Origin of Polarity in AmorphousSrTiO3

Abstract

Although neither SrTiO3 nor BaZrO3 has any polar crystalline polymorphs, they may form noncrystalline pyro- and piezoelectric phases [Adv. Mater. 19, 1515 (2007)10.1002/adma.200602149]. 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.

Structure and optical properties of [Ba1–xY2x/3](Zr0.25Ti0.75)O3 powders

“…5 (b), practically there are not modifications on the relative positions of the pre-edge peaks, suggesting only the existence of hexacoordinated Ti atoms ([TiO 6 ] clusters). In order to qualitatively estimate the displacement presented by the Ti atoms into the [TiO 6 ] clusters, it was employed the method proposed by Frenkel et al [67]. According to these authors, the pre-edge peak area (A) in perovskite-type titanates is proportional to the square of the offcenter Ti displacement (d Ti ), which is described by the following equation:…”

Quasi‐Amorphous Inorganic Thin Films: Non‐Crystalline Polar Phases

“…As a result of these distortions, the Ti-ions are shifted away from the center of the oxygen octahedra. The shift may reach 0.1 Å in SrTiO 3 [36] and 0.44 Å [8] in BaTiO 3 . The off-center displacement of the Ti-ions in the amorphous and quasi-amorphous BaTiO 3 , 0.44 Å , is much larger than in crystalline ferroelectric BaTiO 3 , 0.23 Å .…”

“…[8] Based on these data one can calculate that the TiO 6 groups possess a relatively large dipole moment of 0.5-2 Debye. [8,36] The local environments of the Ba and Sr cations are not phase invariant. Rather, as will be discussed below, they play an important role in the stability of the amorphous and quasi-amorphous phases.…”

“…Amorphous and quasi-amorphous films of SrTiO 3 and BaTiO 3 were the subject of studies using EXAFS [8,36] and XPS. [7,37] The data produced by these techniques are consistent with the view that the amorphous and quasi-amorphous films do not behave like solids containing random close-packed hard spheres, like ionic or metallic glasses.…”

Finite-size scaling of flexoelectricity in Langmuir-Blodgett polymer thin films

“…In the case of polymers, [8][9][10][11] and elastomers, 12 the ordering of the polymer chains depends on the size and rigidity of the side groups and cross-linked functional groups. While in inorganic amorphous materials, 13 a distortion in the random network of local bonding units renders the system polar. In these materials the inversion symmetry breaking is strictly not crystalline but more geometrical in nature.…”