Structural, thermal behaviour and vibrational study of a new mixed cesium–ammonium tellurate

Synthesis, crystal structure, thermal characterization, FTIR and Raman measurements are given for a new mixed compound cesium–ammonium tellurate. X-ray study showed that the [Cs0.92(NH4)0.08]2TeCl4Br2 compound crystallizes in the tetragonal space group P4/mnc, with a = 7.452 (1) A, c = 10.544 (3) A and Z = 2. The refinement converged at room temperature to R p = 0.093 and R wp = 0.076. The structure is considered as isolated octahedral TeCl4Br\(_{2}^{2-}\). These anions show a 4° rotation around the fourfold axis against the cubic arrangement of the K2PtCI6 type structure. The monovalent cations (Cs + /NH\(_{4}^{+}\)) are located between the octahedra ensuring the stability of the structure by ionic and hydrogen bonding contacts: Cs...Cl/Br and N-H......Cl/Br. A DTA/TGA experiment at high temperature reveals one endothermic peak at 780 K for this compound. At low temperature a transition from phase I to phase II was detected at 213 K by DSC. This transition, confirmed by Raman analysis, can be attributed to an order–disorder phase transition.