Defects, Symmetry Breaking, and Electro-mechanical Coupling in Oxide Materials Sina Hashemi Zadeh, Alberto Biancoli and Dragan Damjanovic,* Ceramics Laboratory, Swiss Federal Institute of Technology in Lausanne - EPFL EPFL STI IMX LC, Station 12, 1015 Lausanne, Switzerland *e-mail: [email protected] The breaking of expected centric symmetry either on local or macroscopic level has been reported for ceramics, crystals, polymers, amorphous materials, and thin films with a wide range of chemical compositions. In a large number of cases the origins of the symmetry breaking are not well understood. In this presentation, we discuss breaking of the macroscopic centric symmetry in oxide materials and its relation to charged defects. While focusing on single crystals and ceramics of (Ba1-xSrx)TiO3 solid solution, we also report results on other ferroelectrics, including K(Ta,Nb)O3, Pb(Zr,Ti)O3, Pb(Mg1/3Nb2/3)O3 and others. The noncentric or polar macroscopic symmetry in materials with nominally centric symmetry is identified by detecting piezoelectric and pyroelectric charges or by observing thermally stimulated currents whose direction depends on orientation of unpoled samples. In barium titanate based ceramics, the symmetry breaking is found to be related to strain inhomogeneities formed in the material during the densification of ceramics, and, in crystals and thin films, probably during the growth. The inhomogeneities are associated with a robust and apparently permanent separation of charges in the material. The charge transport and dielectric response of samples are affected by the polarization arising from the symmetry breaking. We shall discuss in detail origins of the broken centric symmetry in examined ceramics and crystals, local versus macroscopic symmetry breaking and interaction of ensuing polarization with defects and charge transport. This work was supported by the Swiss National Science Foundation through NRP62 "Smart materials" (Project No. 406240 -126091). References: 1. A. Biancoli, C. M. Fancher, J. L. Jones, and D. Damjanovic, "Breaking of macroscopic centric symmetry in paraelectric phases of ferroelectric materials and implications for flexoelectricity," Nature Mater., vol. 14, p 224, 2015. 2. A. Biancoli, D. Damjanovic (Dir.). Breaking of the macroscopic centric symmetry in Ba1-xSrxTiO ceramics and single crystals. Thesis EPFL, n° 6366 (2014) http://infoscience.epfl.ch/record/202182/files/EPFL_TH6366.pdf