Structure of BaTiO_3 Phases is Studied by Comparing Neutron Diffraction and Raman Spectroscopy Data

摘要

Considering the structural unit of the rhombohedral BaTiO_3 phase in the form of Ti(—O...Ti)_6 which was constructed by neutron diffraction data, we revealed an atomic group in the form of a pyramidalcomplex anion TiO_3 . Inspection of the Raman spectra of this phase showed symmetry group C_s for this complex anion. From the similarity of the Raman spectra of all ferroelectric BaTiO_3 phases, we inferred that theorthorhombic phase and tetragonal phase are also built of TiO_3~(2-) complex anions. In the paraelectric cubicphase, TiO_3~(2-) complex anions decompose to oxygen ions O~(2-) and Ti~(2+)(O~-)_2 molecules, which are randomly oriented over 12 possible positions. Average Ti—O distances derived from neutron diffraction data were usedto calculate Ti—O~- and Ti=0 bond lengths in TiO_3_ complex anions of the ferroelectric BaTiO_3 phases. Two types of Ti—O...Ti bridges were found to exist; they belong to weak and strong intermolecular bonds, which are similar to weak and strong hydrogen bonds (H-bonds) in ferroelectrics. Weak bonds exist in all of the three ferroelectric phases; strong bonds are only in the orthorhombic phase and the tetragonal phase. As a result of a low potential barrier height in strong bonds, an oxygen atom persistently hops from one potential well to an adjacent one, which is responsible for bands appearing below 100 cm~(-1) in the Raman spectra of the tetragonal phase and orthorhombic phase. The data obtained on the structure of the ferroelectric BaTiO_3 phases were used to interpret their spontaneous polarization, repolarization, and permittivity.