AC-conductivity and Raman spectra of polyiodide inclusion compounds (beta-cyclodextrin)(2)center dot KI7 center dot 16H(2)O and (beta-cyclodextrin)(2)center dot LiI7 center dot 14H(2)O during the dehydration process

摘要

The frequency and temperature dependence of ac-conductivity and phase shift of polycrystalline inclusion compounds (beta-CD)(2) (.) KI7 (.) 16H(2)O and (beta-CD)(2) (.) LiI7 (.) 14H(2)O (beta-CD = beta-cyclodextrin) has been investigated over the frequency and temperature ranges of 0-100 kHz and 240-420 K. A Raman spectroscopic study and calorimetric measurements are also accomplished. The Arrhenius exponential behaviour sigma = sigma(0) exp (-E-W/2K(B)T) of the ac-conductivity for T > 275 K is caused by the contribution of the metal cations K+, Li+. This contribution is facilitated by the water-net via the Grotthuss mechanism. The ac conductivity starts deviating from the exponential behaviour with lower increasing rate, at 347 K for beta-K and at 353 K for beta-Li reaching a maximum value at 371.1 and 361.8 K, respectively, and then decreases rapidly due to the gradual removal of all the water molecules. The values 371.1 and 361.8 K are characterized as semiconductor to metal transition temperatures. The shift of the initial Raman peak at 179 cm(-1) to the final value 165 cm(-1) as the temperature increases reveals the lengthening of 12 units via a charge transfer interaction in 17 units. A second topical maximum value of conductivity appears at 399.7 K for beta-K and 403 K for beta-Li, attributed to the sublimation of I-2. (c) 2006 Elsevier Ltd. All rights reserved.