Search for new materials and properties in the pyrochlore oxides

近十年来，非铜基氧化物超导体的研究得到了扩展，旨在了解电子相关在超导机制中的作用或寻找新的配对机制，希望达到更高的Tc。最近发现的一个有趣的例子是氧化焦绿盐超导体族。在A = Cs、Rb和K条件下，第一个发现的是T_c = 1.0 K的A -焦绿酸Cd_2Re_2O_7，第二个发现的是T_c = 3.3、6.3和9.6 K的β-焦绿酸AOs_2O_6。它们以空间基团Fd-3m的立方焦绿盐结构结晶，通常具有由ReO_6或OsO_6八面体组成的三维骨架。过渡金属离子占据的“焦绿石”亚晶格由共用角的四面体组成，对于具有反铁磁最近邻相互作用的局域自旋系统来说，这是非常令人沮丧的。研究了一类氧化焦氯盐超导体的基本超导参数和正常态性质的化学变化趋势。特别讨论了α-焦绿石Cd_2Re_2O_7的T_c从1.0 K到3.3 K （A = Cs）、6.3 K （Rb）和9.6 K (K)的变化。KOs_2O_6的T_c增强和异常特征归因于可能来自K阳离子的咔嗒声的低能声子。

The study of non-Cu-based oxide superconductors has been extended during the last decade, aiming at understanding the role of electron correlations in the mechanism of superconductivity or searching for a novel pairing mechanism, hopefully to reach a higher Tc. An interesting example recently found is a family of pyrochlore oxide superconductors. The first discovered is a-pyrochlore rhenate Cd_2Re_2O_7 with T_c = 1.0 K and the second β-pyrochlore osmate AOs_2O_6 with T_c = 3.3, 6.3, and 9.6 K for A = Cs, Rb, and K, respectively. They crystallize in the cubic pyrochlore structure of space group Fd-3m and commonly possess a 3D skeleton made of ReO_6 or OsO_6 octahedra. The "pyrochlore" sublattice occupied by the transition metal ions is comprised of corner-sharing tetrahedra that are known to be highly frustrating for a localized spin system with antiferromagnetic nearest-neighbor interactions.Chemical trends of fundamental superconducting parameters and normal-state properties are studied for a family of pyrochlore oxide superconductors. Particularly, the change of T_c from 1.0 K for α-pyrochlore Cd_2Re_2O_7 to 3.3 K (A = Cs), 6.3 K (Rb), and 9.6 K (K) for β-pyrochlore AOs_2O_6 is discussed on the basis of the conventional BCS scheme. Enhanced T_c and anomalous features observed for KOs_2O_6 are ascribed to low-energy phonons probably coming from the rattling of the K cations.