High energy‐resolution <scp>EELS</scp> of ferroelectric and paraelectric <scp>BaTiO3</scp> phases

BaTiO 3 (BTO) is a widely studied material with several potential applications as a result of its intrinsic ferroelectricity. It undergoes multiple structural phase transitions across a range of accessible temperatures, which have an effect on its ferroelectric properties. While BTO is ferroelectric in its low‐temperature phases—rhombohedral below ∼183 K, orthorhombic in the range ∼183–273 K, and tetragonal in the range ∼273–393 K; it becomes paraelectric above ∼393 K [1]. The ferroelectricity of BTO is directly related to the deviation of the TiO 6 octahedra from perfectly undistorted units, which is linked to the off‐centering of the Ti 4+ cation within a octahedron constituted of six O atoms. Nevertheless, the phase transition mechanisms are still widely discussed, and the exact structure of the paraelectric phase remains unclear. Probing the structural distortion within TiO 6 octahedra of the different BTO phases is therefore of particular interest, especially at the nanoscale for BTO thin films and nanostructures. Recently, it was shown that the O‐K energy‐loss near‐edge structures (ELNES) permitted the probing of this subtle structural distortion [2]. The broadening of the ELNES at lower energy is directly related to the Ti 4+ off‐centering. The O‐site symmetry affects the core‐hole potential created during excitation, which then induces the broadening in the ELNES. In this contribution, the structural distortion of BaTiO 3 (BTO) is studied in its ferroelectric (rhombohedral and tetragonal), and paraelectric phases from the O‐K and Ti‐L 23 near‐edge structures in electron energy‐loss spectroscopy [3]. The high energy‐resolution O‐K and Ti L 23 ELNES of ferroelectric and paraelectric BTO are recorded in a monochromated scanning transmission electron microscope (STEM), using cooling and heating stages to reach the phase transitions in a single crystal thin foil. Modifications of the electronic structure are detected in the lowest energy fine structure of the O‐K edge in the ferroelectric phases (Fig. 1a), and are interpreted by core‐hole valence‐electron screening geometry (Fig. 1c). The broader and more asymmetric lowest energy fine structure at low temperature, suggest that the magnitude of the Ti 4+ off‐centering along ⟨111⟩ increases in lower‐temperature phases. Interestingly, the lowest energy fine structure of the paraelectric phase is comparable to the one obtained at room temperature, hence supporting reports in the literature that the paraelectric phase is actually not cubic [4]. First principles calculations support these experimental evidences: they confirm that the lowest energy fine structure of the O‐K edge is broader for a lower O‐site symmetry, but do not reproduce the asymmetry and the overall shape of this fine structure (Fig. 1b). These discrepancies are ascribed to the approximations inherent to the static core‐hole used within the DFT framework. Furthermore, while the Ti‐L 23 ELNES is commonly used to probe and interpret the structural distortions in titanates, we show that they are only as sensitive as the O‐K ELNES to the structural distortions in BTO (Fig. 2). This finding indicates that the O‐K edge can be used instead of, or complementary to, the Ti‐L 23 edge to probe the structural distortion, and therefore the ferroelectricity, in BTO. The sensitivity of the O‐K edge to subtle structural distortions in BTO shows a new way to probe and better understand the ferroelectricity at the nanoscale, on defective or strained BTO thin films for example [5].

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