Fig. 2: Insulator-to-metal transition occurs with the development of a Drude response and screening of the phonons.

a Reflectance of NiPS₃ as a function of frequency showing the development of metallicity under pressure. Dashed lines omit the well-known diamond signature and guide the eye. b Close-up view of the 590 cm⁻¹ P-S stretch which is increasingly screened by the Drude under pressure, consistent with the development of metallicity. The metallicity develops gradually, in line with transport results^12,15 and consistent with a infrared-active second-order transition. This implies that a group-subgroup symmetry analysis is reasonable. c Calculated displacement pattern of the P-S stretching mode. Phosphorus and sulfur centers are purple and yellow, respectively. Black arrows indicate both direction and magnitude of the distortion.