Electric field control of spin orbit coupling and circular photogalvanic effect in a true ferrielectric crystal

Materials possessing long-range ordering of magnetic spins or electric dipoles have been the focus of condensed matter research. Among them, ferri-systems with two sublattices of unequal/non-collinear spins or electric dipoles are expected to combine the properties of ferro- and antiferro-systems, but lack experimental observations in single-phase materials. This is particularly true for the ferrielectric system, since the electric dipoles can usually be redefined to incorporate the two sublattices into one, making it indistinguishable from ferroelectric. This raises doubts about whether or not ferrielectricity can be considered as an independent ferroic order. Here we report the observation of true ferrielectric behaviors in a hybrid single crystal (MV)[SbBr₅] (MV²⁺ = N,N'-dimethyl-4,4'-bipyridinium or methyl viologen), where the two electric dipole sublattices switch asynchronously, and thus cannot be reduced to ferroelectric by redefining the unit cell. Furthermore, the complex dipole configuration imparts circularly polarized light sensitivity to the system. An electric field can modulate the non-collinear dipole sublattices and even induce a transition from ferrielectric to ferroelectric state, thereby tuning the helicity-dependent photocurrent. This study opens a new paradigm for the study of true irreducible ferrielectricity (a new class of polar systems) and provides an effective approach to the electric field control of spin-orbit coupling and circular photogalvanic effect.