Enhanced polarization in epitaxially strained monoclinic potassium niobate for lead-free electromechanical applications

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 betweenε_gapand 〈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-dependentP_son_ifor strained KNbO₃polymorphic phases.