Control of chemical bonding and development of novel polar oxides by high pressure synthesis

在本研究中，我们通过高压合成寻找极性氧化物，并阐明了结构和极性性质以及结构-性质关系。我们成功地合成了几种LiNbO_3（LN）型氧化物，如CdPbO_3和PbNiO_3，并讨论了LN型和钙钛矿（Pv）型氧化物相对于其他结构的氧化物的稳定性。钙钛矿的离子半径容差因子并不是决定稳定性的唯一主导因素，三维过渡金属离子的晶体场效应也很重要。此外，我们还发现了ln型氧化物中b离子从中心对称位置的位移与二次谐波产生强度之间的相关性。在高压下合成的ln型MnMO_3（M=Ti, Sn）表现出极性，弱铁磁性。在弱铁磁转变温度下，两种化合物的介电常数都出现了异常，这表明了磁性和介电性质之间的相关性。这些结果表明，具有磁性阳离子的ln型化合物是多铁材料的新候选材料。在高压下合成的钙钛矿型PbNiO_3相经常压热处理不可逆地转变为ln型相。两相均具有Pb_<4+> Ni_<2+> O_3的特殊价态。pv型和ln型PbNiO_3均发生反铁磁跃迁，跃迁温度与Ni-O距离有关。此外，我们在高压下成功合成了pv型PbMnO_2F，并发现了较大的四方畸变，这表明该化合物可能是具有高自发极化的铁电性化合物。在氧压下合成的Na取代钙矿型Bi_<1/2> Ag_<1/2> TiO_3表现出铁电性。

In this study, we have searched polar oxides by high pressure synthesis and elucidated structure and polar properties and structure-property relation. We have successfully synthesized several LiNbO_3(LN)-type oxides such as CdPbO_3 and PbNiO_3 and discussed the stability of LN-type and perovskite(Pv)-type oxides relative to oxides with other structure for ABO_3 compounds. The tolerance factor of perovskite in terms of ionic radius is not the only predominant one to determine the stability and the crystal field effects of 3d transition metal ions are also important. Furthermore, we found the correlation between the displacements of B-ion from the centro-symmetric position in LN-type oxides and the intensity of second harmonic generation. LN-type MnMO_3(M=Ti, Sn) synthesized under high pressure exhibit polarity, and weak ferromagnetism. Anomalies in the dielectric permittivity were observed at the weak ferromagnetic transition temperature for both the compounds, indicating the correlation between magnetic and dielectric properties. These results indicate that LN-type compounds with magnetic cations are new candidates for multiferroic materials. A perovskite-type PbNiO_3 phase synthesized under high pressure irreversibly transforms to a LN-type phase by heat treatment at ambient pressure. Both phases possess the peculiar valence state of Pb_<4+> Ni_<2+> O_3.The both of Pv-type and LN-type PbNiO_3 undergo antiferromagnetic transition and the transition temperatures depend on the Ni-O distance. Furthermore, we successfully synthesized Pv-type PbMnO_2F under high pressure and found large tetragonal distortion, which indicates that the compound may be a ferroelectric with high spontaneous polarization. Na substituted perovsite-type Bi_<1/2> Ag_<1/2> TiO_3, which was synthesized under oxygen pressure, exhibited ferroelectricity.