For decades, the pursuit of a novel polar phase known as ferrielectricity has been a tantalizing enigma in condensed-matter physics. Ferrielectricity, theoretically posited as an intermediate state bridging ferroelectric and antiferroelectric orders, had remained conspicuously absent in any single-phase crystalline solid—until now. Through groundbreaking work spearheaded by Professors Junling Wang of City University of Hong Kong and Shuai Dong of Southeast University, a definitive breakthrough has been achieved. Their interdisciplinary team has experimentally confirmed the existence of an irreducible ferrielectric state in a hybrid organic-inorganic crystal, (MV)[SbBr₅] (methylviologen antimonium pentabromide), decisively elevating ferrielectricity from theoretical postulate to functional reality.
Ferrielectricity conceptually mirrors ferrimagnetism in magnetic systems, characterized by coexisting sublattices of dipoles which are antiparallel but unequal in magnitude or set at finite angles to one another, giving rise to a nonzero net polarization. However, electric dipoles differ fundamentally from magnetic moments, rendering the microscopic identification of ferrielectric order considerably more challenging. The subtlety arises because electric dipole selection is relative and can be transformed under various structural interpretations, often leading to an oversimplification wherein purported ferrielectrics are reclassified as ordinary ferroelectrics with a single effective dipole. This begs the profound question: can a material demonstrate polarization switching distinctly inconsistent with ferroelectric switching, thus meriting ferrielectric classification as a standalone phase?
(MV)[SbBr₅] serves as an ideal material platform to rigorously interrogate this question. The crystal’s spontaneous polarization emerges from two distinct sources: displacements of bromide ions within the inorganic [SbBr₅]²⁻ lattice and collective motions of the organic methylviologen (MV²⁺) cations. Intriguingly, these two sublattices form non-collinear and nearly antiparallel dipolar arrays with different magnitudes, resulting in a subtle net polarization. Structural refinements combined with second harmonic generation experiments substantiate this picture of coexistence of multiple, competing dipolar orders within a single phase.
Three critical experimental hallmarks distinguish the discovered ferrielectric state in (MV)[SbBr₅]. First and foremost, the material displays macroscopic switchability of its net polarization, fulfilling the essential criterion for any ferroic order. Unlike conventional ferroelectrics, its polarization reversal cannot be attributed to uniform dipole flipping. Instead, asynchronous switching processes emerge from the differing activation energies and relaxation dynamics of the organic and inorganic sublattices. Electrical characterization reveals multiple distinct current peaks upon polarization reversal, evidence that the MV²⁺ and SbBr₅²⁻ dipoles reorient on separate timescales, reflecting low-energy molecular translations versus higher-barrier lattice realignments respectively.
Secondly, the asynchronous nature of switching manifests in a complex sequence of field-driven transitions. At low electric fields, the net polarization reverses without any individual dipole flipping, a phenomenon incompatible with simple ferroelectric behavior. As the field strength increases, the organic methylviologen sublattice first undergoes an antiferroelectric-to-ferroelectric transition, followed closely at higher fields by a similar transition within the inorganic framework. This multistep switching pathway—FiE⁽⁻⁾ → FiE⁽⁺⁾ → FE₁ → FE₂—cannot be encapsulated by a single ferroelectric or antiferroelectric order parameter, underscoring the intrinsic ferrielectric nature of this phase.
Third and perhaps most strikingly, the hybrid crystal exhibits an electric-field-induced polar-to-polar phase transition, specifically a ferrielectric-to-ferroelectric (FiE→FE) transformation. This reversible structural rearrangement triggered by an external field demonstrates tunability of polar order beyond static configurations, offering unprecedented opportunities for the dynamic control of material properties.
The research team employed advanced first-principles computational modeling to demystify the microscopic energetics underlying these phenomena. Density functional theory calculations quantified the hierarchy of dipole switching energy barriers—unveiling that organic MV cation reorientation involves significantly lower activation energies than inorganic lattice displacements. This explains the experimentally observed multi-peak current-voltage (I-E) and capacitance-voltage (C-E) profiles characteristic of sequential dipolar switching and establishes a robust theoretical foundation. The consonance between theory and experiment decisively validates ferrielectricity as more than a semantic construct, confirming its status as a dynamically distinct ferroic order.
The implications of this discovery extend far beyond fundamental classification. By manipulating the FiE→FE transition via an applied electric field, the researchers demonstrated active tuning of the crystal’s spin–orbit coupling (SOC). Remarkably, this led to voltage-modulated changes in the circular photogalvanic effect (CPGE), whereby the material exhibits controllable responses to left- and right-handed circularly polarized light. This coupling of spin, charge, and optical degrees of freedom within a single-phase hybrid crystal heralds a new paradigm for electric-field control of spintronic and optoelectronic functionalities.
This work bridges fundamental physics and device potential, marking a milestone in the design of low-energy multifunctional materials. By defining ferroic states through their dynamic functional behaviors rather than solely static structure, it establishes a transformative framework for identifying and engineering new states of matter. The validated electrically switchable multi-dipole order with asynchronous transitions opens avenues for multistate memory devices, spin-based logic gates, and tunable chiral photonic systems.
In summary, the (MV)[SbBr₅] hybrid crystal is a landmark material, embodying irreducible ferrielectricity as a bona fide ferroic phase with unique switching dynamics and vibrant coupling phenomena. The demonstration of its electric-field controllable SOC and photogalvanic response translates fundamental insights into tangible technological possibilities. This discovery ushers in a novel era where electrical manipulation of charge, spin, and light occurs in concert within a single crystalline matrix, promising transformative advances in quantum materials and functional device engineering.
By establishing ferrielectricity as a fundamentally new, functionally distinct polarization state, this research resolves a decades-old conceptual challenge and extends the horizons of ferroic science. Its fusion of organic and inorganic contributions to polarization, asynchronous dipolar dynamics, and controllable spin-optical effects embodies a new principle for material innovation. Moving forward, the unique multi-dipole architecture of (MV)[SbBr₅] will inspire intense exploration into complex ferroic orders and their integration into next-generation photonic, spintronic, and energy-efficient electronic devices.
Subject of Research: Discovery and characterization of irreducible ferrielectricity in a hybrid organic-inorganic crystal (MV)[SbBr₅]
Article Title: Scientists Capture a New Polar Orders: True Ferrielectric Material Discovered
Web References: http://dx.doi.org/10.1093/nsr/nwaf320
References: National Science Review, experimental and first-principles study led by Professors Junling Wang and Shuai Dong.
Image Credits: ©Science China Press
Keywords
Ferrielectricity, hybrid perovskite, polarization switching, multi-dipole system, spin–orbit coupling, circular photogalvanic effect, ferroelectricity, antiferroelectricity, methylviologen, SbBr₅ framework, ferroic orders, multistate memory, spintronics, chiral photonics
