Large Property Enhancements Through Minute Symmetry Breaking

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