$\eta$-Carbide-type compounds have recently emerged as a diverse class of materials in the study of superconductivity. These phases contribute to a growing family of metal-rich quantum materials that exhibit unusual superconducting properties emerging from complex metallic bonding. Several members of the $\eta$-carbide-type phases have been found to be bulk superconductors, such as ${\mathrm{Nb}}_4{\mathrm{Rh}}_2{\mathrm{C}}_{1-\delta }$, ${\mathrm{Ta}}_4{\mathrm{Rh}}_2{\mathrm{C}}_{1-\delta }$, ${\mathrm{Ti}}_4{\mathrm{Ir}}_2{\mathrm{O}}_{1-\delta }$, and ${\mathrm{Ti}}_4{\mathrm{Co}}_2{\mathrm{O}}_{1-\delta }$–with transition temperatures up to $T_{\mathrm{c}}\approx$ 10 K and upper critical fields as high as ${\mu }_0{H}_{\mathrm{c}2}\left(0\right)\approx$ 30 T. Whereas the transition temperatures may fall within the range typical for intermetallic superconductors, the pronounced violation of the weak-coupling Pauli limit in many of these crystallographically high-symmetry materials is noteworthy. Here, we review recent progress on superconducting $\eta$-carbide-type phases, emphasizing how crystal symmetry, synthetic challenges, transition-metal composition, and electronic structure govern their superconducting properties. Furthermore, we outline open questions and future directions, including the possible discovery of additional $\eta$-carbide-type materials.

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• Carbides
• Superconductors