Electrides are ionic crystals in which the electrons prefer to occupy free space, serving as anions. Because the electrons prefer to be in the pockets, channels, or layers to the atomic orbitals around the nuclei, it has been challenging to find electrides with partially filled d-shells, since an unoccupied d-shell provides an energetically favourable location for the electrons to occupy. We recently predicted the existence of electrides with partially filled d-shells using high-throughput computational screening. Here, we provide an experimental support using X-ray absorption spectroscopy and X-ray and neutron diffraction to show that Sr 3 CrN 3 is indeed an electride despite its partial d-shell configuration. Our findings indicate that Sr 3 CrN 3 is the first known electride with a partially filled d-shell, in agreement with theory, which significantly broadens the criteria for the search for new electride materials.Only a handful of electrides have so far been discovered: 1 organic crown ether-alkalis, 2 and a series of inorganic materials: mayenite Ca 12 Al 14 O 32 , 3 Ca 2 N, 4 Y 5 Si 3 , 5 Y 2 C, 6 or LaH 2 . 7 Yet, in this limited materials set, many fascinating and unusual behaviors have already been uncovered in terms of their chemical, transport, optical and catalytic properties. [8][9][10][11] We recently identified >60 new electrides from a database of 40,000 inorganic materials using high-throughput computational screening. 12 Among the predicted compounds, Sr 3 CrN 3 and Ba 3 CrN 3 stood out as electrides that had transition metals with partially filled 3d-shells. This observation is unusual considering that the redox active chromium could accept the excess electron with a decrease in the formal oxidation state from +4 to +3. Instead, for these Crcontaining nitrides, the valence electron prefers to occupy an off-nuclei site. In fact, of the 60 electrides predicted by Burton et al. and all known electrides, only Sr 3 CrN 3 and Ba 3 CrN 3 contain partially filled dshell transition metals. 12 We experimentally demonstrate herein that Sr 3 CrN 3 is indeed an electride. Our

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