Symmetry and Chemical Stability of M₃X₈ (M = Nb, X= Cl. Br, I) and Other Ionic Kagome Materials

The 2D van der Waals transition metal Kagome halides M₃X₈ are a class of correlated materials showing potential multiferroic properties, spin liquid behavior, and have recently been used in field-free Josephson diodes for quantum computing applications.¹ However, relatively few such materials have been synthesized successfully compared to the 2D dihalides and trihalides (MX₂and MX₃) –only the M=Nb and ( X=Cl, Br, I ) compounds can be found in the literature.² Key to these materials’ properties is a breathing distortion of the Kagome lattice, associated with a triangular M₃ cluster formation coupled to a trimer orbital formation – extended along the three transition metals in the cluster. Using density functional theory simulations, symmetry analysis, and Crystal Orbital Hamiltonian Populations (COHP), we show that the unique chemical stability of the Nb compounds is intrinsically tied to the metal cluster formation in these materials, and the specific bonding and antibonding symmetry of the trimer molecular orbitals associated with it. Using this theoretical method, we propose new possible materials for synthesis in this materials family and extend our theory to understand dynamic electrochemical doping in the Kagome Oxide Li_xScMo₃O₈.³