5 years ago

Emergent Low-Symmetry Phases and Large Property Enhancements in Ferroelectric KNbO3 Bulk Crystals

Emergent Low-Symmetry Phases and Large Property Enhancements in Ferroelectric KNbO3 Bulk Crystals
Martin Holt, Y. Ren, Amit Kumar, Venkatraman Gopalan, Sergei V. Kalinin, Ryan C. Haislmaier, Brian K. VanLeeuwen, J. Leung, Keji Lai, X. Wu, Tom T. A. Lummen
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. Here, the discovery of enhanced material properties associated with a new metastable phase of monoclinic symmetry within bulk KNbO3 is reported. This phase is found to coexist with the nominal orthorhombic phase at room temperature, and 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%, 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. Enhanced functional material properties are reported in a new metastable monoclinic phase of bulk KNbO3. The phase is stabilized by a small strain of ≈0.017% arising from domain microstructure, which lowers the symmetry of the lattice and significantly enhances the nonlinear optical response. Symmetry breaking can be a promising avenue for significant property enhancements in ferroelectrics.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/adma.201700530

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