New oxide group-9 transition metal superconductors in the filled-Ti2Ni type structure
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 . 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 Tc = 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 Tc = 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 μ0Hc2(0) =16.06 T, which is exceeding by far the weak-coupling BCS Pauli paramagnetic limit of μ0Hpauli = 9.86 T.
 Mackay, R.; Miller, G. J.; Franzen, H. F., Journal of alloys and compounds,1994,204, 109–118.
 Ma, K.; Lago, J.; von Rohr, F. O., Journal of Alloys and Compounds, 2019,796, 287–292.