The IV-VI chalcogenides exhibit exotic electronic properties arising from their diverse crystal structures, including both two-dimensional (2D) and three-dimensional (3D) lattices, attracting broad interest for device application and fundamental research. Here we investigate the lattice-mismatched ternary selenide ${\mathrm{PbSnSe}}_2$, an alloy derived from the 3D PbSe and the 2D SnSe, under high pressure in a diamond anvil cell using low-temperature transport measurements, x-ray diffraction, and optical absorption spectroscopy. ${\mathrm{PbSnSe}}_2$ is a semiconductor at ambient conditions. It undergoes a pressure-induced metallization and becomes superconducting near 17.8 GPa, and the superconducting transition temperature $T_{\mathrm{c}}$ reaches 4.4 K at approximately $20.1\mathrm{GPa}$. The $T_{\mathrm{c}}$ was found to be suppressed monotonically under further compression. During decompression, a higher $T_{\mathrm{c}}$ was observed, reaching approximately 6.1 K when the pressure is released to approximately 15.5 GPa. The pressure-dependent properties of ${\mathrm{PbSnSe}}_2$ combine characteristics of its parent compounds SnSe and PbSe: the low-pressure behavior resembles that of SnSe with a Pnma structure and a monotonously decreasing band gap under pressure, whereas the high-pressure behavior is similar to PbSe, with a simultaneous emergence of metallization and superconductivity before the Pnma-to-CsCl phase transition. These results reveal complex structural evolution and emergent superconductivity in a 3D-2D alloy under pressure and provide insight into the versatile physics of this and related material families.

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• Electrical conductivity
• High-pressure studies
• Phase transitions
• Superconductivity
• Alloys
• Semiconductors
• Infrared spectroscopy
• Pressure techniques
• X-ray diffraction