Synthesis of superconducting pyrochlore RbOs2O6

RbOs2O6, the third superconducting pyrochlore oxide (known so far), has been synthesized by encapsulation and by high pressure techniques. Suitable post chemical treatment of the as-prepared sample allowed us to eliminate the impurity phases. Bulk superconductivity with Tc=6.4 K was observed in magnetisation and specific heat measurements. The transition temperature of RbOs2O6 was found to be the same for both preparation methods. Structural investigations showed that Rb atoms occupy the 8b site in the pyrochlore lattice with a lattice parameter of 10.1137(1) A.