# Arsenic trisulfide

Arsenic trisulfide is the inorganic compound with the formula As$_2$S$_3$. It is a dark yellow solid that is insoluble in water. It also occurs as the mineral orpiment (Latin: auripigmentum), which has been used as a pigment called King's yellow. It is produced in the analysis of arsenic compounds. It is a group V/VI, intrinsic p-type semiconductor and exhibits photo-induced phase-change properties.

## Structure

As$_2$S$_3$ occurs both in crystalline and amorphous forms. Both forms feature polymeric structures consisting of trigonal pyramidal As(III) centres linked by sulfide centres. The sulfide centres are two-fold coordinated to two arsenic atoms. In the crystalline form, the compound adopts a ruffled sheet structure. The bonding between the sheets consists of van der Waals forces. The crystalline form is usually found in geological samples. Amorphous As$_2$S$_3$ does not possess a layered structure but is more highly cross-linked. Like other glasses, there is no medium or long-range order, but the first co-ordination sphere is well defined. As$_2$S$_3$ is a good glass former and exhibits a wide glass-forming region in its phase diagram.

## Properties

It is a semiconductor, with a direct band gap of 2.7 eV. The wide band gap makes it transparent to infrared light between 620 nm and 11 μm.

## Synthesis
### From the elements

Amorphous As$_2$S$_3$ is obtained via the fusion of the elements at 390 °C. Rapid cooling of the reaction melt gives a glass. The reaction can be represented with the chemical equation:
    2 As + 3 S → As$_2$S$_3$

### Aqueous precipitation

As$_2$S$_3$ forms when aqueous solutions containing As(III) are treated with H$_2$S. Arsenic was in the past analyzed and assayed by this reaction, which results in the precipitation of As$_2$S$_3$, which is then weighed. As$_2$S$_3$ can even be precipitated in 6 M HCl. As$_2$S$_3$ is so insoluble that it is not toxic.

## Reactions

Upon heating in a vacuum, polymeric As$_2$S$_3$ "cracks" to give a mixture of molecular species, including molecular As$_4$S$_6$. As$_4$S$_6$ adopts the adamantane geometry, like that observed for P$_4$O$_6$ and As$_4$O$_6$. When a film of this material is exposed to an external energy source such as thermal energy (via thermal annealing), electromagnetic radiation (i.e. UV lamps, lasers, electron beams)), As$_4$S$_6$ polymerizes:
    2 (As$_2$S$_3$)$_n$ ⇌ n As$_4$S$_6$
As$_2$S$_3$ characteristically dissolves upon treatment with aqueous solutions containing sulfide ions. The dissolved arsenic species is the pyramidal trithioarsenite anion AsS3−3:
    As$_2$S$_3$ + 6 NaSH → 2 AsS3−3 + 3 H$_2$S
As$_2$S$_3$ is the anhydride of the hypothetical trithioarsenous acid, As(SH)$_3$. Upon treatment with polysulfide ions, As$_2$S$_3$ dissolves to give a variety of species containing both S–S and As–S bonds. One derivative is S$_7$As−S$^−$, an eight-membered ring that contains 7 S atoms and 1 As atom, and an exocyclic sulfido center attached to the As atom. As$_2$S$_3$ also dissolves in strongly alkaline solutions to give a mixture of AsS3−3 and AsO3−3.
"Roasting" As$_2$S$_3$ in air gives volatile, toxic derivatives, this conversion being one of the hazards associated with the refining of heavy metal ores:
    2 As$_2$S$_3$ + 9 O$_2$ → As$_4$O$_6$ + 6 SO$_2$

## Contemporary uses
### As an inorganic photoresist

Due to its high refractive index of 2.45 and its large Knoop hardness compared to organic photoresists, As$_2$S$_3$ has been investigated for the fabrication of photonic crystals with a full-photonic band-gap. Advances in laser patterning techniques such as three-dimensional direct laser writing (3-D DLW) and chemical wet-etching chemistry, has allowed this material to be used as a photoresist to fabricate 3-D nanostructures.
As$_2$S$_3$ has been investigated for use as a high resolution photoresist material since the early 1970s, using aqueous etchants. Although these aqueous etchants allowed for low-aspect ratio 2-D structures to be fabricated, they do not allow for the etching of high aspect ratio structures with 3-D periodicity. Certain organic reagents, used in organic solvents, permit the high-etch selectivity required to produce high-aspect ratio structures with 3-D periodicity.

### Medical applications

As$_2$S$_3$ and As$_4$S$_4$ have been investigated as treatments for acute promyelocytic leukemia (APL).

### For IR-transmitting glasses

Arsenic trisulfide manufactured into amorphous form is used as a chalcogenide glass for infrared optics. It is transparent for light between wavelengths of 620 nm and 11 μm. The arsenic trisulfide glass is more resistant to oxidation than crystalline arsenic trisulfide, which minimizes toxicity concerns. It can be also used as an acousto-optic material.
Arsenic trisulfide was used for the distinctive eight-sided conical nose over the infra-red seeker of the de Havilland Firestreak missile.

### Role in ancient artistry

The ancient Egyptians reportedly used orpiment, natural or synthetic, as a pigment in artistry and cosmetics.

### Miscellaneous

Arsenic trisulfide is also used as a tanning agent. It was formerly used with indigo dye for the production of pencil blue, which allowed dark blue hues to be added to fabric via pencil or brush.
Precipitation of arsenic trisulfide is used as an analytical test for presence of dissimilatory arsenic-reducing bacteria (DARB).

## Safety

As$_2$S$_3$ is so insoluble that its toxicity is low. Aged samples can contain substantial amounts of arsenic oxides, which are soluble and therefore highly toxic.

## Natural occurrence

Orpiment is found in volcanic environments, often together with other arsenic sulfides, mainly realgar. It is sometimes found in low-temperature hydrothermal veins, together with some other sulfide and sulfosalt minerals.