Charge Ordering, Superconductivity and Dimensionality in Perovskite-type Bismuth Oxides

钙钛矿型BaBiO_3具有单斜斜结构，并具有bio_6八面体的倾斜。它是具有Bi^<3+>和Bi^<5+>有序网络的CDW绝缘子。本文研究了Ba_<1-x>Rb_xBiO_3的低温结构与Tc的关系。研究发现，超导的外观倾向于立方对称和小晶格参数。Bi 6s轨道上的局域电子由于传递能量的增加而发生了移动，从而抑制了CDW，并向金属过渡。在高压下，BaBiO_3的晶格参数有望变小。如果BaBiO_3的晶体结构在高压下转变为立方结构，则有望出现高Tc的超导性。测量了半导体材料Ba_<1-x>K_xBiO_3在高达8gpa高压下的电阻率。然而，它仍然是半导体的。这是因为在高压下，bi_6八面体的倾角会变大。制备了电子掺杂Ba^<2+>_<0.6>Bi^<3+>_<0.4>的BiO_3。然而，这个样品是一个绝缘体。非超导性的原因之一可能是氧空位的存在。另一个原因可能是短程杀伤性武器的存在。制备了不显示CDW的层状锂嵌层铌氧化物作为参比材料。发现了Tc ~ 5K的Li_xRbSr_2Nb_3O_<10>和Li_xCsSr_2Nb_3O_<10>的新超导体。目前铌氧化物中Tc值低于铋氧化物的原因尚不清楚。

Perovskite-type BaBiO_3 has a monoclinic structure with a tilt of the BiO_6octahedra. It is a CDW insulator with an ordered network of Bi^<3+>and Bi^<5+>. The relation between Tc and the structure at low temperatures has been investigated for Ba_<1-x>Rb_xBiO_3. It has been found that the appearance of superconductivity prefers a cubic symmetry and small lattice parameters. The localized electrons in Bi 6s orbit are expected to itinerate due to the gain of transfer energy, which leads to the suppression of CDW and the transition to metal.Under high pressures, the lattice parameters of BaBiO_3 are expected to become small. If the crystal structure of BaBiO_3 will transform to cubic under high pressures, the appearance of superconductivity with higher Tc is expected. The electrical resistivity under high pressures up to 8 GPa has been measured for semiconductive Ba_<1-x>K_xBiO_3. However, it still remains semiconductive. This is because the tilt angle of BiO_6 octahedra may become large under high pressures.Electron-doped Ba^<2+>_<0.6>Bi^<3+>_<0.4>BiO_3 has been prepared. However, this sample is an insulator. One of the reasons of non-superconductivity may be the existence of oxygen vacancies. Another may be the existence of the CDW in a short range.Li-intercalated Nb oxides with the layered structure, which does not show the CDW, have been prepared as referential material. New superconductors with Tc〜5K of Li_xRbSr_2Nb_3O_<10> and Li_xCsSr_2Nb_3O_<10> have been found. The reason why the value of Tc in the present Nb oxides is lower than that in Bi oxides is not clear yet.