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 image-pyrochlore structure with space group image and lattice constant image at image: the K atom is located at the image site, not at the image site as in conventional pyrochlore oxides. We found an anomalously large atomic displacement parameter image at image 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 image-pyrochlore oxide superconductors AOs2O6 (image, Rb and K), and is greatest for the smallest K cation.