Large Property Enhancements through Minute Symmetry Breaking

Image Caption: 
Optical Second Harmonic Generation (SHG) microscopy imaging of a KNbO3 single crystal reveals a metastable monoclinic phase (bright yellow regions) with large SHG response embedded in a conventional orthorhombic domain matrix (light and dark brown). The black square, L1, is a metal marker.
Credits/Names: 

National Science Foundation Center for Nanoscale Science grant numbers DMR-1420620, DMR-1210588, and grant DMR-1649490.  Center for Nanophase Materials Sciences (ORNL) and Center for Nanoscale Materials (ANL), through Department of Energy contract no. DE-AC02-06CH11357., Tom T. A. Lummen,1 J. Leung1 A. Kumar,1 X. Wu,2 Y. Ren,2 B. K. Vanleeuwen,1 R. C. Haislmaier,1 M. Holt,3 K. Lai,2 S. V. Kalinin,4 Venkatraman Gopalan,1 1Pennsylvania State University, University Park, PA 16802, 2 U. Texas Austin,  Austin, TX 78712, 3Argonne National Laboratory, Argonne, IL 60439, USA. 4Oak Ridge National Laboratory, Oak Ridge, TN, 37831.

The design of new or enhanced functionality in materials is traditionally viewed as requiring the discovery of new chemical compositions through synthesis. Large property enhancements may however also be hidden within already well-known materials, when their structural symmetry is deviated from equilibrium through a small local strain or field. We discovered a new metastable phase of monoclinic symmetry (Figure) within bulk KNbO3 that is both induced by and stabilized with local strains generated by a network of ferroelectric domain walls. While the local microstructural shear strain involved is only 0.017% , a minute symmetry breaking from the original orthorhombic phase, the concurrent symmetry reduction results in an optical second harmonic generation response that is over 550% higher at room temperature. Moreover, the meandering walls of the low-symmetry domains also exhibit enhanced electrical conductivity on the order of 1 S m-1. This discovery reveals a potential new route to local engineering of significant property enhancements and conductivity through symmetry lowering in ferroelectric crystals.

Advanced Materials, (2017) DOI: 10.1002/adma.201700530

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Year of Research Highlight: 
2018
IRG: 
IRG1 - Layered Ferroics