Dragan Damjanovic

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: dragan.damjanovic@epfl.ch
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