AC-conductivity and Raman spectra of polyiodide inclusion compounds (β-cyclodextrin)2·KI7·16H2O and (β-cyclodextrin)2·LiI7·14H2O during the dehydration process

The frequency and temperature dependence of ac-conductivity and phase shift of polycrystalline inclusion compounds (β-CD)2·KI7·16H2O and (β-CD)2·LiI7·14H2O (β-CD=β-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 σ=σ0exp(-EW/2KBT) 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 β-K and at 353 K for β-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 I2 units via a charge transfer interaction in I−7 units. A second topical maximum value of conductivity appears at 399.7 K for β-K and 403 K for β-Li, attributed to the sublimation of I2.