Arsenic trisulfide is the inorganic compound with the formula . 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 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 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. 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 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: Aqueous precipitation forms when aqueous solutions containing As(III) are treated with . Arsenic was in the past analyzed and assayed by this reaction, which results in the precipitation of , which is then weighed. can even be precipitated in 6 M HCl. is so insoluble that it is not toxic. Reactions Upon heating in a vacuum, polymeric "cracks" to give a mixture of molecular species, including molecular . adopts the adamantane geometry, like that observed for and . 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)), As4S6 polymerizes: characteristically dissolves upon treatment with aqueous solutions containing sulfide ions. The dissolved arsenic species is the pyramidal trithioarsenite anion : is the anhydride of the hypothetical trithioarsenous acid, . Upon treatment with polysulfide ions, dissolves to give a variety of species containing both S–S and As–S bonds. One derivative is , an eight-membered ring that contains 7