Miho Ito

Density Functional Calculation on the Structure of Cordierite

Density functional theory calculations were performed to analyze atomic and electronic structures of cordierite. The most stable configuration of Al and Si atoms in the six-membered ring of a unit cell was decided by geometry optimizations for sixteen different initial structures. A correlation was observed between the number of Al-O-Al bonds in a unit cell and the lattice energy. We concluded that the number of Al-O-Al bonds is a determining factor of the lattice stability. The structural parameters of the most stable crystal structure obtained by geometry optimization agreed well with the experimental observation. According to the distribution of atomic charges and analysis of bond order, the followings were clarified: (i) the covalent bond in a T1 tetrahedron was stronger than that in a T2 tetrahedron, (ii) the covalent bond of a cation in six-membered ring with O atoms in a T1 tetrahedron was the weakest, and (iii) Mg in the cordierite is of ionic nature.

Valence instability of iron oxide ultrafine particles on ferroelectrics studied by Mössbauer spectroscopy

Various types of iron oxides loaded on ferroelectric BaTiO3 and nonferroelectric ZrO2 supports have been studied by 57Fe Mossbauer spectroscopy to investigate the valence instability of iron oxides affected by ferroelectric polarization fluctuation. Although the iron oxide loaded on ZrO2 shows a typical reduction process at high vacuum and high temperature, the iron oxide loaded on BaTiO3 shows an opposite process, i.e., iron oxide oxidization. We propose the dynamics of band structures of ferroelectrics in the paraelectric phase to understand this phenomenon. The oscillation of band bending causes electron transfer from an iron oxide to a ferroelectric support. This result suggests that the surface charges of ferroelectrics are useful in developing new functional materials with metal compounds.