Evolution of crystal structure of dual layered molecular conductor (ET)4ZnBr4(C6H4Cl2) with temperature

Abstract

Electrical resistivity measurements of a dual layered organic conductor (ET)₄ZnBr₄(1,2-C₆H₄Cl₂) above room temperature show abrupt changes in resistivity at 320 K. Single-crystal X-ray diffraction studies in the 100–350 K range show polymorphic transformation at T_p ∼320 K, indicating that the crystals belong to the tetragonal system (high-temperature phase, HT-phase) above T_p and to the triclinic system (low-temperature phase, LT-phase) below T_p with abrupt phase transition from the HT-phase to the LT-phase. During the sequential heating of the crystal, anharmonic effects are detected in the 260–315 K range: i) an increase in growth of rate of parameter a and rate of the mean-square atomic displacements of the [ZnBr₄]²⁻ anions; ii) an increase in the amplitudes of thermal vibrations of ET and solvent molecules. The structure near the phase transition at 315 K reveals splitting of the positions of ethylene groups of ET, demonstrating for the first time that for the (ET)₄MBr₄(1,2-C₆H₄Cl₂) family disordering of the ET occurs in the LT-phase when approaching T_p. The final rearrangement of the structure from the triclinic LT-phase to the tetragonal HT-phase occurs above 315 K.