Structure of amorphous non-molecular CS₂ at high pressure - experiment and simulation

CS2 represents an important simple molecular system with double bonds. It is widely believed that upon compression it converts into a system of (-(C=S)-S-)n polymeric chains (Bridgman's black polymer-BBP). Here we show, combining new Raman, IR and XRD data for pressures up to 40 GPa with ab initio and machine-learned potential simulations, that the structure of BBP is different. Pressure-induced polymerization creates an amorphous system consisting of chains with 3-coordinated C atoms connected via S bridges or double C=C bonds as well as ones with 4-coordinated C atoms within edge-sharing CS4 tetrahedra. Spectral features observed upon compression can be explained without need for decomposition of molecules. Upon decompression, partially reversible transition is observed. Our work uncovers the non-trivial high-pressure structural evolution in one of the simplest and long-studied molecular systems.