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.

Substitution Effects of Gd and Mg on Cubic Structure of Barium Titanate

Barium titanate BaTiO3 is a ferroelectric oxide with low dielectric loss and high permittivity, which makes it a good insulator for industrial uses. However, the pure barium titanate is not always used for electronic devices. Gd- and Mg-doping in barium titanate is effective for the dielectrics in multilayer ceramic capacitors MLCCs, for example, to attain the uniform temperature-variation of dielectric constants in the wide temperature-range where MLCCs are practically used, while the Curie temperature is significantly decreased [1]. In this study, the structural characteristics of Gd- and Mg-substituted barium titanate (Ba,Gd)(Ti,Mg)O3 (BGTM) in the cubic phase were investigated to clarify the effects of Gd- and Mg-substitution on the crystal structure and the chemical bonding. Synchrotron radiation x-ray powder diffraction measurements were carried out using the BGTM samples with the Mg content of x = 0 ~ 0.1. Lattice parameters, substitution sites of the Gd and Mg ions, thermal parameters, and electron charge density distributions in the crystals were analyzed by using the maximum entropy method (MEM)/Rietveld method. The Curie temperature decreased with increasing x. The phase transition did not take place in BGTM with x > 0.05. The Gd ion was confirmed to be substituted for the Ba ion with larger thermal vibration amplitude than that of the Ba ion. The amplitude was almost independent of the Gd content. It was revealed that the Gd ion occupied the off-centered positions slightly form the Ba-site along the <100> directions at lower temperatures. Difference in thermal behaviors of the Gd and Ba ions can be attributed to the size difference between the smaller Gd ion and the larger Ba ion. The Mg ion was observed on the Ti-site. The substitution of the Mg ion for the Ti ion suppresses the ferroactivity of the Ti ion, which causes the lowering of the Curie temperature.

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.

Structure and Anion Exchangeability of Ni-Al-Type Layered Double Hydroxides

Layered double hydroxide (LDH) is one of promising inorganic materials for cleaning the environmental water polluted by toxic anions. The crystal structure of LDH is composed of the positively-charged metal hydroxide nanosheets and the anions with water molecules intercalated between the nanosheet layers. To put LDHs for practical use, it is necessary to understand why only special anions can be intercalated into the crystal structure from the aqueous solution. In this study, the anion exchange experiments and the synchrotron radiation x-ray powder diffraction measurements of Ni-Al-type LDHs of several kinds of Ni/Al ratios with chlorine and nitrate anions were performed to investigate the relationship between the anion exchange selectivity and crystal structures. The nitrate ion selectivity is normally poor in most of LDHs with different metal ions in the hydroxide nanosheet [1]. However, the nitrate ions were preferred to the chlorine ions in Ni-Al-type LDH when Ni/Al = 4, whereas the chlorine ions were selected when Ni/Al = 2. The crystal structure analysis revealed that the interlayer distance decreased and the thermal motion of the nitrate ions suppressed in Ni-Al-nitrate-type LDH with increasing the Ni/Al ratio, whereas those of the chlorine ions in Ni-Al-chlorine-type LDH increased and enhanced with increasing the Ni/Al ratio. These results indicate that the nitrate ions are more stable than the chlorine ions in the crystal structure of the Ni-Al-type LDH when the positive charge of the nanosheet is small, i.e. the number of anions is small. The short interlayer distance and the small thermal vibration of anions in the crystal structure are the key to understand the anion selectivity of LDH.