A transformation I2.I-.I2⇆I-3.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 (alpha-CD)(2)center dot Cd-0.5 center dot I-5 center dot 26H(2)O (alpha-CD=alpha-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 In sigma vs. 1/T plot with activation energy E-a = 0.51 eV. In the range of 335.6-377.0 K a second linear part with E-a = 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 l(-) 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 I-2 center dot I-center dot I-2 and I-3(')center dot I-2 <-> I-2 center dot I-3(-), respectively. Because of the inverse transformation (I-3(-)center dot I-2 <-> I-2 center dot I-3(-)) <-> (I-2 center dot I-center dot I-2) 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 P4(2)2(1)2 and lattice parameters that are in good agreement with the theoretical values. (C) 2007 Elsevier B.V All rights reserved.