Breathing-kagome niobium halides

Nb₃X₈ (X = Cl, Br, I)

Also called Nb₃Cl₈, Nb₃Br₈, Nb₃I₈, niobium halide cluster compounds

van der Waals crystal insulator

Crystals in which the basic magnetic unit is not a single atom but a triangle of three niobium atoms. Each triangle shares one unpaired electron, and the triangles form a kagome-like lattice whose electrons sit in unusually flat bands – ideal conditions for strong electron correlations. Nb₃Cl₈ is a Mott insulator and possible spin liquid that abruptly loses its magnetism below about 90 K, when its layers restack. Nb₃Br₈ became famous in 2022 as the thin barrier in the first reported Josephson junction that superconducts for current in one direction only, without any magnetic field.

Cell: Hexagonal, a = 7.08 Å
Atoms per cell: 11
Nb–Br bonds: 2.55–2.79 Å
Nb–Nb bond: 2.88 Å
Height: 3.72 Å between the outer atom centres

Niobium atoms group into triangles of three, 2.88 Å apart inside a triangle and much further apart between triangles – a kagome lattice squeezed, or breathed, into trimers. Each trimer shares one unpaired electron, so the layer behaves as a triangular lattice of spin-½ moments rather than of individual atoms. The two faces of the layer carry bromine in different arrangements, so a single layer has no centre of inversion. One layer of bulk Nb₃Br₈ (Simon and von Schnering, Journal of the Less-Common Metals 11, 31, 1966; COD 1539108): a = 7.08 Å, Nb–Nb 2.88 Å within a trimer.

Data sheet

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Kind of sheet: van der Waals crystal
Peels off a layered crystal, one sheet at a time
Behaves as: insulator
Band gap: Insulating, with narrow, flat bands near the Fermi level split by electron correlations; Nb₃Cl₈ is described as a single-band Mott insulator
Thickness: Three atomic planes, halogen–niobium–halogen, per layer
Structure: Each layer is a plane of Nb₃ triangles – trimers linked into a ‘breathing’ kagome lattice of alternating small and large triangles – between halogen planes above and below; the layers stack by van der Waals forces, and in Nb₃Cl₈ the stacking changes at low temperature
Space group: Nb₃Cl₈: bulk P3m1, monolayer P3m1
Bulk: the Materials Project crystal linked below. Monolayer: the 2DMatPedia layer, as a periodic slab
Stability: Comparatively robust: ultrathin Nb₃I₈ flakes keep their Raman spectra at room temperature, and exfoliation works under ordinary conditions. Devices that include NbSe₂ still need inert handling.
First made: Compounds known for decades; the 90 K singlet transition of Nb₃Cl₈ characterised in 2017 (Haraguchi and colleagues; Sheckelton and colleagues); field-free Josephson diode with Nb₃Br₈ in 2022 (Wu and colleagues)
Materials Project: Nb₃Cl₈
Computed crystal structure and properties
2DMatPedia: Nb₃Cl₈
Computed monolayer, exfoliated from a bulk crystal of the same compound

Compare with other materials

Key properties

Each Nb₃ trimer carries one unpaired electron (spin 1/2), and the trimers form a frustrated triangular lattice of moments
Nb₃Cl₈ has a structural and magnetic transition near 90 K in which the layer stacking changes and the moments pair into non-magnetic singlets
Flat bands seen in photoemission; Nb₃Cl₈ is described as a cluster, single-band Mott insulator, and its monolayer is discussed as a spin-liquid candidate
NbSe₂/Nb₃Br₈/NbSe₂ junctions act as field-free Josephson diodes, which has been linked to an out-of-plane polarisation of Nb₃Br₈ and its proposed obstructed-atomic-insulator character
The crystals exfoliate readily down to monolayers, and ultrathin Nb₃I₈ shows stable Raman spectra over time at room temperature

How it is made

Bulk crystals by reaction of niobium with niobium halides in sealed ampoules, and by chemical vapour transport
Mechanical exfoliation to few-layer and monolayer flakes
Stacking with NbSe₂ electrodes into van der Waals Josephson junctions, assembled in inert gas because NbSe₂ oxidises

Uses, and how close they are

Superconducting diodes for low-dissipation cryogenic electronicslab
Model systems for flat-band correlations and cluster magnetismlab

Readiness runs lab → prototype → pilot → deployed.

Open problems

What drives the 90 K transition of Nb₃Cl₈ – charge disproportionation between layers, dimerisation of layers, or singlet formation – and does it survive in a monolayer?
Is Nb₃Br₈ an obstructed atomic insulator, and is that what makes its Josephson junctions non-reciprocal?
Do monolayers host a quantum spin liquid on the frustrated lattice of trimers?

Going deeper

Short notes for specialists. Choose a lens in the header and yours comes first.

For theoreticians · your lens

The minimal model has one orbital per Nb₃ trimer on a triangular lattice at half filling; a non-magnetic DFT calculation therefore gives a metal with a half-filled flat band, and the Hubbard U decides the insulating state, which DMFT and cluster approaches reproduce. Stacking matters: dimerisation of neighbouring layers can turn the low-temperature phase into a singlet insulator, so bulk and monolayer ground states may differ. Obstructed-atomic-insulator analyses place the Wannier charge centres at empty sites, producing a polarisation at boundaries.

For experimentalists · your lens

Follow the Nb₃Cl₈ transition with magnetic susceptibility, Raman and X-ray diffraction, and repeat it as a function of thickness. ARPES resolves the flat bands, but the small bandwidth makes gap assignments delicate. For Josephson-diode measurements, sweep both current directions with careful field compensation, because tiny residual fields also produce non-reciprocity.

For engineers · your lens

A superconducting diode that needs no magnetic field is attractive for cryogenic electronics, but Nb₃Br₈ junctions are hand-stacked devices working at a few kelvin. The family is a research platform rather than an engineering material.

In the research tracks

Theory frontiers

Growth methods

Chemical vapour transport (CVT)

Recent news

The newest items tagged Nb₃Cl₈, Nb₃Br₈, Nb₃I₈, from the news feed updated 5 Oct 2026.

Journal 21 Sep 2026 Physical Review B

Anderson lattice in incommensurate Nb₃Cl₈/graphene van der Waals heterostructures

The periodic Anderson model, traditionally realized in rare-earth compounds with limited tunability, have hindered systematic exploration of correlated quantum phenomena. Here, we introduce a strategy for realizing and engineering this model in incommensurate van der Waals heterostructures by coupli… [Phys. Rev. B 114…

TheoryGraphene Nb₃Cl₈, Nb₃Br₈, Nb₃I₈

Preprintnot yet peer reviewed 19 Aug 2026 arXiv

Hysteresis without coexistence: disorder-rounded first-order transitions in a van der Waals magnet

Quenched disorder can profoundly modify phase transitions. In low-dimensional systems, theory predicts that even weak quenched disorder can round the thermodynamic discontinuities associated with a first-order phase transition. Here, we employ time-domain terahertz spectroscopy to investigate the quasi-two-dimensional…

TheoryExperimentNb₃Cl₈, Nb₃Br₈, Nb₃I₈

Preprintnot yet peer reviewed 1 Jul 2026 arXiv

Geometry-Driven Magnetoelectric Coupling in Two-Dimensional Compensated Ferrimagnets

The magnetoelectric coupling in compensated magnets enables stray-field-free manipulation of spin-splitting, holding great promise for spintronics, but inherently hindered by the symmetry mismatch between spatial-inversion-broken ferroelectricity and time-reversal-broken spin states. Here, based on a symmetry-decoupled…

TheoryNb₃Cl₈, Nb₃Br₈, Nb₃I₈

Preprintnot yet peer reviewed 9 Jun 2026 arXiv

Layer-parity-dependent interfacial coupling in Nb₃Cl₈/graphene van der Waals heterostructures

Strongly correlated two-dimensional systems provide compelling platforms for investigating exotic quantum phenomena. Niobium chloride (Nb₃Cl₈), a single-band Mott insulator, exhibits a remarkable out-of-plane polarization in its topmost layer that oscillates with layer parity, manifesting as an odd-even effect. Using…

ExperimentTheoryGraphene Nb₃Cl₈, Nb₃Br₈, Nb₃I₈

Preprintnot yet peer reviewed 24 Mar 2026 arXiv

Dimensionality-Dependent Exciton Dispersion in a Single-Band Mott Insulator

Excitonic band structure is critical for investigating exciton dynamics. Theoretically, quantum effects from exchange scattering between electron-hole pairs significantly modulate exciton dispersion. Here, we report the direct observation of dimensionality-dependent exciton dispersion in a single-band Mott insulator…