This collaborative program between Penn State University, Argonne National Laboratory and the University of Sheffield (UK) is directed towards understanding tilt transitions in perovskite thin films. These tilt transitions occur due to cooperative motions of the oxygen octahedra framework, and are the most common phase transitions in perovskite structured compounds. Such transitions have a profound impact on the temperature coefficient of resonance frequency and on piezoelectric constants. Despite their importance, the physics of size effects and the effect of mechanical constraint (as in thin films) for these tilt transitions is poorly understood in comparison to displacive/ordering transitions. In this program, we seek to answer the following questions that will allow breakthroughs in our understanding of tilt transitions in constrained systems: 1. How do elastic boundary conditions affect the tilt transitions in perovskites? 2. Are ferroelectric size effects affected by tilt transitions? 3. What are the property consequences of changes in the stability of tilt distortions? Epitaxial thin films of PbZr1-xTixO3, AgTa1-xNbxO3 and BiScO3-PbTiO3 will be grown with compositions that yield tilt transitions in the bulk. The phase transition sequence as a function of film thickness, in-plane lateral constraint, temperature and composition will be studied via impact on ferroelectric, dielectric, and electromechanical properties, by conventional x-ray scattering, and by synchrotron x-ray scattering at the Advanced Photon Source (APS). In conjunction with Prof. Ian Reaney of the University of Sheffield, films will also be interrogated by transmission electron microscopy and Raman scattering.
