Chemical-Disorder-Induced Non-metallic Transport in Thermodynamically Metallic Mo4TGa16Ge (T = Co, Rh or Ir)

Flat-band electronic states can be highly sensitive to chemical perturbations, offering opportunities to access a broader range of electronic behaviors beyond that of parent materials. Here, we report the discovery of a new series of compounds, Mo4TGa16Ge (T = Co, Rh, or Ir), derived from the strongly correlated flat-band superconductor, Mo4PtGa17, via nominally preserving the total valence electron counts. All three materials crystallize in a noncentrosymmetric cubic space group, F-43m, with Ge selectively occupying one of the Ga sites in the parent compound. Although the total electron count remains the same, Mo4TGa16Ge exhibits significantly distinct properties from Mo4PtGa17. Theoretical calculations predict metallic electronic structures with narrow, dominant Mo-d states at the Fermi energy, while low-temperature heat capacity results demonstrate finite Sommerfeld coefficients. However, electrical transport measurements show predominantly non-metallic behaviors with small observed activation energies, excluding the possibilities of conventional semiconducting behaviors. Combining chemical bonding analysis, electronic structure and compositional determination from X-ray crystallography and spectroscopy, we propose that the coexistence of the non-metallic transport features and the thermodynamically metallic behaviors can be attributed to the high sensitivity of the narrow-band materials to chemical disorders. Thus, the discovery and investigation of Mo4TGa16Ge provide a chemically tunable platform for studying disorder-controlled transport in flat-band intermetallics.