  The discovery of a new beta-pyrochlore oxide superconductor CsOs2O6 with Tc =
3.3 K is reported. It is the third superconductor in the family of
beta-pyrochlore oxides, following KOs2O6 with Tc = 9.6 K and RbOs2O6 with Tc =
6.3 K. The Tc of this series decreases with increasing the ionic radius of
alkaline metal ions, imposing negative chemical pressure upon the Os pyrochlore
lattice.
