New organic–inorganic bromides[(C3H7)4N]2MBr4 (M=Hg and Cd): Synthesis, crystal structure and vibrational characterization

Abstract

Two new quaternary ammonium mercury bromide and cadmium bromide crystals [(C₃H₇)₄N]₂HgBr₄ and [(C₃H₇)₄N]₂CdBr₄ have been synthesized by slow evaporation met hod at room temperature. Their crystal structure was determined at room temperature by single crystal X-ray diffraction. Both structures consist of three symmetrically inequivalent tetrapropylammonium cations and one discrete tetrahedral [MBr₄]^2- (M=Hg and Cd) anions. The compounds are isomorphous and crystalize in monoclinic system with C 2/c space group. The purity of each preparation was confirmed by X-ray powder diffraction measurements at room temperature. Internal vibration modes of both crystals have been studied at room temperature using Raman spectroscopy. The structural phase transitions in [(C₃H₇)₄N]₂HgBr₄ and [(C₃H₇)₄N]₂CdBr₄ are studied by differential scanning calorimetry (DSC) and two reversible structural phase transitions are detected at 341/330 K and 402/375 K for [(C₃H₇)₄N]₂HgBr₄ and at 345/333 K and 404/377 K for [(C₃H₇)₄N]₂CdBr₄ on heating/cooling process. Electrical measurements showed that both compounds were good semiconductors. The thermal evolution of their conductivity shows two regime changes at the temperatures at which structural transitions occur. The thermal evolution of the dielectric constant shows that the second phase transition for the [(C₃H₇)₄N]₂CdBr₄ compound is ferroelectric in nature.