Molecular-Orbital Degeneracy Lifting and Suppression of a Flat-Band Metal in a Tetrahedral Cluster System NbSeI

The lifting of degenerate electronic states has been extensively studied in transition metal compounds, where diverse quantum phenomena arise from the degrees of freedom of d electrons on individual atoms. In contrast, transition metal compounds containing high-symmetry clusters composed of multiple transition-metal atoms are expected to host a broader range of emergent phenomena due to the entanglement of the electronic degrees of freedom across multiple atoms. Here, we report the discovery of two distinct structural responses associated with molecular-orbital-degeneracy lifting in NbSeI, which comprises Nb4 tetrahedral clusters with molecular orbital degrees of freedom and whose cubic average crystal structure is predicted to host a flat-band metallic state. Below 106 K, NbSeI is a molecular-orbital-ordered nonmagnetic insulator with orthorhombic symmetry. Above this temperature, the average structure becomes face-centered cubic without superlattice formation, while local symmetry breaking associated with pronounced local distortions of the Nb4 tetrahedra lifts the molecular-orbital degeneracy. This local symmetry breaking occurs without cooperative long-range order and stabilizes a nonmagnetic insulating state instead of the flat-band metallic state predicted for the cubic average structure.