Structure of BaTiO3 phases is studied by comparing neutron diffraction and Raman spectroscopy data

Considering the structural unit of the rhombohedral BaTiO3 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 pyramidal complex anion TiO32−. Inspection of the Raman spectra of this phase showed symmetry group Cs for this complex anion. From the similarity of the Raman spectra of all ferroelectric BaTiO3 phases, we inferred that the orthorhombic phase and tetragonal phase are also built of TiO32− complex anions. In the paraelectric cubic phase, TiO32− complex anions decompose to oxygen ions O2− and Ti2+(O−)2 molecules, which are randomly oriented over 12 possible positions. Average Ti-O distances derived from neutron diffraction data were used to calculate Ti-O− and Ti=O bond lengths in TiO32− complex anions of the ferroelectric BaTiO3 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 BaTiO3 phases were used to interpret their spontaneous polarization, repolarization, and permittivity.