Thallous iodide, TlI

Thallous iodide, TlI, is prepared like the corresponding chloride and bromide, and forms a citron-yellow precipitate of density 7.07. The crystalline form is orthorhombic. At 168° the yellow thallous iodide is converted into a red, cubic variety, the compressibility of which is 6.8×10^-6 per atmosphere, and the density 7.10. The two polymorphs are interconvertible. The red form melts at 431° to a dark red liquid.

The conversion of one form into the other is often extremely slow. A hot, filtered, aqueous solution of thallous iodide deposits red crystals when rapidly cooled, but a hot, saturated solution, when slowly cooled in the presence of the yellow form, deposits yellow crystals. A hot solution in aqueous potassium acetate or hydroxide deposits the metastable red modification when cooled, and the red form may also be precipitated by adding potassium iodide to a hot, concentrated solution of a thallous salt. Freshly precipitated yellow thallous iodide becomes green when exposed to sunlight. The green iodide can be recrystallised from water without losing its colour, but on standing it slowly reverts to the yellow modification, the change being hastened by heating with aqueous iodine or potassium iodide.

The solubility of the yellow modification in water, in grams per 100 grams of water, is as follows: -

Temperature, °C	0°	20°	40°	60°	80°	100°
Grams of TlI	0.002	0.006	0.015	0.035	0.070	0.120

The specific conductivity of the saturateduqueous solution at 18° is 22.3×10^-6 reciprocal ohms per cm. cube.

Thallous iodide forms mixed crystals with thallous nitrate; the " liquidus " curve for the system TlI - TlNO₃ consists of two parts meeting at a transition-point.