| Summary: | An in situ high-pressure X-ray diffraction study was performed on Ag<sub>2</sub>S nanosheets, with an average lateral size of 29 nm and a relatively thin thickness. Based on the experimental data, we demonstrated that under high pressure, the samples experienced two different high-pressure structural phase transitions up to 29.4 GPa: from monoclinic <i>P</i>2<sub>1</sub>/<i>n</i> structure (phase I, <i>α</i>-Ag<sub>2</sub>S) to orthorhombic <i>P</i>2<sub>1</sub>2<sub>1</sub>2<sub>1</sub> structure (phase II) at 8.9 GPa and then to monoclinic <i>P</i>2<sub>1</sub>/<i>n</i> structure (phase III) at 12.4 GPa. The critical phase transition pressures for phase II and phase III are approximately 2–3 GPa higher than that of 30 nm Ag<sub>2</sub>S nanoparticles and bulk materials. Additionally, phase III was stable up to the highest pressure of 29.4 GPa. Bulk moduli of Ag<sub>2</sub>S nanosheets were obtained as 73(6) GPa for phase I and 141(4) GPa for phase III, which indicate that the samples are more difficult to compress than their bulk counterparts and some other reported Ag<sub>2</sub>S nanoparticles. Further analysis suggested that the nanosize effect arising from the smaller thickness of Ag<sub>2</sub>S nanosheets restricts the relative position slip of the interlayer atoms during the compression, which leads to the enhancing of phase stabilities and the elevating of bulk moduli.
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