Macroscopic Symmetry Breaking in the Paraelectric Phase of Ferroelectric Materials
A centric macroscopic symmetry and thus absence of the piezoelectric and pyroelectric responses is expected in the paraelectric phase of most ferroelectric materials. In polycrystalline materials with perfectly randomly oriented grains, a global centric symmetry is also expected in ferroelectric phase. Yet, experimental results show that the centric symmetry is absent in many such materials and that they exhibit macroscopic piezoelectric or pyroelectric responses even when no electric field has been applied on them. While in most cases the ensuing electro-mechanical response is small, it is not negligible, and values of apparent piezoelectric coefficients an order of magnitude higher than in quartz single crystals have been observed.
In this presentation different aspects of the symmetry breaking will be discussed, including: (i) macroscopic piezoelectric and pyroelectric responses above the Curie temperature in nominally centrosymmetric paraelectric phases of ferroelectric and relaxor thin films, crystals and ceramics; (ii) switching of the apparent polarization by stress; (iii) the symmetry breaking in ceramics and single crystals; and (iv) breaking of a centric symmetry in non–ferroelectric materials. It will be shown that electro-mechanical coupling that results from the symmetry breaking can be of comparable magnitude to what is expected in piezoelectric meta-materials based on flexoelectric effect. A detailed account of these observations is given, and implications and possible origins of the macroscopic symmetry breaking are discussed. Investigated materials include ceramics and single crystals of (Sr1-xBax)TiO3 solid solution and several other materials.