Copper thiophosphate ferroelectrics

CuInP₂S₆; relatives CuInP₂Se₆, CuCrP₂S₆, AgCrP₂S₆

Also called CuInP₂S₆, CIPS, copper indium thiophosphate, CuInP₂Se₆, CuCrP₂S₆

van der Waals crystal insulator

The layered crystal that made room-temperature ferroelectricity a van der Waals property. Below about 315 K the copper ions in CuInP₂S₆ settle off-centre and give each layer an electric polarisation pointing out of the plane, which an electric field can flip – so a flake a few nanometres thick can store a bit, steepen a transistor’s switching or act as a memristor. The same copper ions can also wander through the crystal, making it both a ferroelectric and an ionic conductor: a combination devices can exploit, and one that complicates every measurement.

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: ~2.7–2.9 eV; a wide-gap insulator that nevertheless conducts copper ions

In colour: this family, 2.7–2.9 eV. In grey: the other 39 families on the catalogue’s chart, each at the middle of its range.
Thickness: ~0.65 nm per layer
Structure: The MPS₃ framework with the metal sites shared by Cu⁺ and In³⁺: each layer is a sulfur sandwich whose octahedral holes hold copper, indium and P–P pairs, stacked by van der Waals forces (monoclinic, Cc). Below the transition copper shifts away from the layer centre, and indium slightly the other way
Stability: Stable in air for exfoliation and device work. The practical problem is copper migration under strong or long-applied fields, which can shift the polarisation loop and leave copper-rich or copper-poor regions behind.
First made: Ferroelectric transition reported in 1994 (Simon and colleagues); ferrielectric ordering resolved in 1997 (Maisonneuve and colleagues); room-temperature switching in ~4 nm flakes in 2016 (Liu and colleagues)

Compare with other materials

Key properties

Ferrielectric below a Curie temperature of ~315 K: the copper and indium sublattices displace in opposite directions by different amounts, so their dipoles only partly cancel
An order–disorder transition: above the Curie temperature copper hops between sites, below it the ions settle off-centre
Switchable out-of-plane polarisation at room temperature in flakes down to ~4 nm
Giant negative longitudinal piezoelectricity – the opposite sign to almost every other piezoelectric, the best-known other example being the polymer PVDF
A quadruple potential well: copper can also sit in the van der Waals gap, giving a second, larger polarisation state
Mobile Cu⁺ ions: a biased probe tip can pull copper out of the lattice and let it return reversibly
CuCrP₂S₆, the magnetic relative, orders antiferromagnetically below ≈ 30 K with ferromagnetic coupling inside each layer; a field of 6–8 T suppresses that order, and broad anomalies between 140 and 200 K mark the antiferroelectric ordering of its copper, which at room temperature already sits off-centre, disordered between positions

How it is made

Bulk crystals by chemical vapour transport or solid-state reaction of the elements in sealed quartz ampoules
CuCrP₂S₆ and AgCrP₂S₆ as large crystals by optimised chemical vapour transport; CuCrP₂S₆ exfoliates down to four layers (2.6 nm)
Mechanical exfoliation to flakes a few nanometres thick; thin-flake ferroelectricity is usually probed by piezoresponse force microscopy
Van der Waals stacking with graphene, MoS₂ or silicon to build diodes, transistors and memristors

Uses, and how close they are

Non-volatile ferroelectric memory: ferroelectric diodes and transistorslab
Negative-capacitance transistors switching below the 60 mV per decade limitlab
Self-rectifying memristors for neuromorphic computinglab
Bulk photovoltaic photodetectors and flexible piezoelectric deviceslab

Readiness runs lab → prototype → pilot → deployed.

Open problems

How thin can CuInP₂S₆ be and stay ferroelectric? Early probe measurements lost polarisation below ~50 nm, later devices switched at ~4 nm – how much do electrodes, screening and mobile copper decide?
In a given switching loop, how much of the signal is ferroelectric polarisation and how much is copper-ion motion?
Can the high-polarisation state, with copper in the van der Waals gap, be stabilised and used at room temperature?
Can the transition be raised far enough above room temperature for memory that must survive chip operating temperatures?

Going deeper

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

For theoreticians · your lens

A clean example of a second-order Jahn–Teller ferroelectric: d¹⁰ Cu⁺ gains energy by moving off-centre in its sulfur cage, and the layered lattice lets that instability order cooperatively. Along the stacking axis the copper potential has four minima – two within the layer and two reaching into the van der Waals gap – so interlayer coupling through copper in the gap matters as much as the single-layer picture. The large displacive instability of copper in a layered lattice also explains its negative piezoelectricity. CuCrP₂S₆ adds magnetism: calculations predict ferromagnetic layers with an antipolar copper arrangement that an electric field can drive into a polar state.

For experimentalists · your lens

A piezoresponse hysteresis loop alone does not prove ferroelectricity here: electrostatic tip effects and copper migration give similar loops. Check that written domains persist and can be imaged without bias, compare with second-harmonic generation or macroscopic polarisation loops, and keep tip voltages and dwell times low, because a biased tip can extract copper and damage the surface. Always report the temperature: with the Curie temperature at ~315 K, a warm laboratory is already close enough to the transition to reduce the polarisation.

For engineers · your lens

One of the few ferroelectrics that stacks onto 2D semiconductors without dangling bonds. It has already made lab-scale negative-capacitance MoS₂ transistors with a minimum subthreshold swing of 28 mV per decade, as well as ferroelectric diodes and memristors. The obstacles are a Curie temperature only ~20 K above room temperature, copper migration that threatens retention and endurance, and the absence of wafer-scale growth. Hafnium-oxide ferroelectrics, already compatible with silicon manufacturing, set the bar it has to clear.

In the research tracks

Growth methods

Chemical vapour transport (CVT)

Readiness assessments

2D memristors and neuromorphic devices · TRL 3

Recent news

The newest items tagged CuInP₂S₆, CuCrP₂S₆, from the news feed updated 5 Oct 2026.

Preprintnot yet peer reviewed 1 Oct 2026 arXiv

Thickness-Controlled Ferroelectric-to-Ionic Transport in CuCrP2S6 at Room Temperature

Ambient-temperature coexistence of ferroelectricity, photoconductivity, and ionic transport in ultra-thin van der Waals (vdW) compound CuCrP2S6 (CCPS) offers significant potential for multifunctional electronic, photonic, and neuromorphic applications. However, ferroelectricity and ionic transport can become…

ExperimentCuInP₂S₆, CuCrP₂S₆

Journal 1 Oct 2026 Advanced Materials

Nonvolatile Ferroelectric Control of Diffusive Spin Transport at Room Temperature in Graphene/CuInP2S6 Heterostructures

ExperimentGraphene CuInP₂S₆, CuCrP₂S₆

Journal 24 Sep 2026 2D Materials (IOP)

Band edge shifts with polarisation switching in a 2D ferroelectric heterostructure

There is rising interest in two-dimensional (2D) ferroelectrics, particularly as functional components in 2D heterostructures using their reconfigurable polarisation to create low-power memory devices. Understanding and optimising their application will require investigations that can unravel, at the nanoscale, the…

ExperimentEngineeringCuInP₂S₆, CuCrP₂S₆ WSe₂

Preprintnot yet peer reviewed 31 Aug 2026 arXiv

Switchable Magnetoelectric Transport in Graphene via a Van der Waals Multiferroic

Electric and magnetic control of transport properties at atomic interfaces is central to the development of next generation electronics and spintronics. Van der Waals multiferroics materials that simultaneously host dielectric and magnetic orders down to the monolayer limit offer a promising platform for such…

TheoryGraphene CuInP₂S₆, CuCrP₂S₆

Preprintnot yet peer reviewed 23 Aug 2026 arXiv

Flexo-Strain Engineering of Phonons and Ferrons in Thin Films of Van der Waals Ferrielectrics

The influence of the flexoelectric coupling on the fluctuations of electric polarization and elastic strains can lead to the principal changes of the dispersion law of soft optical and acoustic phonons and ferrons in a bulk van der Waals ferrielectric. Since the size, gradient and strain effects determine phase…

TheoryCuInP₂S₆, CuCrP₂S₆

All 16 items tagged CuInP₂S₆, CuCrP₂S₆ in the news feed · RSS feed for CuInP₂S₆, CuCrP₂S₆

Key references

Paraelectric–ferroelectric transition in the lamellar thiophosphate CuInP₂S₆Simon et al. · Chemistry of Materials 6, 1575 (1994)cited by 159doi:10.1021/cm00045a016
Ferrielectric ordering in lamellar CuInP₂S₆Maisonneuve et al. · Physical Review B 56, 10860 (1997)cited by 307doi:10.1103/PhysRevB.56.10860
CuInP₂S₆ room temperature layered ferroelectricBelianinov et al. · Nano Letters 15, 3808 (2015)cited by 536doi:10.1021/acs.nanolett.5b00491
Room-temperature ferroelectricity in CuInP₂S₆ ultrathin flakesLiu et al. · Nature Communications 7, 12357 (2016)cited by 1,256doi:10.1038/ncomms12357
Two-dimensional ferromagnetism and driven ferroelectricity in van der Waals CuCrP₂S₆Lai et al. · Nanoscale 11, 5163 (2019)cited by 189doi:10.1039/C9NR00738E
Crystal growth, exfoliation, and magnetic properties of quaternary quasi-two-dimensional CuCrP₂S₆Selter et al. · Physical Review Materials 7, 033402 (2023)cited by 35doi:10.1103/PhysRevMaterials.7.033402
Crystal growth of the quasi-2D quarternary compound AgCrP₂S₆ by chemical vapor transportSelter et al. · Crystals 11, 500 (2021)cited by 18doi:10.3390/cryst11050500
Origin of giant negative piezoelectricity in a layered van der Waals ferroelectricYou et al. · Science Advances 5, eaav3780 (2019)cited by 302doi:10.1126/sciadv.aav3780
Van der Waals negative capacitance transistorsWang et al. · Nature Communications 10, 3037 (2019)cited by 271doi:10.1038/s41467-019-10738-4
Tunable quadruple-well ferroelectric van der Waals crystalsBrehm et al. · Nature Materials 19, 43 (2019)cited by 302doi:10.1038/s41563-019-0532-z
Enhanced bulk photovoltaic effect in two-dimensional ferroelectric CuInP₂S₆Li et al. · Nature Communications 12, 5896 (2021)cited by 301doi:10.1038/s41467-021-26200-3
Manipulation of current rectification in van der Waals ferroionic CuInP₂S₆Jiang et al. · Nature Communications 13, 574 (2022)cited by 168doi:10.1038/s41467-022-28235-6