Synthesis, Calorimetric and X ray diffraction studies in the solid solution Tl2(SO4)1−X(SeO4)X.Te(OH)6 0≤X≤1

The crystal structures of the thallium tellurate solid solutions Tl2(SO4)1−x(SeO4)x.Te(OH)6 were determined by X-ray diffraction method. The Tl2SeO4.Te(OH)6 (TlSeTe) and Tl2(SO4)0.6(SeO4)0.4.Te(OH)6 (TlSSeTe) compounds crystallize in the monoclinic system with P21/c space group. Whereas the Tl2SO4.Te(OH)6 (TlSTe) compound crystallizes in the P21/a one. The following parameters of TlSeTe structure are: a=12.358(3)Å; b=7.231(1)Å; c=11.986(2)Å; β=111.092(2)∘; Z=4.The TlSeTe structure can be regarded as being built of isolated TeO6 octahedra and SeO4 tetrahedra. The Tl+ cations are intercalated between these kinds of polyhedra. The main feature of this structure is the coexistence of two different and independent anions (SeO42− and TeO66−) in the same unit cell. The structure is stable thanks to O–H…O hydrogen bonds which link tetrahedral and octahedral groups.Crystals of Tl2SeO4.Te(OH)6 undergo two endothermal peaks at 373 K and 437 K. These transitions detected by DSC and analysed by dielectric measurements. However, the evolution of the conductivity versus temperature showed the presence of a protonic conduction phase transition at 437 K. The phase transition at 373 K can be related to structural phase transition.