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A Landau–Devonshire thermodynamic energy density function for ferroelectric wurtzite aluminum scandium nitride (Al1−xScxN) solid solution is developed. It is parametrized using available experimental and theoretical data, enabling the accurate reproduction of composition-dependent ferroelectric properties, such as spontaneous polarization, dielectric permittivity, and piezoelectric constants, for both bulk and thin films. The maximum concentration of Sc for the wurtzite structure to remain ferroelectric is found to be 61 at. %. A detailed analysis of Al1−xScxN thin films reveals that the ferroelectric phase transition and properties are insensitive to substrate strain. This study lays the foundation for quantitative modeling of novel ferroelectric wurtzite solid solutions. 

Abstract Ferroelectric nanotubes offer intriguing opportunities for stabilizing exotic polarization domains and achieving new or enhanced functionalities by tailoring the complex interplay among the geometry, surface effects, crystal symmetry, and more. Here, phase‐field simulations to predict the room‐temperature equilibrium polarization domain structure in (001)_pcPbZr_0.52Ti_0.48O₃(PZT) nanotubes are used (pseudocubic (pc)). The simulations incorporate the influence of surface‐tension‐induced strains, which have been ignored in existing computational studies. It is found that (001)_pcPZT nanotubes can host a unique class of topological polarization domain structures comprising non‐planar flux‐closures and anti‐flux‐closures that are inaccessible with ferroelectrics of planar geometry (e.g., thin‐films, nanodots). It is shown that surface‐tension‐induced strain is significantly enhanced in thin‐walled nanotubes and thereby can lead to noticeable modulation of the flux closures. Domain stability map as a function of the nanotube wall thickness and height is established. The results provide a basis for geometrical engineering of domain structures and associated functional (e.g., piezoelectric, electrocaloric) responses in ferroelectric nanotubes. 

Abstract Strong coupling between polarization (P) and strain (ɛ) in ferroelectric complex oxides offers unique opportunities to dramatically tune their properties. Here colossal strain tuning of ferroelectricity in epitaxial KNbO₃thin films grown by sub‐oxide molecular beam epitaxy is demonstrated. While bulk KNbO₃exhibits three ferroelectric transitions and a Curie temperature (T_c) of ≈676 K, phase‐field modeling predicts that a biaxial strain of as little as −0.6% pushes itsT_c> 975 K, its decomposition temperature in air, and for −1.4% strain, toT_c> 1325 K, its melting point. Furthermore, a strain of −1.5% can stabilize a single phase throughout the entire temperature range of its stability. A combination of temperature‐dependent second harmonic generation measurements, synchrotron‐based X‐ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal a single tetragonal phase from 10 K to 975 K, an enhancement of ≈46% in the tetragonal phase remanent polarization (P_r), and a ≈200% enhancement in its optical second harmonic generation coefficients over bulk values. These properties in a lead‐free system, but with properties comparable or superior to lead‐based systems, make it an attractive candidate for applications ranging from high‐temperature ferroelectric memory to cryogenic temperature quantum computing.