Structural Study of Ferroelectric Phase Transition of Sn-doped SrTiO₃

Recently, ferroelectricity was discovered in Sn-doped SrTiO3 (abbreviated by SSTO), in which Sr-atom was substituted by a few percent Sn-atom[1]. The ferroelctricity of SSTO was confirmed by means of the appearance of the dielectric anomaly, that reached several thousands and the clear D-E hysteresis loop in low temperature phase. In order to clarify the mechanism of ferroelectric phase transition of SSTO from the viewpoint of the crystal structure, we investigated the average crystal structure and the local structure around the substitutional Sn-atom of SSTO10 (10% Sn concentration, ferroelectric phase transition temperature 180K) by means of synchrotron-radiation powder X-ray diffraction and transmission XAFS spectrum of Sn:K-edge, respectively. From the results of MEM/Rietveld analysis of powder X-ray diffraction data, it was obtained that crystal structure of paraelectric phase of SSTO10 was cubic perovskite structure with the disorder state of Sn-atom. In ferroelectric phase, the crystal system was tetragonal, which was similar in structure to tetragonal ferroelectric structure of BaTiO3, and Sn-atom was order state. XAFS study revealed that the valence of Sn-ion was +2 charge and the local structure of Sn-atom was seemed as being the self-insistent state of SnO crystal structure. However, strangely, the coordination number of the nearest neighbor atom, that is O-atom, was 2 instead of 4. This is a mystery result and we have been analyzing. We have considered that the ferroelectricity of SSTO is induced by the distortion around the subsitituional Sn-atom. At the meeting, we are planning to discuss the precise crystal structure and the mechanism of the ferroelectric phase transition of SSTO.

Fabrication of Barium Titanate Based Ferroelectrics by Containerless Processing

Lead-free ferroelectric materials are desired in our lives owing to the environmental problem, and are anticipated to replace conventional ferroelectrics such as Pb(Zr,Ti)O₃ (PZT) ceramics. Many researchers have attempted to develop lead-free ceramics whose ferroelectric properties would surpass those of PZT ceramics. Recently, it has been reported that oxygen-deficient hexagonal-BaTiO₃ which is metastable state of perovskite-BaTiO₃ exhibits giant dielectric constants over the wide temperature range. Such a metastable material has capability as a lead-free dielectric material with high performance. The containerless processing has an advantage over the other conventional methods in synthesizing metastable materials. It can suppress heterogeneous nucleation from the container wall, and produce the undercooled state below the solidification point of the metastable phase, which is lower than that of the stable phase. The purpose of this study is to synthesize barium titanate based ferroelectrics by the containerless processing as lead-free metastable materials. We developed the aerodynamic levitation furnace which enables us to levitate and melt a sample about 1-3 mm in diameter in containerless condition. Figure 1 shows a schematic view of the aerodynamic levitation furnace. The samples are levitated by the gas jet nozzle, and are melted using the CO₂ laser radiation. The metastable crystals of the barium titanate based ferroelectrics were fabricated by the aerodynamic levitation furnace. The crystal structure is demonstrated by analyzing high energy synchrotron radiation powder diffraction data. We discuss the prospects of the ferroelectricity on the basis of the determined crystal structure.

Valence Electron Distributions in Ferroelectric Barium Titanate Nanopowders

Barium titanate BaTiO3 is one of the most important perovskite-type electroceramics, which undergoes the phase transition at 130 ⁰C from cubic to tetragonal, and exhibits ferroelectricity at room temperature. The phase transition depends on the particle size. BaTiO3 powders with the particle sizes less than several tens of nanometers are known to show no phase transition and hence no ferroelectricity at room temperature. The size effect of BaTiO3 is the most important issue in designing small ceramic capacitors with high capacitance. Our group has been devoted to visualizing the electron density distributions of perovskite-type oxides by analyzing the synchrotron-radiation x-ray powder diffraction (SXRD) data measured at SPring-8 using the maximum entropy method (MEM)/Rietveld method [1, 2]. In this study, the distributions of valence electrons in the outer shells of atoms are derived accurately from the SXRD data of BaTiO3 nanopowders to prove the characteristic chemical bondings which govern the ferroelectric phase transition. The powder samples used were 500 and 35 nm in particle sizes. The former showed the phase transition whereas the latter showed no phase transition. The MEM valence electron density studies at 200 ⁰C in the cubic structure revealed the clear structural variations that the Ti-O covalent bonding is found in the 500 nm sample, while all the valence electrons are localized at the O sites in the 35 nm sample exactly like an ionic crystal. Ferroelectricity originates from the balance between the long-rage Coulomb force and the short-range repulsion force. The obtained results provide direct experimental evidence that the electron orbitals hybridization on the Ti-O bonds weakens the short-range repulsion force, and causes the second-order Jahn-Teller distortion on the TiO6 octahedron in the 500 nm sample. We consider that the Ti-O bonding in the prototype structure governs the ferroelectric phase transition temperature in BaTiO3.