The rhombohedral unit cell of the NASICON structure contains six formula units (see Fig. ). With the monovalent sodium NASICON structure, at least one Na atom per formula unit is needed to form a charge neutral cell. However, for the divalent Ca and Mg cations only half a cation per formula unit (or one cation per two formula units) is needed. It is still typical to refer the Ca stoichiometry to one formula unit as in Ca y M 2 (PO 4 ) 3 , 12 which then results in a nomenclature in which a site occupancy of 0.5 is assumed. This nomenclature is also motivated by the results of X-ray diffraction (XRD) experiments which yield that the Ca atom can sit at equivalent Na1 sites of the sodium atom in the standard NASICON phase yielding a likelihood of 0.5 for occupation of one particular site. These Na1 sites in a traditional NASICON phase (6B Wyckoff site) sit in an elongated octahedra while the so-called Na2 sites are left empty and only passed during diffusion from one Na1 site to the next in the studied systems.