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 monoclinicPmphase 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 KNbO3polymorphs - 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 monoclinicPmphase, showing an anisotropic nonlinear relationship betweenεgapand 〈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. Usingi, we successfully rationalize and demonstrate the linear dependency of direction-dependentPsonifor strained KNbO3polymorphic phases.
Bibliographical noteFunding Information:
We gratefully acknowledge support from the Ministry of Science and ICT under the Creative Materials Discovery Program (2018M3D1A1058536) and the National Research Foundation of Korea under the Material Convergence Innovation Technology Development Program (2020M3D1A2102913). Computational resources have been kindly provided by the KISTI Supercomputing Center (KSC-2019-CRE-0174) and the Australian National Computational Infrastructure (NCI). We also thank Seungjae Yoon for helpful discussions and some preliminary DFT calculations.
© The Royal Society of Chemistry 2021.
All Science Journal Classification (ASJC) codes
- Materials Chemistry