Crystal structure of the pyrochlore oxide superconductor KOs ₂O₆

We report the single-crystal X-ray analysis of the structure of the pyrochlore oxide superconductor KOs₂O₆. The structure was identified as the β-pyrochlore structure with space group Fd3̄m and lattice constant a=10.089(2)Å at 300K: 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 Uiso=0.0735(8)Å2 at 300K 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 β-pyrochlore oxide superconductors AOs₂O ₆ (A=Cs, Rb and K), and is greatest for the smallest K cation.