersion (relaxational effect) in the audio frequency range [5].

However, pure PZT ceramic materials are rarely applied to electronic devices and a doping process enhances their properties [2], [6]. For example, La³⁺ cations have substituted the Pb²⁺ cations and formed a Pb_1−xLa_xZr_1−yTi_yO₃ (PLZT) system. This substitution induces a peculiar diffuse phase transition with frequency dispersion and both La³⁺ aliovalent ions and/or oxygen vacancies, necessary to preserve charge neutrality, are believed to break the translational symmetry of the lattice and represent a type of disorder responsible for the formation of polar nanodomains, hence, a relaxor feature [5].

The substitution of Pb²⁺ by Ba²⁺ cations that form the Pb_1−xBa_xZr_1−yTi_yO₃ (PBZT) system has been also studied in details. Since the publication of the PBZT phase diagram [7], several studies have been conducted due to its variety of interesting physical properties of technological and fundamental importance and the relaxor ferroelectric behavior exhibited by certain PBZT compositions [5], [8]. In comparison with PZT samples containing lanthanum, a higher amount of barium is required for the occurrence of a relaxor behavior [5], [9]. Although this difference has not been completely understood, it has been related to vacancies created by the heterovalent substitution of Pb²⁺ by La³⁺. The high amount of Ba²⁺ in the PBZT system necessary for the occurrence of the relaxor behavior would be related to defects in the structure caused only by the difference between Pb and Ba ionic radius [5].

X-ray diffraction (XRD) results of PBZT ferroelectric relaxor materials showed the existence of a long-range order cubic symmetry with no phase transition above and below the temperature of maximum dielectric permittivity (T_m) [10], [11]. However, such symmetry is not compatible with a ferroelectric relaxor state.

Short-range order structure characterization techniques, such as X-ray absorption spectroscopy (XAS) and Raman spectroscopy, have revealed the existence of a high degree of local disorder above and below T_m, which is apparently not compatible with the long-range order cubic symmetry detected by XRD [12], [13], [14]. In fact, XAS technique is a powerful tool for the investigation of local structures and provides meaningful additional structural information on materials [15]. Although the local structural data afforded by XAS are usually not sufficient for the construction of a whole structural model, they often provide valuable information about the local structural peculiarities [16]. Pair distribution function analysis (PDF) is also able to give valuable local information [17]. However, we have decided to complete our preceding studies with the same synchrotron structural tool, EXAFS.

Pb atoms are of particular importance in PZT-based systems, as they hybridize with oxygen states, which leads to a large off-center displacement for the B atoms in the ABO₃ structure, hence, a high polarization and ferroelectric order [18], [19]. Recent studies have also shown the importance of the B atom site displacement in the BO₆ octahedron in the normal-to-relaxor ferroelectric transition [20], [21], [22]. Larger B atom off-center displacements cause the overbonding of oxygen atoms to be alleviated by the motion of B atoms away from the O atom, which leads to the stabilization of the normal ferroelectric phase [21]. The Zr displacement will produce a small Zr polarization that can be aligned with a modest electric field and create a favorable energetic situation for the switching of the strongly polar Ti–O dipole [18]. Therefore, a Zr shift can also play an important role for the large ferroelectric order in PZT-based systems.

We have carefully studied the local and electronic structure around Ti atoms in Ti-rich PBZT samples by X-ray absorption near edge structure (XANES) and observed a large Ti off-centering displacement as a function of Ba content in PBZT samples, even for the samples characterized as cubic by XRD Rietveld refinement and as relaxors from dielectric measurements [23].

Due to the important role played by Pb and Zr atoms in the normal to relaxor ferroelectric phase transition and to complete our previous structural study of PBZT ferroelectric ceramics, in which only the short-range order around Ti atoms was characterized, we present a short-range order analysis for Pb²⁺ and Zr⁴⁺ atoms. To the best of our knowledge, Zr and Pb short-range order in a Ti-rich PBZT system has not been characterized by XAS and Raman spectroscopy techniques.