Discontinuous transition from insulator to semiconductor induced by phase change of the new organic- inorganic hybrid [(CH2)7(NH3)2]CoBr4

Abstract

[(CH2)7(NH3)2]CoBr4, triclinic crystal has a space group P1¯ with two molecules per asymmetric unit cell (Z = 2). Unit cell dimensions are: a = 7.6588 (3) Å, b = 10.5160 (3) Å, c = 11.3319 (5) Å, α = 66.745 (2) °, β = 77.2258 (13) °, γ = 88.001 (2) °, volume = 816.37(5) Å3 and calculated density = 2.078 Mgm−3. The structure consists of [CoBr4]2−tetrahdera separated by heptane diammonium layers. The organic hydrocarbon layers' pack in a stacked herring-bone manner hydrogen bonded to the halide ions. Lattice potential energy is Upot = 1827.7 kJ/mol, and molecular volume Vm = 0.408 nm3. Calorimetric (DSC) measurements indicate phase transitions at T1 = 350 ± 1 K (ΔS = 22.7 J/K-mol) and 335 K (ΔS = 10.5 J/K-mol). Dielectric and conductivity as a function of temperature (300 < T K < 420) and frequency (0.21 < f (kHz) < 60) were investigated. Abrupt change from an insulator to semiconductor behavior takes place around T1. At low temperatures, thermally activated Arrhenius behavior is observed with frequency dependent, temperature independent activation energy. At high temperatures conductivity and activation energy are frequency and temperature dependent implying non-Arrhenius type behavior where the charged particles experience different potentials for local and long distance movements. Conduction takes place via correlated barrier hopping with maximum barrier height WM = 0.015 eV.