The thallium chalcogenides include all compounds of thallium with the chalcogen elements, oxygen, sulfur, selenium and tellurium. (Polonium is excluded as little is known about its compounds with thallium). Some compounds that have been reported have not been substantiated by later researchers. The list of compounds below shows compounds that have been reported, and those compounds that have not had their structure determined, or whose existence has not been confirmed by the latest structural investigations, are in italics. In contrast to the indium chalcogenides the thallium(I) compounds are well characterised, there are no compounds with a Tl₂²⁺ unit, and the only genuine thallium(III) chalcogenide is the oxide, Tl₂O₃

All of these compounds are thallium(I) compounds. There are all black with a metallic appearance. Tl₂S occurs as the mineral carlinite and has been used in infrared detectors. Tl₂Se has metallic conductivity, and is a semiconductor in the liquid phase.^[1] The structure of Tl₂Te has been reinvestigated and is shown to be a new structure type related to Tl₅Te₃. ^[2]

Tl₅Se₃ has the Cr₅B₃ structure. Tl₅Te₃ has a structure related to In₅Bi₃, and has been formulated as 9Tl⁺ Tl³⁺ 6Te^{2− [3]}

TlS and TlSe are both black mixed valence compounds containing thallium in oxidation states +1 and +3. For example TlS can be formulated Tl⁺ Tl³⁺ 2S²⁻. TlS has two crystalline forms, a tetragonal chain structure and a monoclinic layer structure. The room temperature monoclinic form is ferroelectric. ^[4]. TlTe on the other hand is not a mixed valence compound but is a thallium(I) polytelluride, with a complex structure. TlTe is described as semimetallic.^[5]

Tl₂O₃ is the mineral avicennite. Its normal form has a bixbyite, Mn₂O₃, structure ^[6] and the high pressure form has a corundum, α-Al₂O₃, structure. It is a degenerate n-type semiconductor. Tl₂S₃ and Tl₂Se₃ have both been reported in the past however they have not been confirmed. Tl₂Te₃ is a polytelluride with a complex structure ^[7]

These are all thallium(I) polysulfides.

The gallium chalcogenides include all compounds of gallium with the chalcogen elements, oxygen, sulfur, selenium and tellurium. (Polonium is excluded as little is known about its compounds with gallium). Some compounds that were reported and have found their way into text books have not been substantiated by later researchers. The list of compounds below shows compounds that have been reported, and those compounds that have not had their structure determined, or whose existence has not been confirmed by the latest structural investigations, are in italics.