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MM: Fachverband Metall- und Materialphysik

MM 11: Topical Session: TEM-Symposium - STEM

MM 11.5: Vortrag

Montag, 11. März 2013, 17:00–17:15, H4

Microscopic origin of the giant ferroelectric polarization in strained BiFeO3 thin films — •Marta D. Rossell — Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf, Switzerland

Because of their astounding electromechanical properties, BiFeO3 (BFO) thin films are promising candidates for the replacement of lead-based ceramics in microelectromechanical system devices. However, a full understanding of the piezoelectric properties reported for these ceramic materials is still missing. In particular, polymorphs of BFO stabilized under epitaxial strain are not yet fully understood. Two distinct structures are known to evolve above and below a 4.5% critical compressive strain. They are pseudotetragonal (T phase) and pseudorhombohedral (R phase), respectively. The T phase shows a unique structure characterized by a strongly elongated unit cell with a c/a axial ratio close to 1.3. The structural information of this metastable polymorph is particularly relevant because it is predicted to have a giant polarization roughly 1.5 times of the bulk material.

We determine the atomic structure of the T phase and the R phase in highly strained BFO thin films by using a combination of atomic-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS). The coordination of the Fe atoms and their displacement relative to the O and Bi positions are assessed by direct imaging. These observations allow us to interpret the electronic structure data derived from EELS and provide evidence for the giant spontaneous polarization in strained BFO thin films.