High-Pressure Stabilization of New Transition Metal Oxides with the Perovskite Structure and Their Physical Properties

高压合成已被用于稳定具有钙钛矿结构的新过渡金属氧化物，从而实现有趣的物理性质，如铁磁性和超导性。在4GPa和600 ℃下合成了T_c为105 K的铁磁体BiMnO_3。Sr取代Bi使电阻率显著降低，但没有实现金属态，不像金属铁磁体（La，Sr）MnO_3。磁矩随Sr浓度的增加而减小，铁磁性在Sr浓度为40%左右完全消失。用Rietveld分析的粉末衍射图进行的结构精修表明，由于Bi^<3+>离子的6s 2孤对极化，Bi原子明显偏离了规则的12配位位置。所有Mn^<3+>离子均表现出Jahn-Teller畸变。相邻Mn^<3+>离子之间的超交换相互作用预计2/3平行，1/3反平行，因此铁磁相互作用被认为是主导的。在<2-y2>1.5 -2.5GPa和1000 ℃左右进行了Ti基铜酸盐的高压合成，得到了新的高Tc氧化物TlSr_2Ca_2Cu_3O_y和<yp>TlSr_2Ca_3Cu_4O_y，它们的Tc分别为97 K和90 K。高温高压有利于防止Ti在长时间退火过程中的挥发，而长时间退火是建立Ca、Sr有序排列的必要条件。用<6.5>冲击加载法成功地合成了cla= 3.0的四元EuBa_2Cu_2O_3，但氧处理不能实现超导电性。只有在900 ℃以上退火后，才出现超导电性，这建立了Eu和Ba之间的完美秩序。

High pressure synthesis has been utilized to stabilize new transition metal oxides with the perovskite structure, thereby achieving interesting physical properties like ferromagnetism and superconductivity. A ferro-magnet BiMnO_3 with T_cof 105K was synthesized at 4 GPa and 600ﾟC.Sr substitution for Bi reduces electrical resistivity considerably, but no metallic states were realized, unlike metallic, ferromagnet (La, Sr)MnO_3. Magnetic moment rather decreased with increasing Sr concentration and ferromagnetism completely disappeared around 40% Sr concentration.Crystal structure of monoclinically distorted perovskite was determined by neutron diffraction. Structure refinement using Rietveld analysis of powder pattern revealed that the Bi atom deviated remarkably from regular 12-coordinated sites owing to polarized 6s2 lone pair of Bi^<3+> ions. Every Mn^<3+> ions showed Jahn-Teller distortion. Superexchange interaction between adjacent Mn^<3+> ions were expected to be parallel for 2/3 and anti-parallel for 1/3, hence ferromagnetic interactions were considered to be dominated. Unlike LaMnO_3, rather homogeneous distribution of vacant dx_<2-y2> orbitals would explain the ferromagnetism of BiMnO_3.High pressure synthesis of TI-based cuprates was carried out for 1 .5-2.5GPa and around 1000ﾟC.New high Tc oxides of TlSr_2Ca_2Cu_3O_y and TlSr_2Ca_3Cu4O_<yp> wiht T_c of 97K and 90K were obtained. High pressure condition was considered to be favorable for preventing TI evaporation in long time annealing which was necessary for establishing ordered arrangement of Ca and Sr.Shock synthesis of high Tc cuprates was also tried. Tetragonal EuBa_2Cu_2O_<6.5> with cla= 3.0 was successfully synthesized by shock loading of the component mixture, but no superconductivity was realized by oxygen treatment. Superconductivity appeared only after annealing the as-shock-synthesized product above 900ﾟC, which established perfect order among Eu and Ba.