Crystal structure of the pyrochlore oxide superconductor KOs2O6

We report the single-crystal X-ray analysis of the structure of the pyrochlore oxide superconductor KOs2O6. The structure was identified as the beta-pyrochlore structure with space group Fd (3) over barm and lattice constant a = 10.089(2) angstrom at 300 K: the K atom is located at the 8b site, not at the 16d site as in conventional pyrochlore oxides. We found an anomalously large atomic displacement parameter U-iso = 0.0735(8) angstrom(2) at 300 K for the K cation, which suggests that the K cation weakly bound to an oversized OS12O18 cage exhibits intensive rattling, as recently observed for clathrate compounds. The rattling of A cations is a common feature in the series of flpyrochlore oxide superconductors AOs(2)O(6) (A = Cs, Rb and K), and is greatest for the smallest K cation. (c) 2005 Elsevier Inc. All rights reserved.