The intercalation of electron donor species into graphite belongs to the class of two-dimensional topotactic reactions. During the intercalation, which requires an oxidoreduction between the reagents, the graphene planes, which form the skeleton of the host material, remain indeed quasi unaltered while their spacing is strongly expanded allowing the setting up of the guest reagent layers. Intercalation into graphite is therefore the prototype of the topotactic reaction. The intercalation mechanisms of these electron donors such as met-[a] Carbon-4800 Scheme 2. Schematic representation of the intercalation into graphite of calcium starting from a liquid Li-Ca binary alloy. First step: selective intercalation of lithium by staging mechanism (1 st stage LiC 6 ); second step: evacuation of lithium, which is replaced by calcium (1 st stage CaC 6 with Ic = 453 pm).