Tuning piezoelectric properties through epitaxy of La₂Ti₂O₇ and related thin films

Current piezoelectric sensors and actuators are limited to operating temperatures less than ~200 °C due to the low Curie temperature of the piezoelectric material. Strengthening the piezoelectric coupling of high-temperature piezoelectric materials, such as La₂Ti₂O₇ (LTO), would allow sensors to operate across a broad temperature range. The crystalline orientation and piezoelectric coupling direction of LTO thin films can be controlled by epitaxial matching to SrTiO₃(001), SrTiO₃(110), and rutile TiO₂(110) substrates via pulsed laser deposition. The structure and phase purity of the films are investigated by x-ray diffraction and scanning transmission electron microscopy. Piezoresponse force microscopy is used to measure the in-plane and out-of-plane piezoelectric coupling in the films. The strength of the out-of-plane piezoelectric coupling can be increased when the piezoelectric direction is rotated partially out-of-plane via epitaxy. The strongest out-of-plane coupling is observed for LTO/STO(001). Deposition on TiO₂(110) results in epitaxial La_2/3TiO₃, an orthorhombic perovskite of interest as a microwave dielectric material and an ion conductor. La_2/3TiO₃ can be difficult to stabilize in bulk form, and epitaxial stabilization on TiO₂(110) is a promising route to realize La_2/3TiO₃ for both fundamental studies and device applications. Overall, these results confirm that control of the crystalline orientation of epitaxial LTO-based materials can govern the resulting functional properties.