Chalcohalide chains SbSI and BiSI

SbSI, SbSBr; the bismuth analogues BiSI, BiSeI

Also called SbSI, BiSI, BiSeI, antimony sulfoiodide

van der Waals crystal semiconductor

A ferroelectric that is also a semiconductor and a photoconductor, made of weakly bonded chains. SbSI was one of the first materials where light and polarisation were seen to be strongly coupled: its Curie point sits at 22 °C, right at room temperature, its dielectric constant climbs towards 50,000 near the transition, and its absorption edge shifts strongly with an applied field. The bismuth version BiSI is not ferroelectric but is a non-toxic, earth-abundant candidate solar absorber, which is why the family keeps returning as a lead-free alternative to halide perovskites.

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: semiconductor
Band gap: ~1.8–2.0 eV (SbSI); ~1.6 eV (BiSI)

In colour: this family, 1.6–2 eV. In grey: the other 39 families on the catalogue’s chart, each at the middle of its range.
Thickness: Chains are ~1 nm across; crystals grow as needles and are split into nanowires rather than sheets
Structure: Double chains of metal, chalcogen and halogen running along one axis, bonded covalently within the chain and by van der Waals forces to their neighbours (orthorhombic); in SbSI the chains lose their mirror symmetry below the Curie point, which is where the polarisation comes from
Space group: SbSI: bulk Pnma
BiSI: bulk Pnma
Bulk: the Materials Project crystal linked below
Stability: Stable in air at room temperature, but the ferroelectric state in SbSI is only just below it – ordinary warming crosses the transition, which matters for any device relying on the polarisation.
First made: Ferroelectricity in SbSI reported in 1962 (Fatuzzo and colleagues) and its structure refined in 1966 (Iwata and colleagues); BiSI and BiSeI proposed as photovoltaic absorbers from first principles in 2016 (Ganose and colleagues)
Materials Project: SbSI
BiSI
Computed crystal structure and properties

Compare with other materials

Key properties

SbSI is ferroelectric below 22 °C, with a spontaneous polarisation of ~25 μC cm⁻² and a coercive field of only ~100 V cm⁻¹
Its dielectric constant reaches ~50,000 at the Curie point – among the largest known
Photoconducting and strongly photoferroelectric: the absorption edge moves with applied field, tying optics to polarisation
Large piezoelectric response: nanowire composites report d₃₃ ≈ 650 pC N⁻¹ and an electromechanical coupling k₃₃ ≈ 0.9
BiSI and BiSeI are centrosymmetric – no ferroelectricity – but have band gaps and band structures suited to photovoltaics
Calculations attribute the poor efficiency of BiSI devices to band misalignment with standard contact layers rather than to the absorber itself

How it is made

Vapour transport and solution or sonochemical growth, both of which give needles and nanowires
Nanowires dispersed into polymer composites for piezoelectric devices
Thin films by solution or vapour routes for photovoltaic test structures

Uses, and how close they are

Piezoelectric nanogenerators and vibration sensorslab
Photodetectors and photoferroelectric deviceslab
Lead-free solar absorbers (BiSI, BiSeI)lab

Readiness runs lab → prototype → pilot → deployed.

Open problems

Can SbSI’s Curie point be pushed comfortably above room temperature by substitution or strain, without losing the response?
Are the poor BiSI solar efficiencies really a contact-alignment problem, and do the suggested device stacks fix them?
What do isolated chains do – can a single chain or a few-chain ribbon be measured, and does the polarisation survive?
How much of the reported piezoelectric response belongs to the material and how much to the composite it is measured in?

Going deeper

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

For theoreticians · your lens

A displacive ferroelectric in a quasi-one-dimensional lattice: the polar distortion runs along the chain, so interchain coupling – which is dispersion-bound – sets the ordering temperature, and calculations need van der Waals corrections to get it right. In the bismuth compounds, relativistic effects widen the conduction band as they do in lead halide perovskites, which is the origin of the interest in them as absorbers; hybrid functionals with spin–orbit coupling are the working standard, and defect calculations put the efficiency problem at the interfaces rather than in the bulk.

For experimentalists · your lens

Control the temperature carefully: with a transition at 22 °C, whether a sample is ferroelectric depends on the room. State the sample form – most SbSI numbers come from nanowires embedded in polymer, where the matrix, alignment and filling fraction all shape the measured piezoelectric response. For photoferroelectric measurements, separate the field-induced absorption shift from heating, and for BiSI photovoltaics measure band positions rather than only efficiency, because the reported losses are alignment losses.

For engineers · your lens

SbSI is attractive on paper for sensors and harvesters – large piezoelectric and pyroelectric coefficients, simple chemistry – but its transition sits at room temperature and its devices are composites, not films. BiSI is a lead-free absorber whose measured efficiencies are still far below what its band gap allows. Both are research materials today.

In the research tracks

Theory frontiers

Sliding and in-plane ferroelectricity

Growth methods

Chemical vapour transport (CVT)

Key references

Ferroelectricity in SbSIFatuzzo et al. · Physical Review 127, 2036 (1962)cited by 265doi:10.1103/PhysRev.127.2036
Refined crystal structure and type of phase transition in ferroelectric SbSIIwata et al. · Journal of the Physical Society of Japan 21, 1846 (1966)cited by 23doi:10.1143/JPSJ.21.1846
Relativistic electronic structure and band alignment of BiSI and BiSeI: candidate photovoltaic materialsGanose et al. · Journal of Materials Chemistry A 4, 2060 (2016)cited by 169doi:10.1039/C5TA09612J
Defect engineering of earth-abundant solar absorbers BiSI and BiSeIGanose et al. · Chemistry of Materials 30, 3827 (2018)cited by 101doi:10.1021/acs.chemmater.8b01135
Photoactive piezoelectric energy harvester driven by antimony sulfoiodide (SbSI): A AVBVICVII class ferroelectric-semiconductor compoundPurusothaman et al. · Nano Energy 50, 256 (2018)cited by 64doi:10.1016/j.nanoen.2018.05.058
SbSI composites based on epoxy resin and cellulose for energy harvesting and sensorsToroń et al. · Materials 13, 902 (2020)cited by 27doi:10.3390/ma13040902