Fig. 3: Copper migration under pressure.

Pressure-dependent evolution of unit cell volume (a) and the occupancies of interlayer sites (b) from 0 to 2.00 GPa. The black line in a is the fitting of the pressure-cell volume curve and the green dots are the contribution of cell volume reduction to polarization increase, the error bars were estimated from the APEX3 software. c Highly smeared distribution of Cu atoms at ambient conditions. The size of Cu atoms represents the occupancy of different sites. d Below 0.26 GPa, the Cu atoms move toward interlayer sites according to the ferroelectric hysteresis loops measurements and Raman experimental results. e The atomic sites occupation of Cu is close to the ground state at 2.00 GPa. f The Cu atoms are completely located in the center of the S octahedron at 2.68 GPa, and ferroelectricity completely disappears.