Quasi-amorphous inorganic thin films: Non-crystalline polar phases

Quasi-amorphous thin films of BaTiO₃, SrTiO₃, and BaZrO₃ are the only known examples of inorganic, non-crystalline, polar materials. The conditions under which they are formed and the origin of their polarity set these materials apart from other classes of inorganic materials. The most important feature of the quasi-amorphous phase is that the polarity is the result of the orientational ordering of local bonding units but without any detectable spatial periodicity. This mechanism is reminiscent ofthat observed in ferroelectric polymers and permits compounds that do not have polar crystalline polymorphs, such as SrTiO₃ and BaZrO₃, to form polar noncrystalline solids. In the present report, we provide an overview of the essential features of these materials including preparation, structure, and chemical composition. The report also reviews our current level of understanding and offers some guidelines for further development and application of non-crystalline inorganic polar materials.