Colossal Strain Tuning of Ferroelectric Transitions in KNbO₃ Thin Films

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 KNbO₃ thin films grown by sub-oxide molecular beam epitaxy is demonstrated. While bulk KNbO₃ exhibits 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 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 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.