GeS as an Emerging OTS Material: Atomic Structure and Modeling of ON/OFF Switching

Time‐of‐flight neutron and high‐energy x‐ray diffraction, together with Raman spectroscopy performed on tiny splat‐quenched GeS droplets (2.6 mg total), supported by first‐principles molecular dynamics and DFT modeling of vibrational properties, were employed to decipher the atomic and electronic structure of this promising OTS material for next‐generation high‐density memory architectures. A half‐century‐old structural controversy, between a continuous 4:2 random network of tetrahedra and a distorted trigonal model derived from orthorhombic GeS, is resolved, revealing instead a patchwork of mixed tetrahedral and trigonal units. Finite‐field FPMD simulations predict that, under a high electric field driving the ON state, a mild structural transformation begins, characterized by a partial conversion of mixed tetrahedra into strongly deformed defect‐octahedral units with elongated interatomic distances and a reduced population of Ge─Ge homopolar bonds. The simulations reveal that emerging under‐ and over‐coordinated Ge and S species, accompanied by three‐membered rings, form an interconnected defect network associated with overlapping, localized, and extended mid‐gap states near the Fermi level and with bandgap closure. These computational results suggest that the resulting donor‐like and acceptor‐like centers and amphoteric defect sites enable high‐rate electron hopping and tunneling, thereby generating strong electrical contrast between the low‐conducting OFF and highly conductive ON states.