Local structure of crystalline materials has been increasingly recognized to play a crucial role in understanding their functional properties. One important example is perovskite ferroelectrics where the structure often involves the presence of chemical short-range order, correlated atomic displacements and/or oxygen octahedral tilting disorder. In this case electron diffraction can prove to be a suitable technique for probing the local structure. The strong electron-matter interaction makes it possible to easily record weak superlattice reflections and diffuse scattering with good signal-to-noise ratio.

The ternary system xBi_0.5Na_0.5TiO₃–yBi_0.5K_0.5TiO₃–zBaTiO₃ (BNT-BKT-BT) is a potential candidate for replacing Pb-based materials in piezoelectric applications¹. Even though it is one of the most extensively studied lead-free piezoelectric systems, structural information about local distortions in this material is scarce. In this study Transmission Electron Microscopy (TEM) was employed in order to investigate the long- and short-range structure of this system in both real and reciprocal space. Two and three dimensional electron diffraction data was recorded. The true features of diffuse electron scattering in 3D as well as Bragg reflections were obtained by Rotation Electron Diffraction (RED) method².

A projection of the three-dimensional reconstructed reciprocal space volume along <001>_pc, is shown in Fig. 1(a). Superlattice reflections of both ½ooo and ½ooe type can be observed indicating the coexistence of two different octahedral tilting systems, anti-phase a^–a^–a^– and in-phase a⁰a⁰c⁺ respectively (in Glazer notation³). The anti-phase a^–a^–a^– tilting system is consistent with the long-range order space group, namely R3c. But, the in-phase a⁰a⁰c⁺ tilting system is consistent with P4bm space group indicating local deviations from the average space group. Furthermore two types of diffuse scattering features could be observed: (i) “asymmetric L-shaped” features around Bragg peaks extending along <100>_pc directions clearly visible for Bragg peaks close to the direct beam, Fig. 1(b) and (ii) linear {h00}*, {0k0}* and {00l}* diffuse scattering ’rods’ that pass through the superlattice reflections as highlighted in Fig. 1(c). A possible explanation for the observed diffuse features is correlated displacements of the A- and B-cations along <111>_pc and <100>_pc chains coupled with disordered (in-phase/anti-phase) rotations of the oxygen octahedral⁴.

References

¹ J. Shieh, K.C. Wu, and C.S. Chen, Acta Materialia 55, 3081–3087 (2007)

² W. Wan, J. Sun, J. Su, S. Hovmöller and X. Zou, J. Appl. Crystallogr. 46, 1863-1873 (2013).

³ A. M. Glazer, Acta Cryst. B28, 3384 (1972).

⁴ J. Kreisel, P. Bouvier, B. Dkhil, P. A. Thomas, A. M. Glazer, T. R. Welberry, B. Chaabane and M. Mezouar, Physical Review B 68, 014113 (2003)

Acknowledgments

The Knut and Alice Wallenberg (KAW) Foundation is acknowledged for providing the electron microscopy facilities and financial support under the project 3DEM-NATUR

Figures:

Fig. 1 Overview of the three-dimensional reconstructed reciprocal space volume projected along <001>pc (a) Enlargement of the area around the origin showing Bragg peaks together with asymmetric L-shaped diffuse features (b) Linear diffuse scattering rods passing through 3/2 3/2 0 superlattice reflection (c).

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