Structure and properties of alkali-doped C₆₀

The structure of superconducting face-centered cubic (f.c.c.) M₃C₆₀ compounds has been precisely studied for binary alkali-metal-doped C₆₀ molecules, where M is Na, K, Rb, Cs and their binary mixtures. Refinements of the observed X-ray diffraction patterns show that M_S2M_LC₆₀ compounds are stable and preferentially ordered compared with the crystal phases for M_SM_L2C₆₀, which are very dependent on the alkali metals used, where M_S denotes the smaller alkali metal and M_L the larger metal. A new relationship between the superconducting transition temperature (T_c) and the f.c.c. lattice constant (a₀) is demonstrated over a wide range of lattice parameters, using the binary alkali-metal-doped f.c.c. M₃C₆₀ superconductors. The new T_c-a₀ relationship is compared with the previously reported pressure experiments and the prediction based on the simple Bardeen-Cooper-Schriefter theory.