Polymeric alkali fullerides are stable in air

Infrared transmission, electron spin resonance, and x‐ray diffraction measurements show unambiguously that RbC60 and KC60 are stable in air, in contrast to Rb6C60 which decomposes rapidly upon exposure. The specimens studied transform to pure C60 and other by‐products when heated above 100 °C, approximately the temperature of the orthorhombic‐fcc phase transition. The stability of these compounds raises the possibility of applying them as protective layers for the superconducting fullerides.