[(CH3)3N(CH2)2Br]2[CoBr4] halogenometallate complex: crystal structure, high-temperature reversible phase transition, electrical and optical properties

Abstract A new Co(II)-based organic-inorganic hybrid compound, [(CH3)3N(CH2)2Br]2[CoBr4], was synthesized by slow evaporation at room temperature. The compound was characterized by single crystal X-ray diffraction, thermal analysis, optic and dielectric measurements. [(CH3)3N(CH2)2Br]2[CoBr4] belongs to the monoclinic P21/c space group at 150K. Molecular structure of the compound consists of discrete [CoBr4]2− anions and (2-bromoethyl) trimethylammonium cations linked together via non-covalent interactions including C-H...Br hydrogen bonding and weak halogen…halogen interactions which lead to a three-dimensional supramolecular network. The thermal analysis reveals a reversible phase transition at 366/337 K upon heating/cooling respectively, which is giving rise to a thermal hysteresis of ∼26 K. In addition, the UV/Vis optical spectrum indicates that the material has an indirect band gap of 3.7 eV which can be used in the application of the semiconductor materials. The electrical properties as a function of frequency and temperatures were studied and confirmed the phase transition showed in the thermal analysis. The equivalent circuit was determined to separate grain and grain boundaries response and determine the grain resistivity. The modulus studies confirm the presence of grain and grain boundaries. The AC conductivity was analyzed by Jonscher power law and showed that the conduction mechanisms are well described by the small polaron tunnel (SPT) and overlapping large polaron tunneling (OLPT) models in the phases I and II, respectively. The variation of σg, fp and σdc from the simulation of Nyquist diagram, Modulus and AC conductivity proves the transition in the DSC and allows to determinate the activation energies for the two phases. These results will provide positive clues to explore the semiconductor and electrical properties of hybrid halogenometallate complexes.