Colossal strain tuning of ferroelectric phases in KNbO₃ thin films

A strong coupling of polarization and strain in ferroelectric complex oxides offers unique opportunities to dramatically tune their properties. Here we demonstrate colossal strain tuning of ferroelectricity in epitaxial KNbO₃ thin films grown by sub-oxide molecular beam epitaxy. While bulk KNbO₃ exhibits three ferroelectric transitions and a Curie temperature (T_c) of ~676 K, our phase-field modeling predicts that a biaxial strain of as little as -0.6% pushes its T_c >975 K, its decomposition temperature in air, and for -1.4% strain, to T_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, synchrotron-based x-ray reciprocal space mapping, ferroelectric measurements, and transmission electron microscopy reveal a single tetragonal phase from 10 K-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.