The joint impact of crystal-cell thickness and biaxial ([110]) strain on the ferroelectricity of KNbO[formula omitted] in bulk/thin-film form

Abstract

Due to robust spontaneous Polarization (P), ferroelectric materials are extensively used in numerous applications such as power consumption, exhaustion-free capacitor, and energy harvesting. Herein, we focus on the possibility of controlling the ferroelectric properties of the bulk/thin films of KNbO3 perovskite oxide via unit cell thickness and strain engineering using ab-initio calculations. First, the thermodynamic stability of the bulk and thin film structures is determined by computing the formation enthalpies and surface energies using Convex Hull analysis, respectively, which affirms their experimental realization. It is demonstrated that bulk structure is stable against its competing KO and NbO2 phases, while NbO2-NbO2 symmetric film is more energetically stable to grow in the lab as compared to others. Moreover, mechanical stability of the structures is confirmed by calculating the elastic constants. The estimated value of (P) for the bulk motif is 37.4 μC cm−2, which is in a good agreement with experimentally observed value. Thin films with larger slab thickness display more structural distortion, which enhances the P magnitude. For possible [KO-NbO2] (asymmetric) and [KO-KO]/[NbO2-NbO2] (symmetric) slabs, P gradually increases from 11.77 to 26.87 μC cm−2 and 8.59 to 16.43/23.38 to 27.42 μC cm−2 as the KNO unit cell thickness increases from 1 to 5 (1.86 nm) and 1.5 to 5.5 (2.05 nm), respectively. Interestingly, it is found that -5% compressive strain increases the P value of bulk KNO up to 31% than that of unstrained one. Similarly, the P amplitude in KO-NbO2 (1.86 nm thick) and KO-KO/NbO2-NbO2 (2.05 nm thick) slabs also improve up to 10.6% and 10.8%/11% as compared to unstrained ones for −5% compressive strain, respectively. Hence, the present work yields deep observations into the combined effect of crystal cell thickness and strain on the ferroelectricity of KNO, which provides a possible way to tune the P to enhance its potential realization in the oxide industry.