Abstract
Electromechanical properties such as d33 and strain are significantly enhanced at morphotropic phase boundaries (MPBs) between two or more different crystal structures. Many actuators, sensors and MEMS devices are therefore systems with MPBs, usually between polar phases in lead (Pb)-based ferroelectric ceramics. In the search for Pb-free alternatives, systems with MPBs between polar and non-polar phases have recently been theorized as having great promise. While such an MPB was identified in rare-earth (RE) modified bismuth ferrite (BFO) thin films, synthesis challenges have prevented its realization in ceramics. Overcoming these, we demonstrate a comparable electromechanical response to Pb-based materials at the polar-to-non-polar MPB in Sm modified BFO. This arises from ‘dual’ strain mechanisms: ferroelectric/ferroelastic switching and a previously unreported electric-field induced transition of an anti-polar intermediate phase. We show that intermediate phases play an important role in the macroscopic strain response, and may have potential to enhance electromechanical properties at polar-to-non-polar MPBs.
Highlights
The enhanced electromechanical response observed at a morphotropic phase boundary (MPB) is a critical phenomenon in ferroelectrics[1,2]
The peaks are labelled according to their space group and corresponding peak indices. (e) High resolution (HR) transmission electron microscopy (TEM) image of the 15.5 mol% Sm sample with closest proximity to the PB region. (f–h) Show selected area fast Fourier transforms (FFTs) in [001]pc zone axis of specific phase regions corresponding respectively to R3c, Pbam with 1⁄4 (100)pc reflections, and Pbnm with 1⁄2 (110)pc reflections
Through a systematic study across the MPB, with compositions x = 8–18 mol% Sm, we show how an anti-polar phase[26,27] underpins the evolution of the phase composition and crucially, drives a nano-scale domain topology with increasing Sm content towards the MPB
Summary
The enhanced electromechanical response observed at (or near) a morphotropic phase boundary (MPB) is a critical phenomenon in ferroelectrics[1,2]. (f–h) Show selected area fast Fourier transforms (FFTs) in [001]pc zone axis of specific phase regions corresponding respectively to R3c, Pbam with 1⁄4 (100)pc reflections (ringed), and Pbnm with 1⁄2 (110)pc reflections (ringed) These regions are marked in the HR-TEM image by dashed boxes. A strain-electric-field study reveals that the anti-polar phase facilitates dual strain mechanisms; both ferroelectric/ferroelastic domain switching and a previously unreported electric-field induced phase transition This transition of an anti-polar intermediate phase is the key to understanding their role in RE-BFO and discloses an important opportunity to achieve large electromechanical properties at polar-to-non-polar MPBs. By using polycrystalline materials we bring new understanding to RE-BFO, complementing the existing picture of atomic-scale evolution in thin films[28], and in doing so we highlight the potential of polar-to-non-polar MPB systems as Pb-free piezoelectrics
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