Transition-metal oxyhalides

MOX (M = Ti, V, Cr, Fe) and MOX₂ (M = Nb, Mo); X = Cl, Br, I

Also called NbOI₂, NbOCl₂, CrOCl, TiOCl, FeOCl, VOCl, MoOCl₂

van der Waals crystal depends on form

The niobium oxide dihalides are ferroelectric within each layer and double the frequency of light far more efficiently than almost any other thin crystal; a flake of NbOCl₂ a few tens of nanometres thick has served as a source of entangled photon pairs. The chromium, iron, vanadium and titanium oxychlorides are layered magnets – TiOCl is a textbook case of spins pairing up along chains – and CrOCl reshapes the electronic states of graphene laid on top of it. MoOCl₂ is a metal that conducts so differently along its two in-plane directions that it guides visible light as hyperbolic plasmons.

Cell: Rectangular, a = 3.78 Å, b = 3.30 Å
Atoms per cell: 6
Fe–O bonds: 1.96 and 2.10 Å
Fe–Cl bond: 2.37 Å
Height: 5.23 Å between the outer atom centres

A buckled double layer of iron and oxygen sandwiched between two sheets of chlorine, with each iron in a distorted octahedron of four oxygens and two chlorines. The chlorine sheets face each other across the van der Waals gap. The cell is rectangular rather than hexagonal, so the two in-plane directions differ – the origin of the anisotropy this family shows. One layer of bulk FeOCl (Lind, Acta Crystallographica B 26, 1058, 1970; COD 2106381): a = 3.78 Å, b = 3.30 Å, chlorine sheets 2.61 Å above and below the centre of the layer.

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: depends on form
Band gap: NbOX₂: indirect gaps of about 1–2 eV in calculations, depending on the method; MOX compounds: magnetic Mott insulators; MoOCl₂: metallic
Thickness: ~0.65–0.8 nm per layer
Structure: Two main structure types. In MOX (FeOCl type, orthorhombic Pmmn) each layer is a buckled metal–oxygen double layer sandwiched between halogen sheets. In NbOX₂ (monoclinic C2) niobium sits in octahedra linked into sheets; it is displaced off-centre along one in-plane axis, which makes the layer ferroelectric, and pairs up along the other
Space group: FeOCl: bulk Pmmn, monolayer Pmmn
CrOCl: bulk Pmmn, monolayer Pmmn
NbOI₂: bulk C2, monolayer Pmm2
Bulk: the Materials Project crystal linked below. Monolayer: the 2DMatPedia layer, as a periodic slab
Stability: Varies across the family and has been studied little. CrOCl is commonly handled in air as a device substrate, while compounds of easily oxidised ions such as Ti³⁺ in TiOCl call for inert handling and encapsulation.
First made: Bulk oxyhalides studied for decades; spin-Peierls transition in TiOCl reported in 2003 (Seidel and colleagues); giant second-harmonic generation in NbOI₂ in 2022 (Abdelwahab and colleagues); entangled photon pairs from NbOCl₂ in 2023 (Guo and colleagues)
Materials Project: FeOCl
CrOCl
NbOI₂
Computed crystal structure and properties
2DMatPedia: FeOCl
CrOCl
NbOI₂
Computed monolayer, exfoliated from a bulk crystal of the same compound

Compare with other materials

Key properties

NbOI₂ and NbOCl₂ are in-plane ferroelectrics: niobium is displaced by ~0.14 Å along the polar axis and paired along the other axis (a Peierls distortion)
Giant second-harmonic generation in NbOI₂, with absolute conversion efficiencies above 0.2%; because neighbouring layers barely interact, the response grows with thickness instead of cancelling
Entangled photon pairs by spontaneous parametric down-conversion from NbOCl₂ flakes about 46 nm thick
CrOCl orders antiferromagnetically at ~13.5 K in a first-order transition that also distorts the lattice; FeOCl and VOCl order near 80 K
TiOCl: spin-Peierls physics along titanium chains, with transitions at ~91 K (incommensurate) and ~66 K (commensurate, dimerised)
MoOCl₂: in-plane hyperbolic plasmon polaritons at visible and near-infrared frequencies

How it is made

Chemical vapour transport of bulk crystals such as NbOI₂, NbOCl₂, CrOCl and MoOCl₂
Mechanical exfoliation to thin flakes; exfoliated CrOCl also serves as a substrate for graphene devices
Chemical vapour deposition of monolayer CrOCl films

Uses, and how close they are

Ultrathin nonlinear-optical and entangled-photon sourceslab
Substrates that tune the electronic states of graphenelab
Hyperbolic nanophotonics at visible frequencieslab

Readiness runs lab → prototype → pilot → deployed.

Open problems

Can the ferroelectric polarisation of NbOX₂ be switched electrically in thin flakes, and how does switching change the nonlinear optical response?
What happens at the graphene/CrOCl interface: charge transfer into CrOCl states, correlations within graphene, or both?
What makes plasmons in MoOCl₂ low-loss even though its electron transport looks like that of a bad metal, and can the effect be found in other crystals?

Going deeper

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

For theoreticians · your lens

NbOX₂ combines a polar distortion along one axis with Peierls dimerisation along the other, a clean case of ferroelectricity coupled to bond order; calculated indirect gaps range from about 1 to over 2 eV depending on the functional. Weak interlayer coupling lets monolayer-like excitons survive in bulk, which is why the second-order response scales with thickness. The MOX compounds are low-dimensional Mott magnets – TiOCl as S = 1/2 chains with a spin-Peierls instability – and MoOCl₂ calls for a strongly anisotropic, correlated description of its free-carrier response.

For experimentalists · your lens

Find the polar axis before any optical measurement: second-harmonic signals and Raman modes of NbOX₂ depend strongly on polarisation relative to it. For CrOCl, transport and heat conduction change sharply at the 13.5 K transition, and graphene on CrOCl is best compared with graphene on hBN in the same cooldown. Near-field optical microscopy (s-SNOM) is the standard way to image plasmons in MoOCl₂.

For engineers · your lens

The niobium oxide dihalides are the most application-ready members: their nonlinear responses are very strong for their thickness, which makes them candidates for on-chip frequency conversion and photon-pair sources, although wafer-scale growth does not yet exist. The magnetic oxychlorides remain research materials.

In the research tracks

Theory frontiers

Sliding and in-plane ferroelectricity

Growth methods

Chemical vapour transport (CVT)

Recent news

The newest items tagged NbOI₂, CrOCl, TiOCl, from the news feed updated 5 Oct 2026.

Preprintnot yet peer reviewed 28 Sep 2026 arXiv

Tensor-Engineered Van der Waals NbOCl2 Resonant Metasurface for Polarization-entangled Bell State Generation

Polarization-entangled photon pairs are essential resources for quantum information technologies, yet realizing compact sources with intrinsically controllable entanglement remains challenging. Van der Waals (vdW) nonlinear materials such as NbOCl2 provide atomically thin platforms for quantum light generation, yet…

ExperimentNbOI₂, CrOCl, TiOCl

Preprintnot yet peer reviewed 10 Sep 2026 arXiv

Hilbert-space selected switch of helical edges in an artificial quantum Hall insulator

Quantum Hall effects (QHE) host one-dimensional topologically-protected edge channels, which can serve as an essential ingredient in exotic quantum electronic systems. Yet the manual reconstruction of Landau-level topology, by electrostatic confinement or symmetry breaking, remains experimentally challenging. Here, we…

TheoryExperimentTwisted graphene NbOI₂, CrOCl, TiOCl Graphene

Preprintnot yet peer reviewed 10 Sep 2026 arXiv

A substrate booster for P-type 2D ferromagnetic semiconductor

Spin transistors with its both charge and spin properties tuned via electrostatic gating are believed capable for widespread use, which however have proven challenging due to the extreme rareness of their physical base -- magnetic semiconductors. The latter are limited within very few systems including diluted magnetic…

EngineeringExperimentTheoryCr₂Ge₂Te₆ NbOI₂, CrOCl, TiOCl

Preprintnot yet peer reviewed 7 Jul 2026 arXiv

Correlated Insulating States in Twisted Double Bilayer Graphene Enhanced by Interfacial Effect on CrOCl

Interaction between different two dimensional materials can give rise to many exotic physical phenomena which are rarely observed in intrinsic materials. Recently, several theoretical and experimental works have revealed that magnetic proximity effect between pristine graphene and magnetic substrates can lead to the…

ExperimentTheoryNbOI₂, CrOCl, TiOCl Graphene

Preprintnot yet peer reviewed 13 May 2026 arXiv

Giant optical spin-orbit interactions in ferroelectric van der Waals waveguides

Optical spin-orbit interactions (SOI) link photonic spin to momentum, offering a route toward on-chip polarization control and beam steering. Nevertheless, achieving sufficient optical SOI and nonlinearities on sub-micrometer scales - a prerequisite for dense photonic integration - remains an outstanding challenge.…

TheoryExperimentNbOI₂, CrOCl, TiOCl

All 18 items tagged NbOI₂, CrOCl, TiOCl in the news feed · RSS feed for NbOI₂, CrOCl, TiOCl

Key references

S=1/2 chains and spin-Peierls transition in TiOClSeidel et al. · Physical Review B 67, 020405 (2003)cited by 161doi:10.1103/PhysRevB.67.020405
Giant second-harmonic generation in ferroelectric NbOI₂Abdelwahab et al. · Nature Photonics 16, 644 (2022)cited by 241doi:10.1038/s41566-022-01021-y
Quantum Hall phase in graphene engineered by interfacial charge couplingWang et al. · Nature Nanotechnology 17, 1272 (2022)cited by 67doi:10.1038/s41565-022-01248-4
Ultrathin quantum light source with van der Waals NbOCl₂ crystalGuo et al. · Nature 613, 53 (2023)cited by 327doi:10.1038/s41586-022-05393-7
Visible-frequency hyperbolic plasmon polaritons in a natural van der Waals crystalVenturi et al. · Nature Communications 15, 9727 (2024)cited by 55doi:10.1038/s41467-024-53988-7
Good plasmons in a bad metalRuta et al. · Science 387, 786 (2025)cited by 45doi:10.1126/science.adr5926