Origin of polarity in amorphous SrTiO3

Although neither SrTiO3 nor BaZrO3 has any polar crystalline polymorphs, they may form noncrystalline pyro- and piezoelectric phases [Adv. Mater. 19, 1515 (2007)]. These phases and the similar phase of BaTiO3 have been called quasiamorphous. In this Letter, the structure of the quasiamorphous phase of SrTiO3 is examined by the x-ray absorption fine structure technique and found to be built of a random network of polar octahedral TiO6 local bonding units. While in crystalline SrTiO3 all TiO6 octahedra are apex sharing only, in its amorphous and quasiamorphous phases, some octahedra share edges. The polarity of the quasiamorphous phase is due to the partial alignment of the TiO6 octahedra. Such a mechanism is completely different from that of inorganic polar crystals. This mechanism should be possible in a large variety of other compounds that contain similar local bonding units.