Dielectric and Raman spectroscopy of the heptaiodide complex (β-Cyclodextrin)2·CsI7·13H2O

The frequency and temperature dependence of the real and imaginary parts of the dielectric constant (ε′,ε″), the phase shift (φ) and the ac-conductivity (σ) of the polycrystalline (β-Cyclodextrin) 2·CsI7·13H2O (β-Cs) have been investigated over the frequency and temperature ranges of 0-100 kHz and 140-425 K, respectively, in combination with Raman spectroscopy and DSC. The ε′(T), ε″(T) and φ(T) variations at frequency 300 Hz in the range 140 K<T<300 K show two sigmoids, two bell-shaped curves and two minima, respectively, revealing the existence of two kinds of water molecules, tightly bound and easily movable ones. β-Cs shows the transition of normal hydrogen bonds to those of flip-flop type at 199.9 K. As the temperature increases most of the thirteen water molecules per cyclodextrin dimer remain tightly bound and only a small number of them become easily movable. The DSC trace shows a small endothermic peak with an onset temperature of 80 °C, which is related to the easily movable water molecules. Strong peaks at 115 and 135 °C are caused by the tightly bound water molecules and the sublimation of iodine, respectively. The Cs+ ions contribute to the ac-conductivity via a Grotthuss mechanism with an activation energy 0.64 eV when all the water molecules exist in the crystal lattice and 0.45 eV when the easily movable water molecules start to escape. The Raman peaks at 179, 170 and 166 cm-1 are due to the I2·I-3·I2 polyiodide chains consisting of I-3 units indicating charge transfer interactions and lengthening of I2 units, respectively. The charge of I-7 units remains localized with negligible contribution to the conductivity until the sublimation of iodine starts. © 2005 Elsevier SAS. All rights reserved

The electrical properties and quality factor of nickel electrodeposits

The quality factor Q, of the nickel electrodeposits prepared under different conditions (bath pH, current density, etc.), was measured by a Q-meter and the in-series method was utilized. The inductance (L) and ohmic resistance (R) were obtained through the resonance frequency (F) by use of appropriate equations. The measurements can be shown on C, L, R and Q against F plots. The above electrical properties of nickel electrodeposits are affected by the electrodeposition conditions and slightly by the deposits' thickness. © 1996 Chapman & Hall

Impedance spectroscopy study of nickel electrodeposits

Nickel electrodeposits prepared in NiSO4 and NiCl2 electrolytes with thicknesses of 43-49.4 μm were examined by the impedance spectroscopy (IS) method for a-c frequencies 1 Hz-100 kHz. The Nyquist diagrams were single, almost perfect semicircles, suggesting the applicability of the in parallel combination of the Ra-Ca elements characterised by a single relaxation time, 8.27 × 10-6 s, combined in series with another Rb element, Rb ≪ Ra. This relaxation time predicted that the incorporated hydrogen atoms are involved in the process of conduction by a hopping/diffusion mechanism, which is differentiated in the cases of NiSO4-Ni and NiCl2-Ni deposits. The diffusion coefficient of hydrogen, 1.25 × 10-11 cm2 s-1, was determined. The microstructure of Ni electrodeposits was also studied and characterised by electron microscopy. Finally, a general model for the microstructure of Ni electrodeposits, consistent with the results of impedance spectroscopy, was formulated. © 2002 Elsevier Science B.V. All rights reserved

Electron hopping mechanism in hematite (α-Fe2O 3)

The frequency dependence of the real (ε′) and imaginary (ε″) parts of the dielectric constant of polycrystalline hematite (α-Fe2O3) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O2- ion into one of the magnetic ions Fe3+) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the -ln τ vs 10 3/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy -0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature TMo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains. © 2004 Elsevier Ltd. All rights reserved

Metal-heptaiodide interactions in cyclomaltoheptaose (β-cyclodextrin) polyiodide complexes as detected via Raman spectroscopy

The Raman spectra of the cyclomaltoheptaose (β-cyclodextrin, β-CD) polyiodide complexes (β-CD)2·NaI7·12H2O, (β-CD)2·RbI7·18H2O, (β-CD)2·SrI7·17H2O, (β-CD)2·BiI7·17H2O and (β-CD)2·VI7·14H2O (named β-M, M stands for the corresponding metal) are investigated in the temperature range of 30-140 °C. At room temperature all systems show an initial strong band at 178 cm-1 that reveals similar intramolecular distances of the disordered I2 units (∼2.72 Å). During the heating process β-Na and β-Rb display a gradual shift of this band to the final single frequency of 166 cm-1. In the case of β-Sr and β-Bi, the band at 178 cm-1 is shifted to the final single frequencies of 170 and 172 cm-1, respectively. These band shifts imply a disorder-order transition of the I2 units whose I-I distance becomes elongated via a symmetric charge-transfer interaction I2←I3-→I2. The different final frequencies correspond to different bond lengthening of the disordered I2 units during their transformation into well-ordered ones. In the Raman spectra of β-V, the initial band at 178 cm-1 is not shifted to a single band but to a double one of frequencies 173 and 165 cm-1, indicating a disorder-order transition of the I2 molecules via a non-symmetric charge-transfer interaction I2←I3- → I2. The above spectral data show that the ability of I3- to donate electron density to the attached I2 units is determined by the relative position of the different metal ions and their ionic potential q/r. The combination of the present results with those obtained from our previous investigations reveals that cations with an ionic potential that is lower than ∼1.50 (Cs+, Rb+, Na+, K+ and Ba2+) do not affect the Lewis base character of I3-. However, when the ionic potential of the cation is greater than ∼1.50 (Li+, Sr2+, Cd2+, Bi3+ and V3+), the Mn+⋯I3- interactions become significant. In the case of a face-on position of the metal (Sr2+, Bi3+) relative to I3-, the charge-transfer interaction is symmetric. On the contrary, when the metal (Li+, Cd2+, V3+) presents a side-on position relative to I3-, the charge-transfer interaction is non-symmetric. © 2007 Elsevier Ltd. All rights reserved

An insight into the disorder properties of the α-cyclodextrin polyiodide inclusion complex with Sr2+ ion: Dielectric, DSC and FT-Raman spectroscopy studies

At T<250K, the polyiodide inclusion complex (α-cyclodextrin) 2Sr0.5I517H2O displays two separate relaxation processes due to both the frozen-in proton motions in an otherwise ordered H-bonding network and the order-disorder transition of some normal H-bonds to flip-flop ones. At T>250K, the AC-conductivity is dominated by the combinational contributions of the disordered water network, the mobile Sr 2+ ions, the polyiodide charge-transfer interactions and the dehydration process. The evolution of the Raman spectroscopic data with temperature reveals the coexistence of four discrete pentaiodide forms. In form (I) (I-3·I2↔I2· I-3), the occupancy ratio (x/y) of the central I - ion differs from 50/50. In form (IIa) (I2·I -·I2) x/y=50/50, whereas in its equivalent form (IIb) (I2·I-·I2)* as well as in form (III) (I-3·I2), x/y=100/0 (indicative of full occupancy). Through slow cooling and heating, the inverse transformations (I)→(IIa) and (IIa)→(I) occur, respectively. © 2010 Taylor & Francis

Significant modification of the I - 3 Lewis base character in the β-cyclodextrin polyiodide inclusion complex with Co 2+ ion: An FT-Raman investigation

The β-cyclodextrin (β-CD) polyiodide inclusion complex (β-CD) 2·Co 0.5·I 7· 21H 2O has been synthesized, characterized and further investigated via FT-Raman spectroscopy in the temperature range of 30-120 °C. The experimental results point to the coexistence of I - 7 units (I 2·I - 3·I 2) that seem not to interact with the Co 2+ ions and I - 7 units that display such interactions. The former units exhibit a disorder-order transition of both their I 2 molecules above 60 °C due to a symmetric charge-transfer interaction with the central I - 3 [I 2 ← I - 3 → I 2], whereas in the latter units only one of the two I 2 molecules becomes well-ordered above 30 °C. The other I 2 molecule remains disordered presenting no charge-transfer phenomena. The Co 2+ ion induces a considerable asymmetry on the geometry of the I - 3 anion and a significant modification of its Lewis base character. Complementary dielectric measurements suggest no important involvement of H⋯I contacts in the observed modification of the I - 3 electron-transfer properties. © 2011 Elsevier B.V