Structural, vibrational and dielectric properties of the new Li-doped material [Li0.08(NH4)0.92]2TeCl4Br2

The crystal structure of the new Li-doped material [Li0.08(NH4)0.92]2TeCl4Br2, has been determined by X-ray single crystal analysis at room temperature. The title compound crystallizes in the tetragonal space group P4/mnc, with a lattice parameters a = 7.3269(3) Å and c = 10.3916(5) Å, Z = 2. The structure of [Li0.08(NH4)0.92]2TeCl4Br2 consists of isolated TeCl4Br2 octahedra slightly deformed in c-direction. These octahedra show a slightly rotation around the fourfold axis against the cubic arrangement of the K2PtCl6 type structure. The monovalent cations (Li+/NH4+) are located between the octahedra ensuring the stability of the structure by ionic and hydrogen bonding contacts: Li ⋯ Cl/Br and N–H ⋯ Cl/Br. Three endothermic peaks in thermal behavior were detected for this compound at 350 K, 419 K and 558 K, by DSC experiment. The first transition was confirmed by Raman spectroscopic and IR studies. Hydrogen bonding, the nature and the degree of structure (dis)order and the mechanisms of the transitions are discussed. The dielectric constant at different frequencies and temperatures revealed a phase transition at 350 K related to NH4+ reorientation and H+ diffusion.