Group-9 Transition-Metal Suboxides Adopting the Filled-Ti₂Ni Structure: A Class of Superconductors Exhibiting Exceptionally High Upper Critical Fields

Ti2Ni and the related ν-carbide structure are known to exhibit various intriguing physical properties. The Ti2Ni structure with the cubic space group Fd3¯ m is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related ν-carbide compounds correspond to a filled version of the Ti2Ni structure. Here, we report on the structure and superconductivity in the ν-carbide-type suboxides Ti4M2O with M = Co, Rh, and Ir. We have successfully synthesized all three compounds in the single-phase form. We found all three compounds to be type-II bulk superconductors with transition temperatures of Tc = 2.7, 2.8, and 5.4 K and with normalized specific heat jumps of ΔC/γTc = 1.65, 1.28, and 1.80 for Ti4Co2O, Ti4Rh2O, and Ti4Ir2O, respectively. We found that all three superconductors exhibit high upper critical fields. Particularly noteworthy in this regard is Ti4Ir2O with an upper critical field of μ0Hc2(0) = 16.06 T, which exceeds by far the weak-coupling Pauli limit-widely considered as the maximal upper critical field-of μ0HPauli = 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 Ti2Co. In contrast to the metallic ν-carbide-type suboxides Ti4M2O, we found that Ti2Co displays a semimetallic behavior down to 0.75 K. Below 0.75 K, we observe a broad decrease in the resistivity, which can most likely be attributed to an onset of a superconducting transition at lower temperatures. Hence, the octahedral void-filling oxygen plays a crucial role in 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 Ti2Ni-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.