Crystal structure, phase transitions, dielectric and conduction mechanisms of the new long chain organic-inorganic hybrid [(CH2)8(NH3)2]CoCl4

Abstract

Two dimensional long chain organic inorganic hybrid [NH3(CH2)8NH3]CoCl4, (2C8CoCl) crystallizes in triclinic system, (P1/P 1̅ ), lattice parameters: a = 8.368 Ǻ, b = 12.888 Ǻ, c = 15.045 Ǻ, α = 97.56°, β = 86.82°, γ = 89.82°, Z = 4, d = 1.44 gm cm−3 and V = 1597.3 Ǻ3. Calculated lattice potential energy (ΔUpot) and change of enthalpy (ΔH) are ΔUpot = 1835.01 (kJ mol−1) and ΔH = 425.92 (kJ mol−1) respectively. Thermal study using differential thermal analysis (DTA) and thermogravimetric analysis (TGA) confirm different phase transitions, in the temperature range (300 < T(K) < 500) with onset temperatures at T1 = 401 ± 1 K, T2 = 358.3 K and T3 = 328.9 K, such transitions are reflected in impedance spectroscopy. The transition at T3 corresponds to chain melting. Dielectric constant and ac-conductivity are studied in the temperature range (300 < T (K) < 420) and frequency range (0.11 < f(kHz) < 65). Three different thermally activated conduction regions were observed corresponding to phases I (404–414) K; (ΔEo~ 0.83 eV), II (360–400) K; (ΔEo~ 1.19 eV) and III (T < 348 K); ΔEo = (0.03 eV– 0.006 eV). Equivalent circuit derived from impedance analysis, is composed of combination of resistor and two series of parallel (R//CPE) circuits. Super-linear power law dominates for ac- conduction in the measured temperature ranges. Conduction mechanism is analyzed on the basis of different conduction models which varies as the phases change. Quantum mechanical tunneling prevails at low temperatures with s value ~0.77 while correlated barrier hopping dominates in the range (363 < T (K) < 400) with barrier height WM = 0.34 eV. Comparison to previously studied long chain hybrids [NH3(CH2)nNH3]CoCl4, n = 6 and 7 is discussed. A linear decrease in lattice potential energy and enthalpy values is observed as chain length increases. The room temperature conductivity reveals that σ ac(2C7CoCl) > σ ac(2C6CoCl) >σ ac(2C8CoCl), which is discussed in light of structural characteristics.