In general, electrides are expected to meet specific conditions that distinguish them from conventional materials. First, they must have excess electrons, which can be inferred from the apparent oxidation state of the chemical formula. Second, the material must have free space in which the excess electron can reside, hence the 0D, 1D, 2D nomenclature mentioned earlier. Finally, it must contain a cation from which the excess electron will be pushed into the crystal structure and not recombine. For example, 𝐶𝑎 2 𝑁 the first compound to be confirmed as 2D electride material, conforms to all three Please do not adjust margins Please do not adjust margins criteria simultaneously, see Figure . 𝐶𝑎 2 𝑁 has 2 𝐶𝑎 2+ cations which provide 4 electrons and 1 𝑁 3-anion which can only accept 3 electrons; thus one electron remains unassigned. Together, the [𝐶𝑎 2 𝑁] + 𝑒 -system is electroneutral but the chemical formula alone denotes an apparent valence of +1. Secondly, the structure of the 𝐶𝑎 2 𝑁 is layered allowing space for the anionic electrons between the bonded sheets. And finally, the donor ion calcium is extremely electropositive. However, the degree to which these design principles can be relied upon to define an electride in the general sense has recently been called in to question with the emergence of socalled intermetallic electrides, such as La3Cu2Si4, Y3Pd2, and LaRuSi reported in the literature. If the design principles are not adhered to, what distinguishes an electride from a metal or mixed alloy? But on the other hand, to what extent are the design principles limiting the identification and use of these fascinating systems?