Dielectric relaxation of α-cyclodextrin-polyiodide complexes (α-cyclodextrin)2 · BaI2 · I2 · 8H2O and (α-cyclodextrin)2 · KI3 · I2 · 8H2O

Abstract

The frequency and temperature dependence of the real (ε′) and imaginary (ε″) parts of the dielectric constant of the polycrystalline complexes (α-CD)2 · Bal2 · I2 · 8H2O and (α-CD)2 · KI3 · I2 · 8H2O (α-CD = α-cyclodetrin) have been investigated over the frequency and temperature ranges 0–100 kHz and 120–300 K, respectively. The temperature dependences of ε′, ε″ and the phase shift φ show two steps, two peaks and two minima, respectively, revealing the existence of two kinds of water molecule, the tightly bound and the easily movable water molecules, in both complexes. The first peak of (T) or the first minimum of φ(T) presents the transformation of flip-flop hydrogen bonds to the normal state. The second ε″ (T) peak or φ(T) minimum corresponds to the easily movable water molecules or to a partial transformation of tightly bound to easily movable water molecules. For T > 270K both samples show semiconductive behaviour with energy gaps of 1.84eV for the (α-CD)2 · BaI2 · I2 · 8H2O complex and 1.36eV for the (α-CD)2 · KI3 · I2 · 8H2O complex. The conductivity at room temperature decreases in the order: (α-CD)2 · BaI2 · I2 · 8H2O > (α-CD)2 · LiI3 · I2 · 8H2O > (α-CD)2 · KI3 · I2 · 8H2O > (α-CD)2 · Cd0.5 · I5 · 26H2O. The relaxation time varies in a Λ-like curve (from 120 to 250 K) and rises rapidly for temperatures greater than 250 K, indicating the process of ionic movements. The activation energies around the transition temperature 0.98–1.09 k B T trans for (α-CD)2 · BaI2 · I2 · 8H2O and 1.06-1.55 k B T trans for (α-CD)2 · KI3 · I2 · 8H2O reveal the greater stability of the α-K complex against that of the α-Ba complex.