Scientists capture a new polar orders: True ferrielectric material discovered

For decades, condensed-matter physics has pursued a long-hypothesized polar state known as ferrielectricity—predicted to bridge the properties of ferroelectrics (FE) and antiferroelectrics (AFE), yet remaining experimentally elusive in any single-phase crystal.

A collaboration led by Professor Junling Wang (City University of Hong Kong) and Professor Shuai Dong (Southeast University)  has now brought this picture into clear view. Writing in National Science Review, the team reports the first unambiguous evidence of irreducible ferrielectricity in a hybrid crystal, (MV)[SbBr₅] (MV²⁺ = methylviologen), establishing ferrielectricity as a functionally distinct polar order rather than a mere re-labeling of ferroelectricity.

What makes ferrielectricity special?

Ferrielectricity was originally formulated by analogy to ferrimagnetism: microscopically, multiple dipoles coexist within a unit cell—either antiparallel but unequal in magnitude, or oriented at a finite angle—thereby producing a net polarization; macroscopically, the net polarization must be switchable by an electric field. However, unlike magnetic moments, the selection of an electric dipole is relative and non-unique, so particular structural analyses may obscure the underlying physics—many so-called ferrielectrics can be reduced to ordinary ferroelectrics by merging multiple dipoles into a single one.

Hence the central question: if a material switches exactly like a ferroelectric, should it be classified as a new ferroic order merely because its microscopic description is complex? More fundamentally: does a truly “irreducible ferrielectric” exist whose switching behavior is intrinsically different from ferroelectrics—that is, a ferrielectric worthy of separate classification by virtue of distinctive functional functionality?

(MV)[SbBr₅]: multi-dipole system

The team designed (MV)[SbBr₅] as an ideal platform to host this state. Its polarization arises from two sources: Br displacements within the [SbBr₅]^2- framework and the concerted motion of the MV cation. These organic–inorganic contributions form non-collinear, anti-parallel dipolar sublattices of unequal magnitude, producing a small net polarization, as confirmed by structural analysis and second-harmonic generation (SHG).

Three hallmarks set this ferrielectric state apart.

First, (MV)[SbBr₅] shows a switchable net polarization, satisfying the macroscopic switchability criterion highlighted in earlier definitions.

Second, it displays asynchronous switching: temperature- and frequency-dependent electrical measurements reveal multiple current peaks during polarization reversal, a fingerprint that the organic and inorganic sublattices respond on different timescales. Microscopically, the MV²⁺ sublattice switches via lower-energy reorientations/translations, while the [SbBr₅]^2- framework requires higher-barrier lattice displacements—naturally producing distinct relaxation times and activation energies.

Third, the crystal undergoes a field-driven polar-to-polar transition (FiE→FE).

Together, these observations map a stepwise pathway—FiE⁽⁻⁾ → FiE⁽⁺⁾ → FE1 → FE2—that cannot be compressed into a single ferroelectric or antiferroelectric order parameter. At low fields, the net polarization can be reversed without switching individual dipoles; at intermediate fields the organic sublattice first undergoes an AFE→FE transition; and at higher fields the inorganic sublattice follows, forming an inherently asynchronous sequence.

Computation meets experiment.

First-principles calculations reproduce the hierarchy of switching barriers (organic < inorganic), explaining the observed multi-peak I–E/C–E profiles and the sequential nature of the transitions. The theoretical and experimental narratives align: ferrielectricity here is not a semantic construct—it is a dynamical reality with distinct, testable signatures.

Function follows order.

Beyond the “what,” the study highlights the “so what.” By driving the FiE→FE transition with an electric field, the team reconfigures crystal structure and tunes spin–orbit coupling (SOC). This directly modulates the circular photogalvanic effect (CPGE)—the material’s left-/right-handed light response—demonstrating voltage control over a SOC. Such coupling among structure, spin, and light in a single-phase material points to electric-field control of spin, circularly polarized photodetectors, and reconfigurable chiral photonics.

Why it matters.

This work not only resolves a long-standing fundamental question but also establishes a new paradigm: we should define new states of matter by their functional behavior, not just their static structure. It provides a novel approach for the electric field control of spins and light response, with potential applications in spintronics, low-energy multi-state memory, and chiral optoelectronics.

The hybrid crystal (MV)[SbBr₅] thus serves as a foundational material platform, showcasing a new principle for simultaneously controlling charge, spin, and light degrees of freedom via electric fields.

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