  Rattling-induced superconductivity in the {\beta}-pyrochlore oxide KOs2O6 is
investigated under high pressure up to 5 GPa. Resistivity measurements in a
high-quality single crystal reveal a gradual decrease in the superconducting
transition temperature Tc from 9.7 K at 1.0 GPa to 6.5 K at 3.5 GPa, followed
by a sudden drop to 3.3 K at 3.6 GPa. Powder X-ray diffraction experiments show
a structural transition from cubic to monoclinic or triclinic at a similar
pressure. The sudden drop in Tc is ascribed to this structural tran-sition, by
which an enhancement in Tc due to a strong electron-rattler interaction present
in the low-pressure cubic phase is abrogated as the rattling of the K ion is
completely suppressed or weakened in the high-pressure phase of reduced
symmetry. In addition, we find two anomalies in the temperature dependence of
resistivity in the low-pressure phase, which may be due to subtle changes in
rattling vibration.
