Geometrical frustration and piezoelectric response in oxide ferroics

Despite years of investigation, the exact structural origin of the increased piezoelectric response of oxide ferroics is still unclear. We will present results from resonant high-energy x-ray diffraction experiments on exemplary sodium-potassium niobate ferroics. In particular, we will show that their increased piezoelectric response is due to a geometrical frustration in the underlying perovskite lattice induced by local fluctuations in the tilt pattern of the constituent niobium-oxygen octahedra, and not to a crystal-crystal phase transition or distinct nanodomains. The fluctuations peak when the sodium to potassium ratio approaches one, leading to softening of the perovskite lattice and easing of polarization rotation under electric field. Based on the experimental findings and model calculations, we will also show that the fluctuations are driven by the mismatch between the radii of sodium and potassium atoms, and the increased piezoelectric response of sodium-potassium niobates indeed scales with the variance in the distribution of these radii about their average value. The relevance of our work to the ongoing search for oxide ferroics with improved functionality will be briefly discussed.