Chemical effects of alkali atoms on critical temperature in superconducting alkali-doped fullerides

Alkali metal doped fullerides (A₃C₆₀) are superconductors with critical temperatures, T_c, extending up to 38 K. T_c is known to depend strongly on the lattice parameter a, which can be adjusted by physical or chemical pressure. In the latter case an alkali atom is replaced by a different sized one, which changes a. We have collected an extensive data base of experimental data for T_c from very early up to recent measurements. We disentangle alkali atom chemical effects on T_c, beyond the well-known consequences of changing a. It is found that T_c, for a fixed a, is typically increased as smaller alkali atoms are replaced by larger ones, except for very large a. Possible reasons for these results are discussed. Although smaller in size than the lattice parameter contribution, the chemical effect is not negligible and should be considered in future physical model developments.