Group-9 Transition Metal Suboxides Adopting the Filled-Ti${}_2$Ni Structure: A Class of Superconductors Exhibiting Exceptionally High Upper Critical Fields

The Ti${}_2$Ni and the related $\eta$-carbide structure are known to exhibit various intriguing physical properties. The Ti${}_2$Ni structure with the cubic space group $Fd\bar{3}m$ is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related $\eta$-carbide compounds correspond to a filled version of the Ti${}_2$Ni structure. Here, we report on the structure and superconductivity in the $\eta$-carbide type suboxides Ti${}_4$M${}_2$O with M = Co, Rh, Ir. We have successfully synthesized all three compounds in single phase form. We find all three compounds to be type-II bulk superconductors with transition temperatures of $T_{\mathrm{c}}$ = 2.7, 2.8, and 5.4 K, and with normalized specific heat jumps of $\mathrm{\Delta }C\mathrm{/}\gamma T_{\mathrm{c}}$ = 1.65, 1.28, and 1.80 for Ti${}_4$Co${}_2$O, Ti${}_4$Rh${}_2$O, and Ti${}_4$Ir${}_2$O, respectively. We find that all three superconductors, exhibit high upper-critical fields. Particularly noteworthy is Ti${}_4$Ir${}_2$O with an upper critical field of $\mu_0 H_{\rm c2}{\rm (0)}$ =~16.06~T, which exceeds by far the weak-coupling Pauli limit of 9.86~T. The role of the void filling light atom X has so far been uncertain for the overall physical properties of these materials. Herein, we have successfully grown single crystals of Ti${}_2$Co. In contrast to the metallic $\eta$-carbide type suboxides Ti${}_4$M${}_2$O, we find that Ti${}_2$Co displays a semimetallic behavior. Hence, the octahedral void-filling oxygen plays a crucial role for the overall physical properties, even though its effect on the crystal structure is small. Our results indicate that the design of new superconductors by incorporation of electron-acceptor atoms may in the Ti${}_2$Ni-type structures and other materials with crystallographic void position be a promising future approach. The remarkably high upper critical fields, in this family of compounds, may furthermore spark significant future interest.