The room temperature form of TlI is yellow and has an orthorhombic structure  which can be considered to be a distorted NaCl structure. The distorted structure is believed to be caused by favourable thallium-thallium interactions, the closest Tl-Tl distance is 383 pm. At 175 °C the yellow form transforms to a red CsCl form.
The room temperature phase of TlI appears yellow and exhibits an orthorhombic structure that somewhat resembles a NaCl lattice. This slight deviation is attributed to favorable thallium-thallium interactions, with the nearest Tl-Tl distance being 383 pm. At 175 °C, this form transforms into a red CsCl phase.