Giant polarization in super-tetragonal thin films through interphase strain

Strain engineering has emerged as a powerful tool to enhance the performance of known functional materials. Here we demonstrate a general and practical method to obtain supertetragonality and giant polarization using interphase strain. We use this method to create an out-of-plane-to-in-plane lattice parameter ratio of 1.238 in epitaxial composite thin films of tetragonal lead titanate (PbTiO3), compared to 1.065 in bulk. These thin films with super-tetragonal structure possess a giant remanent polarization, 236.3 microcoulombs per square centimeter, which is almost twice the value of known ferroelectrics. The supertetragonal phase is stable up to 725 degrees C, compared to the bulk transition temperature of 490 degrees C. The interphase-strain approach could enhance the physical properties of other functional materials.