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The Ti$_2$Ni and the related $\eta$-carbide structure are known to exhibitvarious intriguing physical properties. The Ti$_2$Ni structure with the cubicspace group $Fd\bar{3}m$ is surprisingly complex, consisting of a unit cellwith 96 metal atoms. The related $\eta$-carbide compounds correspond to afilled version of the Ti$_2$Ni structure. Here, we report on the structure andsuperconductivity in the $\eta$-carbide type suboxides Ti$_4$M$_2$O with M =Co, Rh, Ir. We have successfully synthesized all three compounds in singlephase form. We find all three compounds to be type-II bulk superconductors withtransition temperatures of $T_{\rm c}$ = 2.7, 2.8, and 5.4 K, and withnormalized specific heat jumps of $\Delta C/\gamma T_{\rm c}$ = 1.65, 1.28, and1.80 for Ti$_4$Co$_2$O, Ti$_4$Rh$_2$O, and Ti$_4$Ir$_2$O, respectively. We findthat all three superconductors, exhibit high upper-critical fields.Particularly noteworthy is Ti$_4$Ir$_2$O with an upper critical field of $\mu_0H_{\rm c2}{\rm (0)}$ =~16.06~T, which exceeds by far the weak-coupling Paulilimit of 9.86~T. The role of the void filling light atom X has so far beenuncertain for the overall physical properties of these materials. Herein, wehave successfully grown single crystals of Ti$_2$Co. In contrast to themetallic $\eta$-carbide type suboxides Ti$_4$M$_2$O, we find that Ti$_2$Codisplays a semimetallic behavior. Hence, the octahedral void-filling oxygenplays a crucial role for the overall physical properties, even though itseffect on the crystal structure is small. Our results indicate that the designof new superconductors by incorporation of electron-acceptor atoms may in theTi$_2$Ni-type structures and other materials with crystallographic voidposition be a promising future approach. The remarkably high upper criticalfields, in this family of compounds, may furthermore spark significant futureinterest.

chemistry of materials

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