Relaxor-ferroelectric materials find application in a broad range of technological devices, including ultrasonic imaging transducers, nano-positioning and high-performance capacitors.They generally exhibit occupationally disordered structures creating local polar fluctuations that are highly sensitive to applied electric or stress fields. The sensitivity of the material structure to external field and stress conditions also makes them likely to develop skin or surface phases that are unique from the bulk. Surface layers can adjust the material response and also lead to ambiguity in structural characterisation. Here, we show using a combination of X-ray diffraction methods that a ~20 m skin structure commonly exists in the lead-free relaxor-ferroelectric ceramic (Na 1/2 Bi 1/2 )TiO 3 -BaTiO 3 . We show using experiments and DFT calculations that the combined action of oxygen vacancies providing internal chemical pressure and the surface plane stress state dictates the stability and structure of the skin layer. This work can be extended to all perovskite relaxor ferroelectrics and provides new insightsThis article is protected by copyright. All rights reserved. 2 into the origin of skin layers in these materials. The opportunity exists to further enhance the functionality of these materials through engineering of surface structures using the methods outlined here.

We demonstrate a unique capability to control the formation and properties of skin layer structures in relaxor ferroelectrics by adjusting defect concentration. It is shown that the skin layer is polar and both electrically and optically active.

A reduction in the orientation distribution of grains, via crystallographic texturing, is known to significantly enhance the electro-mechanical response in polycrystalline piezoelectrics. Through calculations of the electric-field-induced strain in phase-changing ceramics, the local strain heterogeneity was also found to increase with crystallographic texture. The increase in grain-scale strain heterogeneity potentially indicates greater stress concentrations within the polycrystal and may degrade the expected lifetime and fatigue behavior in these ceramics. In crystallographic symmetries with higher numbers of polarization directions, it was found that the magnitude of heterogeneity in the response was lower.