Crossover from overlap large polaron to small polaron tunneling: A study of conduction mechanisms and phase transitions in a new long chain organic inorganic hybrid

Abstract

AC-conductivity study of the new long chain hybrid [NH3 (CH2)8 NH3]Cl2 in the temperature range 210 K–420 K showed the following consecutive phase transitions:(PhaseVIII)T7=232.5K(PhaseVII)T6=253K(PhaseVI)T5=287.9K(PhaseV)T4=316.5K(PhaseIV)T3=330.7K(PhaseIII) T2=378.8 K(PhaseII)T1=399 K(PhaseI).Transitions are confirmed by differential scanning calorimetry (DSC). Room temperature X-ray powder diffraction indicated a monoclinic P21/c with crystallographic parameters a = 18.179 Å, b = 15.380 Å, c = 4.881 Å and β = 109.54(0), V = 1286 (Å)3, Z = 4. Frequency dependent ac-conductivity at different temperatures is interpreted in terms of the jump relaxation model and is fitted to super-linear power law σtot = σdc + A1ωs1+ A2ωs2. The fit parameters lnA1/s1 and lnA2/s2 are sensitive to the type of conduction mechanism. Temperature and frequency dependence of ac-conductivity indicated that in the low (T < 231 K) and intermediate (290 K–316 K) temperature ranges, conduction takes place via quantum mechanical tunneling (phases (VIII) and (V)). Crossover from large polaron to non-overlapping small polaron tunneling takes place associated with transition from phase (VII) to phase (VI). Correlated barrier hopping (CBH) prevails at high temperature (phases (I, II, III and IV)). Activation energy and its frequency dependence were calculated for the different phases. Minimum tunneling and hopping distances, effective barrier height WM as well as the density of states at the Fermi level (N(Ef)) were evaluated from ac-conductivity results.