Uncovering Exotic Topological Quantum States in Pure and Magnetically‐Doped Pyrite OsS ₂

Discovering topological quantum materials that combine nontrivial topology with large bandgaps and experimentally accessible signatures remains a central pursuit in condensed matter physics. We demonstrate, through first‐principles calculations and tight‐binding modeling, that pyrite‐type osmium disulfide (OsS 2 ) is a fragile topological insulator (FTI) characterized by an exceptionally large direct bandgap of 602 meV, placing it among the highest‐gap FTIs reported. Contrary to typical fragile phases that lack prominent boundary states, OsS 2 features distinct, symmetry‐protected gapless surface states across multiple cleavage planes, enabling direct experimental verification via angle‐resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). Additionally, the presence of van Hove singularities in the electronic structure further distinguishes OsS 2 as a unique 3D quantum material. By systematically varying the magnetic moment from 0 to 4 µB through substitutional doping with Pd, Fe, Ni, and Co, we map out a rich topological phase diagram. This diagram reveals transitions from the FTI phase to a strong topological insulator (STI), a topological semimetal, a three‐dimensional quantum anomalous Hall (3D‐QAH) insulator, and ultimately a trivial magnetic semiconductor, each phase corresponding to a specific magnetic moment. The structural similarity and near‐identical lattice constants of PdS 2 , FeS 2 , NiS 2 , and CoS 2 with OsS 2 , all of which crystallize in the cubic pyrite phase, suggest the practical realization of these phases. Collectively, our findings establish OsS 2 as a promising platform for exploring fragile topology, tunable quantum phase transitions, and novel boundary phenomena within a single material system.