Thallium sesquibromide Tl 2 Br 3 is dimorphic. Scarlet coloured crystals of α-Tl 2 Br 3 were obtained by reactions of aqueous solutions of TlBr 3 and Tl 2 SO 4 in agarose gel. In case of rapid crystallisation of hydrous TlBr 3 /TlBr solutions and from TlBr/TlBr 2 melts ß-Tl 2 Br 3 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) Tl 3 [TlBr 6 ]. In the monoclinic unit cell of α-Tl 3 [TlBr 6 ] (a ϭ 26.763(7) Å ; b ϭ 15.311(6) Å ; c ϭ 27.375(6) Å ; ß ϭ 108.63(2)°, Z ϭ 32, space gr. C2/c) the 32 Tl III -cations are found in strongly distorted octahedral TlBr 6 groups. The 96 Tl I cations are surrounded either by four or six TlBr 6 groups with contacts to 8 or 9 Br neighbors. Crystals of ß-Tl 3 [TlBr 6 ] by contrast show almost hexagonal metrics Gemischtvalente Thallium(I,III)-bromide. Die Kristallstruktur von α-Tl 2 Br 3InhaltsübersichtThalliumsesquibromid Tl 2 Br 3 ist dimorph. Dunkelrote Kristallblättchen von α-Tl 2 Br 3 wurden durch Umsetzungen von wässrigen Tl 2 SO 4 /TlBr 3 -Lösungen in Agarose-Gel erhalten. β-Tl 2 Br 3 entsteht in Form extrem dünner, roter Lamellen bei raschen Kristallisationen aus wässrigen TlBr/TlBr 3 -Lösungen oder TlBr/ TlBr 2 -Schmelzen. Beide Formen besitzen einen ähnlichen Aufbau; sie sind als gemischtvalente Thallium(I)-hexabromothallate(III) Tl 3 [TlBr 6 ] zu beschreiben. In der monoklinen Elementarzelle von α-Tl 2 Br 3 (a ϭ 26,763(7) Å ; b ϭ 15,311(6) Å ; c ϭ 27,375(6) Å ; β ϭ 108,63(2)°, Z ϭ 32, Raumgruppe C2/c) sind 32 Tl III -Kationen in stark verzerrten, oktaedrischen TlBr 6 -Baugruppen enthalten. Die 96 Tl I -Kationen sind von vier oder sechs TlBr 6 -Baugruppen umgeben und bilden Kontakte zu 8 oder 9 Br-Nachbarn aus. 2675(a ϭ 13.124(4) Å , b ϭ 13.130(4) Å , c ϭ 25.550(7) Å , γ ϭ 119.91(9)°, Z ϭ 12, P2 1 /m). Refinements of the parameters revealed structural disorder of TlBr 6 units, possibly resulting from multiple twinning. Both structures are composed of Tl 2 [TlBr 6 ] Ϫ and Tl 4 [TlBr 6 ] ϩ multilayers, which alternate parallel (001). The structural relationships of the complicated structures of α-and ß-Tl 3 [TlBr 6 ] to the three polymorphous forms of Tl 2 Cl 3 as well as to the structures of monoclinic hexachlorothallates M 3 TlCl 6 (M ϭ K,Rb) and the cubic elpasolites are discussed.

Structure Structure D 2000Mixed-Valence Thallium(I,III) Bromides. The Crystal Structure of α-Tl 2 Br 3 . -Single crystals of α-Tl 2 Br 3 are prepared by reaction of aq. solutions of TlBr 3 and Tl 2 SO 4 in agarose gel. β-Tl 2 Br 3 is obtained by rapid crystallization of aq. TlBr 3 /TlBr solutions or from TlBr/TlBr 2 melts. The structures of both modifications are very similar and can be described as mixed-valent thallium(I) hexabromothallates(III), Tl 3 [TlBr 6 ]. The α-Tl 3 [TlBr 6 ] modification crystallizes in the monoclinic space group C2/c with Z = 32, while the β-modification shows almost hexagonal metric (space group P2 1 /m, Z = 12). Both structures are composed of Tl 2 [TlBr 6 ]and Tl 4 [TlBr 6 ] + multilayers alternating parallel (001). -(ACKERMANN, R.; HIRSCHLE, C.; ROTTER, H. W.; THIELE*, G.; Z.