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Acta Cryst. (2014). A70, C61
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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.

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Acta Cryst. (2014). A70, C77
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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.
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