A transformation I2·I−·I2 ↔ I3−·I2 in the pentaiodide complex (α-Cyclodextrin)2·Cd0.5·I5·26H2O, detected via dielectric and Raman spectroscopy

The ac-conductivity and the phase shift of the polycrystalline complex (α-CD)2·Cd0.5·I5·26H2O (α-CD = α-Cyclodextrin) have been investigated over the frequency and temperature ranges of 0–100 kHz and 240–425 K. A Raman spectroscopy study is also accomplished in the temperature ranges of (i) 153–293 K and (ii) 303–383 K. From 276.2 up to 335.6 K – where all the water molecules of the crystal lattice exist – the transformation (H2O)tightlybound → (H2O)easilymovable takes place, resulting in the linear increment of the ac-conductivity in the lnσ vs. 1/T plot with activation energy Ea = 0.51 eV. In the range of 335.6–377.0 K a second linear part with Ea = 0.67 eV is observed attributed to the contribution of Cd2+ ions via the water-net. At T > 377.0 K, the abrupt decrease of the ac-conductivity up to 401.6 K is due to the removal of all the water molecules from the lattice. The phase shift presents a topical minimum at 404.3 K directly related to an order–disorder transition of the I− ions in some pentaiodide units and an abrupt decrease at T > 412.9 K due to the sublimation of iodine molecules. The Raman spectra at room temperature present two bands at 160 and 168 cm− 1 indicating the coexistence of two kinds of pentaiodide units I2·I−·I2 and I3−·I2 ↔ I2·I3−, respectively. Because of the inverse transformation (I3−·I2 ↔ I2·I3−) ↔ (I2·I−·I2) the band at 168 cm− 1 disappears as the temperature decreases whereas the band at 160 cm− 1 disappears during the heating process. The X-ray powder diffraction and the Rietveld analysis revealed a tetragonal crystal form with space group P42212 and lattice parameters that are in good agreement with the theoretical values.