The spin-induced ferroelectricity in quasi-1D spin chain system is little
known, which could be fundamentally different from those in three-dimensional
(3D) system. Here, we report the ferroelectricity driven by a tilted screw spin
order and its exotic dynamic in the spin-chain compound Ba9Fe3Se15. It is found
that the spin-induced polarization has already occurred and exhibits
magnetoelectric coupling behavior far above the long-range spin order (LRSO) at
TN = 14 K. The polarized entities grow and their dynamic responses slow down
gradually with decreasing temperature and permeate the whole lattice to form 3D
ferroelectricity at TN. Our results reveal that the short-range spin orders
(SRSOs) in the decoupled chains play a key role for the exotic dynamic in this
dimension reduced system. Ba9Fe3Se15 is the only example so far which exhibits
electric polarization above LRSO temperature because of the formation of SRSOs