Thallium is a silvery-white, soft, extremely toxic scattered metal; in nature it almost never forms independent deposits but is associated in the smelter dust of lead-zinc ores, iron ores and certain polymetallic sulfide ores. Its modern industrial uses are highly restricted, concentrated mainly in two areas: infrared optics and specialty electronic materials. In infrared optics, single crystals of thallium bromoiodide (KRS-5) and thallium bromochloride have excellent transmission in the mid-infrared and are relatively soft, used as prisms, windows and lenses in infrared spectrometers and thermal-imaging equipment. In superconductivity and specialty materials, thallium-based high-temperature superconductors (such as the Tl-Ba-Ca-Cu-O system) once attracted research interest, with critical temperatures above the liquid-nitrogen range, but because thallium is highly toxic and difficult to prepare, they never reached large-scale commercial use. Historically, thallium salts were used as rodenticides and insecticides; because they are equally toxic to humans and livestock and poisoning incidents occurred repeatedly, such agricultural uses have long been banned worldwide. Thallium also has small medical and research uses in a few specialty alloys, high-refractive-index optical glasses and the Tl-201 radioisotope cardiac imaging agent. Because of its severe toxicity and the lack of a specific antidote, thallium is under strict international control, and its civilian use has been shrinking.
Thallium(I) oxide (Tl2O) is one of the main oxides of thallium, a black powder used mainly to prepare other thallium compounds and as a dopant in certain specialty optical glasses, raising their refractive index and density. Thallium(I) chloride (TlCl) is a white, poorly soluble crystal, an important intermediate in thallium metallurgy and one of the key raw materials for the infrared crystal KRS-5 (thallium bromoiodide), with stable but very small use in infrared-optical component manufacturing. Thallium(I) sulfate (Tl2SO4) is a colorless, odorless, extremely toxic white crystal once widely used as a rodenticide and insecticide; because it repeatedly poisoned humans and livestock and polluted the environment, its agricultural use has long been explicitly banned in all countries, and it is now used only in tiny quantities in the laboratory and special analysis. In addition, thallium bromide and thallium iodide are raw materials for infrared-optical crystals, and thallium nitrate is used in a few chemical analyses and luminescent-material research. Overall, because of toxicity controls, there is essentially no civilian market for thallium compounds.
