Symmetry progression and possible polar metallicity in NiPS3 under pressure

van der Waals solids are ideal platforms for the discovery of new states of matter and emergent properties under external stimuli. Under pressure, complex chalcogenides like MPS₃ (M = Mn, Ni, Co, V) host sliding and structural transitions, insulator-to-metal transitions, the possibility of an orbitally-selective Mott state, piezochromism, and superconductivity. In this work, we bring together diamond anvil cell techniques, infrared and Raman scattering spectroscopies, and X-ray diffraction with a detailed symmetry analysis and first-principles calculations to uncover a series of high-pressure phases in NiPS₃. Remarkably, we find five different states of matter between ambient conditions and 39 GPa—quite different than in the other MPS₃ materials. Even more strikingly, infrared spectroscopy and X-ray diffraction combined with a symmetry analysis reveal both metallicity and loss of the inversion center above ~23 GPa suggesting that NiPS₃ may be a polar metal with a P3m1 space group under these conditions and P1 symmetry under maximum compression. In addition to identifying a candidate polar metal ripe for further inquiry, we suggest that pressure may tune other complex chalcogenides into this elusive state.