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 variant of TlI shows yellow color and an orthorhombic structure that somewhat resembles NaCl. This slight variation is believed to be due to favorable thallium-thallium interactions, with the closest Tl-Tl distance at 383 pm. At 175 °C, this form transitions into a red CsCl phase.