Probing the Ba 5d states in BaTiO3 and BaSO4: A resonant x-ray emission study at the Ba-L3 edge

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Abstract

We have directly probed the Ba 5d states in the ferroelectric barium titanate BaTiO3 using two bulk-sensitive spectroscopic probes, resonant x-ray emission spectroscopy (RXES) and x-ray absorption spectroscopy in the partial fluorescence mode (PFY-XAS) at the Ba-L3 edge. The results are compared with those of the non-ferroelectric barium sulfate BaSO4. While the RXES spectra point to a localized character for the Ba 5d states in both compounds, the main peak of the PFY-XAS spectrum, corresponding to the dipolar transitions from 2p to 5d, is found to be significantly broader for BaTiO3 than for BaSO4. On the basis of band structure calculations, this broadening is ascribed to strong hybridization between the unoccupied Ba 5d and O 2p states in the ferroelectric. This suggests that the hybridization between the conduction states of the Ba2+ and O2− ions, and not only Ti4+ and O2−, plays a central role in determining the electronic structure of BaTiO3, and is therefore likely to be indirectly correlated with the occurrence of ferroelectricity in this material.

Highlights

► Resonant X-ray emission measurements for BaTiO3 and BaSO4. ► Hybridization between Ba and O in BaTiO3. ► Possible important role of Ba in ferroelectricity in BaTiO3.

Introduction

Barium titanate BaTiO3 is a well-known ferroelectric material, and has been extensively studied because of its promising technological applications [1], [2]. This material is classified into the so-called displacive-type ferroelectrics. At temperatures higher than the Curie temperature (TC) of ∼130 °C, BaTiO3 is stable in a paraelectric (non-ferroelectric) cubic perovskite structure (space group Pm-3m) [3]. On the other hand, at temperatures lower than TC, the structure changes into a tetragonal ferroelectric type (space group P4mm), in which the cations (Ba2+ and Ti4+) are displaced to the direction opposite to the anion (O2−), thus forming electric dipoles.

Although numerous experimental studies were reported on the ferroelectric behavior of this material, the nature of the ferroelectric phase transitions was unclear until first-principles calculations were employed about 20 years ago [4], [5]. It was proposed that the existence or absence of the displacement is determined by a balance between short-range repulsions between adjacent electron clouds, which favor the non-ferroelectric symmetric structure, and additional bonding characteristics which may stabilize the ferroelectric phase [6]. The ferroelectricity in BaTiO3 was attributed to the strong hybridization between Ti4+ and O2−, which allows off-centering of Ti4+ in the TiO6 octahedron while avoiding short-range repulsion [4], [5]; hence, experimental and theoretical studies have mainly focused on the plausible importance of Ti4+ and the surrounding O2− ions [7], [8], [9]. On the other hand, the Ba2+ ion has been considered as being rather isolated [4], [5], therefore having little or no correlation with the occurrence of ferroelectricity.

In the present study we have used resonant x-ray emission spectroscopy (RXES) to study the 5d states of Ba2+ in two Ba-containing materials, ferroelectric BaTiO3 and non-ferroelectric barium sulfate BaSO4.In both compounds, Ba sites are surrounded by 12 oxygen nearest neighbors, with shorter distances in the former. RXES has recently emerged as a valuable tool to probe the bulk electronic structure of solids, with which it is possible to resonantly enhance excitations to specific intermediate states by properly choosing the incident energy. It has proven instrumental in the study of the degree of valence mixing and electronic localization of the 3d and 4f states [10], [11], [12], [13], and more recently of the 5f states too [14]. We here utilized RXES at the Ba-L3 edge in order to directly observe the empty 5d states of Ba. Information about the degree of localization and hybridization of the Ba 5d states is obtained by analyzing the experimental spectra in view of band structure calculations. Our results show that the Ba 5d states in BaTiO3, while retaining a localized character, strongly hybridize with the O 2p band. This suggests that the hybridization not only between the Ti4+ and the O2− ions, but also between the Ba2+ and O2− ions governs the electronic structure of BaTiO3, and could therefore be indirectly involved in the stabilization of the ferroelectric phase.

Section snippets

Experiment and calculation

The experiments were performed using synchrotron radiation at the BL15XU beamline of SPring-8 [15]. The undulator beam was monochromatized using a Si (111) double-crystal monochromator. The energy analysis of the emitted x-rays was performed using a two-crystal spectrometer equipped with two single crystals of Ge(111). The RXES spectra were measured for several incident photon energies across the Ba-L3 (2p3/2) absorption edge at room temperature. The BaTiO3 sample was a commercially available

X-ray absorption spectra in the partial fluorescence yield

Fig. 1 shows the Ba-L3 x-ray absorption spectra for BaTiO3 and BaSO4. The measurements were carried out by monitoring the intensity of the maximum of the Ba-Lα1 emission peak (∼4467 eV) while sweeping the incident photon energy (Ein) across the Ba-L3 edge. While the overall spectral lineshape resembles that of the spectrum measured in the transmission mode [18], the spectrum obtained in the so-called partial fluorescence yield (PFY) mode benefits from a higher energy resolution owing to the

Summary

We have performed a comparative study of the Ba 5d states in the ferroelectric BaTiO3 and the non-ferroelectric BaSO4 using two complementary bulk-sensitive spectroscopies, RXES and PFY-XAS. Our data, analyzed with the aid of band structure calculations, provide evidence for the formation of hybridized Ba 5d–O 2p band states in both compounds, although the Ba 5d states retain a somewhat localized character. The overlap between the conduction states of the Ba2+ and O2− ions is found to be larger

Acknowledgment

This work was partially supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Culture, Sports, Science, and Technology.

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