Site selectivities of alkali ions in CsRb2C60 and Cs2RbC60 superconductors

The temperature-dependent site selectivities of Cs and Rb ions in CsRb2C60 and Cs2RbC60 superconductors are computed using the free energy obtained from the configuration entropy and the total energy calculated using the ab initio pseudopotential density-functional theory. It is found that in CsRb2C60 the smaller Rb ions can occupy a considerable number of large octahedral interstitial sites at high temperatures, however, the transition to random occupation never takes place. For Cs2RbC60 the Cs occupancy of the large octahedral interstitial sites is almost always 100%, and, interestingly, the two tetrahedral interstitial sites are randomly occupied by Cs and Rb ions even at very low temperatures.