A second phase transition and superconductivity of the β-pyrochlore oxide KOs2O6

Another phase transition that is probably of first order is found for the beta-pyrochlore oxide superconductor KOs2O6 with a superconducting transition temperature T-c of 9.6 K. It takes place at T-p = 7.5 K in the superconducting state in zero magnetic field. On applying magnetic fields of up to 140 kOe, the T-c gradually decreased to 5.2 K, while T-p changed little, eventually breaking through the H-c2 line at approximately 65 kOe in the H - T diagram. Both the normal- state resistivity and H-c2 change slightly but significantly across the second phase transition. It is suggested that the transition is associated with the rattling of potassium ions located in an oversized cage of osmium and oxide ions.