Overlap large polaron sensitivity to phase changes in the first alkali earth organic-inorganic hybrid: [(CH2)7(NH3)2]CaCl4

Abstract

Crystal structure characterization, lattice potential energy, conductivity relaxation and electric conduction mechanisms of the new alkaline earth metal organic-inorganic hybrid, [(CH2)7(NH3)2] CaCl4 are presented. The new hybrid crystallizes in a triclinic system (S.G. P‾1) with two molecules per asymmetric unit cell (Z = 2). Crystallographic parameters are: a = 7.029 Å, b = 8.003 Å, c = 14.039 Å, α = 65.43°, β = 69.99°, γ = 61.39°, the calculated volume and density are 620.3 A3 and 1.68 Mgm−3 respectively. Room temperature lattice potential energy is ΔUpot = 1932.34 (kJ/mol.) and enthalpy change ΔH = 453.8 (kJ/mol.). DSC results indicated weak peaks at T7 = 239 ± 1 K, T6∼ 258.6 ± 0.5 K, a sharp λ-like peak typical of first order T5∼ 266.5 ± 0.5 K, a commensurate to incommensurate at T4 = 279 K, an incommensurate to unmodulated at T3 = 283 ± 1 K, and a chain melting transition is noted with minor peak at T2 = 326.5 K and major one at T1 = 329.3 K. Dielectric constant measurements in the temperature range (170–420) K and frequency range 60 Hz- 40 kHz confirmed the DSC transitions. It is characterized by a nearly temperature and frequency independent background of low dielectric constant (ε’) except at the transitions and at T ≥ 344 ± 2 K where frequency dispersion takes place. At high temperatures, calculated conductivity relaxation energy and relaxation time, are ΔE = 0.78 eV and (το) = 7.81614 × 10−14 s respectively. Fit of frequency dependent ac-conductivity to super-linear power law σtot = σdc + A1ωs1+ A2ωs2 shows different conduction mechanisms in the different temperature ranges (phases). Overlap large polaron tunneling (OLPT) of small radius dominates in the temperature range (240–290) K, it is very sensitive to the phase transitions in this temperature range where the polaron radius varies as the phase changes, which is reflected as small variations of the fitting parameters. In phase VIII (T < 239 K), phase III (295 < T(K) < 326.5) and phase II (327 < T(K) < 339); quantum mechanical tunneling (QMT) is the conduction mechanism. The lower value of the fitting parameters in phase (II) indicates stronger interaction between mobile ions with the lattice than phase (III). At higher temperatures correlated barrier (CBH) hopping prevails. Comparison to other transition metal hybrids of the same organic chain is discussed.