Non-Crystalline Pyroelectric and Piezoelectric Quasi-Amorphous Ionic Films
Non-crystalline piezoelectric and pyroelectric inorganic thin film of BaTiO_3, SrTiO_3 and BaZrO_3, called quasi-amorphous, were investigated. The quasi-amorphous phase of BaZrO_3 and SrTiO_3 is the first known instance when a compound that does not form polar crystalline polymorphs (BaZrO_3 and SrTiO_3) can form a non-crystalline polar phase. Several experimental measurements, including XAFS, XPS and stress measurements, were preformed. Base on these measurements a model describing formation of these materials was developed. This model of “Random Network of Local Bonding Units” is very different from a continuous random network of covalent glasses and a random close packing of hard spheres of metallic glasses. The model poses that the amorphous films of BaTiO_3, SrTiO_3 and BaZrO_3 are composed from asymmetrically distorted TiO_6 or ZrO_6 octahedra that randomly share apices, edges or faces. If an amorphous substrate-supported film is pulled through temperature gradient, it does not crystallize due to substrate clamping. The temperature gradient causes partial alignment of the octahedra, imparting the film with macroscopic dipole moment. Existence of three different compounds forming quasi-amorphous phases strongly suggests that formation of the quasi-amorphous phases is a general phenomenon.