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

钙钛矿型BaBiO_3具有单斜晶系结构，具有BiO_6八面体的倾角。它是一种具有Bi^&lt；3+&gt；和Bi^&lt；5+&gt；有序网络的CDW绝缘子。本文研究了Ba_xBiO_3的低温结构与T_c的关系，发现超导电性的出现更倾向于立方对称性和较小的晶格参数。由于转移能的增加，在Bi6S轨道上的定域电子有望游移，导致CDW的抑制和向金属的转变，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体，则有望出现更高T_c的超导电性。测量了半导体Ba_(1-x)&Gt；K_xBiO_3在高达8 Gpa高压下的电阻率，但仍保持半导性。这是因为在高压下，BiO_6八面体的倾角可能会变大。我们已经制备了电子掺杂的Ba^&lt；2+&gt；_&lt；0.6&gt；Bi^&lt；3+&gt；_&lt；0.4&gt；BiO_3。然而，这个样品是绝缘体。非超导电性的原因之一可能是氧空位的存在。另一种可能是在短范围内存在CDW，已经制备了具有层状结构的锂插层Nb氧化物，但没有显示CDW，作为参考材料。发现了Li_xRbSr_2Nb_3O_(10&gt)和Li_xCsSr_2Nb_3O_(10&gt)的新超导体，其T_c~5K。目前Nb氧化物中Tc值低于Bi氧化物中Tc值的原因尚不清楚。

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.