Local Structure of Ba_1-xSr_xTiO₃ and BaTi_1-yZr_yO₃ Nanocrystals Probed by X-ray Absorption and X-ray Total Scattering

Journal Article · Tue Feb 18 00:00:00 EST 2014 · ACS Nano

DOI:https://doi.org/10.1021/nn405629e· OSTI ID:1114848

Rabuffetti, Federico A.; Brutchey, Richard L. ^[1]

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1. USC

The effect of isovalent chemical substitution on the magnitude and coherence length of local ferroelectric distortions present in sub-20 nm Ba_1–xSr_xTiO₃ (x = 0.0, 0.30, 0.50, 1.0) and BaTi_1–yZr_yO₃ (y = 0.0, 0.15, 0.50, 1.0) nanocrystals synthesized at room temperature is investigated using X-ray absorption near edge structure (XANES) and pair distribution function analysis of X-ray total scattering data (PDF). Although the average crystal structure of the nanocrystals is adequately described by a centrosymmetric, cubic Pm3¯m space group, local ferroelectric distortions due to the displacement of the titanium atom from the center of the perovskite lattice are observed for all compositions, except BaZrO₃. The symmetry of the ferroelectric distortions is adequately described by a tetragonal P4mm space group. The magnitude of the local displacements of the titanium atom in BaTiO₃ nanocrystals is comparable to that observed in single crystals and bulk ceramics, but the coherence length of their ferroelectric coupling is much shorter (≤20 Å). Substitution of Sr²⁺ for Ba²⁺ and of Zr⁴⁺ for Ti⁴⁺ induces a tetragonal-to-cubic transition of the room temperature local crystal structure, analogous to that observed for single crystals and bulk ceramics at similar compositions. This transition is driven by a reduction of the magnitude of the local displacements of the titanium atom and/or of the coherence length of their ferroelectric coupling. Replacing 50% of Ba²⁺ with Sr²⁺ slightly reduces the magnitude of the titanium displacement, but the coherence length is not affected. In contrast, replacing 15% of the ferroelectrically active Ti⁴⁺ with Zr⁴⁺ leads to a significant reduction of the coherence length. Deviations from the ideal solid solution behavior are observed in BaTi_1–yZr_yO₃ nanocrystals and are attributed to an inhomogeneous distribution of the barium atoms in the nanocrystal. Composition–structure relationships derived for Ba_1–xSr_xTiO₃ and BaTi_1–yZr_yO₃ nanocrystals demonstrate that the evolution of the room temperature local crystal structure with chemical composition parallels that of single crystals and bulk ceramics, and that chemical control over ferroelectric distortions is possible in the sub-20 nm size range. In addition, the potential of PDF analysis of total scattering data to probe compositional fluctuations in nanocrystals is demonstrated.

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
• Sponsoring Organization: DOE - BASIC ENERGY SCIENCES
• OSTI ID: 1114848
• Journal Information: ACS Nano, Vol. 7, Issue (12) ; 12, 2013; ISSN 1936-0851
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: ENGLISH