The Ti₂Ni and the related η-carbide-type structures are known to exhibit various interesting physical properties [1, 2]. The Ti₂Ni structure is surprisingly complex for an intermetallic structure-type crystallizing in the space group Fd-3m with a unit cell containing 96 metal atoms [1]. The related η-carbide-type compounds of the general formula A₄B₂X or A₃B₃X correspond to a filled version of the Ti₂Ni structure. The role of the void filling light atom X, which can be carbon, oxygen, or nitrogen, has so far been unclear for the overall physical properties of these materials. Herein, we have successfully grown single crystals of Ti₂Co with the Ti₂Ni-type structure and single crystals of η-carbide-type oxide Ti₄Co₂O. We show that Ti₂Co displays a semimetallic behavior down to 0.75 K, and then a superconducting transition seems to occur at lower temperatures. Its oxygen filled-version Ti₄Co₂O is a bulk superconductor with a critical temperature of T_c = 2.7 K. We find that the interstitial oxygen plays a crucial role for the overall physical properties. By extending this concept to the other group 9 transition metals, we have successfully synthesized two new compounds, Ti₄Rh₂O and Ti₄Ir₂O. We, furthermore, show that both are new bulk type-II superconductors with superconducting transitions at T_c = 2.8 K and 5.3 K, respectively. We present detailed measurements on all three superconductors, showing that all three have remarkably high upper critical field in comparison with their critical temperatures. Most noteworthy Ti₄Ir₂O has an upper critical field of μ₀H_c2(0) =16.06 T, which is exceeding by far the weak-coupling BCS Pauli paramagnetic limit of μ₀H_pauli = 9.86 T.

[1] Mackay, R.; Miller, G. J.; Franzen, H. F., Journal of alloys and compounds,1994,204, 109–118.
[2] Ma, K.; Lago, J.; von Rohr, F. O., Journal of Alloys and Compounds, 2019,796, 287–292.