Thallium(I) Iodide
Properties
| State | Solid (thermochromic: yellow below 170 °C, red above) |
| Color | Yellow (low-temperature form) |
| Solubility | Very slightly soluble in water (0.006 g/100 mL at 20 °C); soluble in KI solutions as [TlI2]⁻ |
| Melting Point | 441 °C |
| Boiling Point | 824 °C |
About Thallium(I) Iodide
Thallium(I) iodide is a yellow crystalline salt (TlI, 331.28 g/mol) and one of the cleaner examples of a thermochromic inorganic compound. Below 170°C it crystallizes in an orthorhombic structure that absorbs blue light and looks yellow. Push it past 170°C and the lattice rearranges into a cubic CsCl-type structure with each Tl⁺ in 8-coordinate cubic iodide, the bandgap drops from about 2.8 eV to 2.3 eV, and the crystal turns red. Cool it back down and it returns to yellow — a structural phase transition that's sharp, reversible, and visible to the naked eye. Beyond the color tricks, TlI has the broadest mid-IR transparency window of any alkali-iodide-type salt (about 2–30 µm), which made it the standard window material for FTIR spectroscopy in the 1960s before AR-coated ZnSe and Ge took over. The compound's biggest legacy use is as the activator dopant in CsI:Tl scintillator crystals: doping CsI with about 0.1 mol% of Tl⁺ creates luminescent recombination centers that convert ionizing radiation into 550-nm visible photons at 54 photons per keV — the highest light yield of any common scintillator. CsI:Tl is the detector behind essentially every flat-panel digital X-ray imaging system in modern radiology. The unalloyed acute toxicity of thallium, however, means TlI is handled only in licensed crystal-growth facilities with rigorous waste tracking.
Where you'll encounter it
If you've ever had a digital X-ray taken at a dentist's office or stood in front of a millimeter-wave airport scanner, the photon-to-electron conversion happened in a CsI:Tl scintillator screen — Varex Imaging and Hamamatsu both make these for the medical-imaging market. Crystal growers at Saint-Gobain Crystals and Hilger Crystals pull CsI:Tl boules from melt at around 620°C in dedicated facilities with isolated waste streams, Tl-specific HEPA filtration, and biological monitoring of every operator (urine Tl every 90 days). In a research spectroscopy lab, the older generation of FTIR systems had TlI windows and beam-splitters; you can still find them in surplus, but most operators replace them with ZnSe at the first opportunity because of the toxicity headache. Thallium iodide also shows up as the green-emitting additive (535 nm Tl line) in a small subset of high-intensity discharge lamps used for plant-growth lighting and theatrical photography backlighting.
Common Uses
- Activator dopant in CsI:Tl scintillator crystals for medical X-ray flat panels
- Activator dopant in NaI:Tl scintillators for gamma-ray spectroscopy and SPECT imaging
- Mid-infrared (2–30 µm) optical window material for legacy FTIR spectrometers
- Green-emitting additive (535 nm) in specialty high-intensity discharge lamps
- Precursor to thallium(I) coordination compounds in research synthesis
- Photocathode and photodetector material in narrow research applications
- Component in some halide perovskite solar cell research formulations
Safety Information
EXTREMELY TOXIC — handle as a thallium poisoning emergency for any suspected exposure. GHS: Acute Toxicity Category 2 (oral H300, dermal H310, inhalation H330), STOT-RE Category 1 (H372, nervous system), Reproductive Toxicity Category 1B (H360), Aquatic Acute/Chronic Category 1 (H400/H410). OSHA PEL is 0.1 mg/m3 (8-hr TWA, as Tl, skin notation); ACGIH TLV identical. Although TlI's aqueous solubility is low (0.006 g/100 mL), gastric acid, sweat, and KI-rich biological fluids dissolve it as [TlI2]⁻ and absorbed Tl⁺ enters cells via K⁺ transporters. Lethal dose in adults is approximately 12 mg/kg (about 1 g for a 70-kg adult). Symptoms appear hours to days after exposure: severe gastrointestinal distress, ascending peripheral neuropathy, and diagnostic alopecia at 2–3 weeks. Treatment uses Prussian blue (ferric ferrocyanide) chelation under physician supervision — not self-administered. For any suspected exposure, contact Poison Control immediately (U.S.: 1-800-222-1222) without waiting for symptoms. Industrial handling requires sealed glove boxes, dedicated PPE, and biological monitoring.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.