Abstract: | Thallium sesquibromide Tl2Br3 is dimorphic. Scarlet coloured crystals of α‐Tl2Br3 were obtained by reactions of aqueous solutions of TlBr3 and Tl2SO4 in agarose gel. In case of rapid crystallisation of hydrous TlBr3/TlBr solutions and from TlBr/TlBr2 melts ß‐Tl2Br3 is formed as scarlet coloured, extremely thin lamellae. The crystal structures of both forms are very similar and can be described as mixed‐valence thallium(I)‐hexabromothallates(III) Tl3[TlBr6]. In the monoclinic unit cell of α‐Tl3[TlBr6] (a = 26.763(7) Å; b = 15.311(6) Å; c = 27.375(6) Å; β = 108.63(2)°, Z = 32, space gr. C2/c) the 32 TlIII‐cations are found in strongly distorted octahedral TlBr6 groups. The 96 TlI cations are surrounded either by four or six TlBr6 groups with contacts to 8 or 9 Br neighbors. Crystals of β‐Tl3[TlBr6] by contrast show almost hexagonal metrics (a = 13.124(4) Å, b = 13.130(4) Å, c = 25.550(7) Å, γ = 119.91(9)°, Z = 12, P21/m). Refinements of the parameters revealed structural disorder of TlBr6 units, possibly resulting from multiple twinning. Both structures are composed of Tl2[TlBr6]— and Tl4[TlBr6]+ multilayers, which alternate parallel (001). The structural relationships of the complicated structures of α‐ and β‐Tl3[TlBr6] to the three polymorphous forms of Tl2Cl3 as well as to the structures of monoclinic hexachlorothallates M3TlCl6 (M = K, Rb) and the cubic elpasolites are discussed. |