Enhanced polarization in epitaxially strained monoclinic potassium niobate for lead-free electromechanical applications. Issue 38 (8th September 2021)

• Record Type: Journal Article
• Authors: Hwang, Woohyun; Lee, Ji-Hwan; Soon, Aloysius
• Is Part Of: Journal of materials chemistry. Volume 9:Issue 38(2021)
• Page Start: 13420
• Page End: 13431
• Publication Date: 2021-09-08
• Subjects: Materials -- Periodicals; Chemistry, Analytic -- Periodicals; Optical materials -- Research -- Periodicals; Electronics -- Materials -- Research -- Periodicals; 543.0284
• DOI: 10.1039/d1tc03191k
• Languages: English
• ISSNs: 2050-7526

Abstract

Abstract : We have performed density-functional theory calculations to understand how applied epitaxial strain influences the thermodynamic, electronic, and (anisotropic) polarization properties in polar KNbO3 polymorphs for next-generation Pb-free piezoelectric applications. Abstract : To further our understanding of how complex anisotropic structure–property relationships may be rationalized by their local atomic arrangements in ferroelectric materials, using the newly found metastable monoclinic Pm phase of potassium niobate (KNbO3 ) as an example, we perform first-principles density-functional (perturbation) theory calculations to understand how applied epitaxial strain may influence their structural, thermodynamic, electronic, and (anisotropic) polarization properties in polar KNbO3 polymorphs – a potential contender for Pb-free piezoelectric applications. Here, we find that the displacement of the center metal cation (niobium, Nb) relies on more complex anisotropic properties than the commonly used isotropic scalar quadratic elongation, 〈 λ 〉 for the monoclinic Pm phase, showing an anisotropic nonlinear relationship between ε gap and 〈 λ 〉. We also show how anisotropic ferroelectric distortion under strain may strongly influence the direction-dependent chemical bonding character in monoclinic KNbO3 . Lastly, building on the isotropic 〈 λ 〉 index, we propound a revised definition of this key structural descriptor – the modified bond elongation index (  i ), which contains vectorial structural information. Using  i, we successfully rationalize and demonstrate the linear dependency of direction-dependent P s on  i for strained KNbO3 polymorphic phases.