Re-examination of the crystal structure of the ββ-pyrochlore oxide superconductor KOs2O6 by X-ray and convergent-beam electron diffraction analyses

The crystal structure of the ββ-pyrochlore oxide superconductor KOs2O6 is re-examined. A single-crystal X-ray diffraction (XRD) analysis at room temperature first revealed that the compound crystallizes in a cubic structure with the centrosymmetric space group Fd3̄m, as in conventional pyrochlore oxides. Later, however, Schuck et al. claimed a different non-centrosymmetric F4̄3m structure based on their single-crystal XRD analysis. To unambiguously determine the true crystal structure of KOs2O6, we carried out high-resolution synchrotron powder X-ray and convergent-beam electron diffraction measurements at room temperature. The space group was determined with high reliability to be centrosymmetric Fd3̄m, not F4̄3m. This confirms the importance of the K atom location in a high-symmetry site, which causes unusually large rattling of the K atom.