Ag₂S belongs to a large family of silver chalocogenides Ag₂M (M = S, Se, and Te) that have been actively investigated for novel topological properties. For example, β-Ag₂Te has been identified as a topological insulator with gapless Dirac-type surface states [1]. The orthorhombic Cmce−Ag₂S is predicted to be a topological nodal-line semimetal with a single glide symmetry-protected nodal loop [2]. Unfortunately, this phase is yet to be synthesized and the stable α-Ag₂S is just a conventional semiconductor at ambient pressure. Previous works reported that pressure can tune the lattice and electronic structures of different Ag₂S phases away from the pristine states [3,4]. However, the crystal structures and electronic properties of these high-pressure (HP) phases are still under debate because of the difficulties in structural solutions based on HP diffraction data, calling for more accurate experiments toward this issue.

Here, we present a comprehensive and multifaceted investigation of the HP behaviors of α-Ag₂S.We report two reversible pressure-induced isosymmetric phase transitions in α-Ag₂S that are accompanied by two compressive anomalies at 7.5 and 16 GPa. The first transition arises from a sudden and drastic puckering of the wrinkled Ag-S layers, which leads to an anomalous structural softening at high pressure and gives rise to the ultrahigh compressive ductility in α-Ag₂S. The second transition stems from a pressure-driven electronic state crossover from a conventional semiconductor to a topological metal. Our findings not only reveal the underlying mechanism responsible for the ultrahigh ductility in this class of inorganic semiconductors, but also provide a distinctive member to the growing family of topological metals and semimetals.

[1] W. Zhang, R. Yu, W. Feng, Y. Yao, H. Weng, X. Dai, and Z. Fang, Topological aspect and quantum magnetoresistance of β−Ag₂Te, Phys. Rev. Lett. 106, 156808 (2011).

[2] H. Huang, K. H. Jin, and F. Liu, Topological nodal-line semimetal in nonsymmorphic Cmce-phase Ag₂S, Phys. Rev. B 96, 115106 (2017).

[3] Z. Zhao, H. Wei, and W. L. Mao, Pressure tuning the lattice and optical response of silver sulfide, Appl. Phys. Lett. 108, 261902 (2016).

[4] D. Santamaria-Perez, M. Marques, R. Chulia-Jordan, J. M. Menendez, O. Gomis, J. Ruiz-Fuertes, J. A. Sans, D. Errandonea, and J. M. Recio, Compression of silver sulfide: x-ray diffraction measurements and total-energy calculations, Inorg. Chem. 51, 5289 (2012).