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

本研究利用高压合成方法寻找极性氧化物，并阐明其结构、极性及结构与性质的关系。我们已经成功地合成了几种LiNbO_3（LN）型氧化物如CdPbO_3和PbNiO_3，并讨论了LN型和钙钛矿（Pv）型氧化物相对于其它结构的ABO_3化合物的稳定性。钙钛矿的离子半径容限因子不是决定稳定性的唯一主要因子，3d过渡金属离子的晶场效应也很重要。此外，我们还发现了LN型氧化物中B离子偏离中心对称位置的位移与二次谐波强度之间的相关性。在高压下合成的LN型MnMO_3（M=Ti，Sn）具有极性和弱铁磁性。两种化合物在弱铁磁转变温度下均观察到介电常数的异常，表明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型化合物是多铁性材料的新候选者。在高压下合成的钙钛矿型PbNiO_3相经常压热处理后不可逆地转变为LN型相。Pb_（4+）Ni_（2+）O_3的价态为Pb_（4+）Ni_（2+）O_3的价态。Pb_（4+）Ni_（2+）O_3的Pb_（4+）Ni_（2+）O_3的Pb_（2+）O_此外，我们还成功地在高压下合成了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一种具有高自发极化的铁电体。在氧气压力下合成的Na取代钙钛矿型Bi_（1/2）Ag_（1/2）TiO_3具有铁电性。

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.