Differential Raman scattering from impurity soft modes in mixed crystals of K1−xNaxTaO3 and K1−xLixTaO3

Impurity modes are observed in crystals of KTaO₃ doped with Na and Li, using differential Raman spectroscopy. One of the modes is shown to soften as the temperature approaches the ferroelectric transition temperature. Analysis of the results shows that the Na and Li ions do not occupy center of inversion positions. The roll of this observation in the inducement of ferroelectricity in these crystals is briefly discussed.     

Conditions for the existence of double resonance effects in Raman scattering

The condition for the existence of Double Resonance effects in Raman scattering are analyzed. It is shown that the existence of points in the electronic momentum space for which both denominators in the expression for Raman scattering vanish is not sufficient. Based on this analysis, it is shown that double resonance effects cannot show up in Resonant Raman scattering in the vicinity of the E₁ point in InSb.     

Preferential Solvation in Mixed Solvents X. Completely Miscible Aqueous Co-Solvent Binary Mixtures at 298.15 K

Summary.  The Kirkwood-Buff integrals for 18 completely miscible aqueous co-solvent binary mixtures have been recalculated from thermodynamic data, and the volume-corrected preferential solvation parameters derived from them are presented. Also presented are these latter quantities for 15 additional such mixtures, for which the volume correction has not been applied previously. The self-interaction of the water, the mutual interaction of the water and the co-solvent, and the self-interaction of the co-solvent at infinite dilution derived from these integrals and parameters are then discussed. The systems studied include aqueous hydrogen peroxide, methanol, ethanol, 1- and 2-propanol, 2-methyl-2-propanol, 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoro-2-propanol, ethane-1,2-diol, glycerol, 2-methoxyethanol (at 313 and 343 K), 2-ethoxyethanol, 2-butoxyethanol, 2-aminoethanol, N-methyl- and N,N-dimethyl-2-aminoethanol, tetrahydrofuran, 1,4-dioxane, acetone, formic, acetic, and propanoic acids, piperidine, pyridine, acetonitrile, formamide, N-methyl- and N,N-dimethylformamide, N-methylacetamide, N-methylpyrrolidin-2-one (at 303 K), hexamethyl phosphoric triamide, dimethylsulfoxide, and tetramethylenesulfone (at 303 K). Received January 10, 2001. Accepted (revised) February 20, 2001     

Use of differential impurity induced spectra for the investigation of phonon dispersion relations

The use of differential impurity induced spectra for the investigation of elementary excitations in solids is discussed. It is shown theoretically that the differential spectra are expected to have peaks at energies corresponding to energies of the excitation at critical points of the pure crystal independent of the nature of the impurity used. As an example we present experimental results of the derivative of the intensity of Raman scattered light with respect to wavelength from Tl doped KBr crystals.     

On the preferential solvation of drugs and PAHs in binary solvent mixtures

The preferential solvation parameters, i.e., the differences between the local and bulk mole fractions of the solvents in solutions of certain drugs and polycyclic aromatic hydrocarbons (PAHs) are derived from their solubilities in binary solvent mixtures by means of the inverse Kirkwood-Buff integral (IKBI) and the quasi-lattice quasi-chemical (QLQC) methods. The solutes include caffeine, niflumic acid, diazepam, benzocaine, phenacetin, paracetamol, nalidixic acid, anthracene, and tr-stilbene and both aqueous and non-aqueous mixtures are considered. The findings are rationalized in terms of the interactions between the solute and solvents and the solvent components among themselves.     

Mixed magnesium and nickel nitrate hexahydrates: an unexpected restructuring

Single crystals of the congruently melting equimolar mixture of magnesium nitrate hexahydrate and nickel nitrate hexahydrate have been grown. The compound crystallizes in a structure foreign to both components, namely in the monoclinic cobalt nitrate hexahydrate structure. The cations are located at random on the cation sub-lattice of the latter crystalline structure.     

Titration calorimetric determination of the pairwise interaction parameters of glycerol,D-threitol,mannitol, and D-glucitol in dilute aqueous solutions

Enthalpic pairwise interaction parameters, h_jj, were determined by titration calorimetry at 25°C for dilute solutions of glycerol, D-threitol, manitol, and D-glucitol in water. The parameters for these and other polyols conform to the expression h_jj (J-kg^–1)=145+135n_OH–21.5n _OH ² –41.7 _dl ² -160.5n_dlld, where n_OH=n_C is the number of hydroxyl groups (carbon atoms), n_dl is the number of dl configuration of a vicinal pair of OH-groups, and n_dlld is the number of such configurations in the polyol molecules. A rationalization of this expression is given.     

Solubility and preferential solvation of indomethacin in 1,4-dioxane + water solvent mixtures

The solubilities of indomethacin (IMC) in 1,4-dioxane + water cosolvent mixtures were determined at several temperatures, 293.15–313.15 K. The thermodynamic functions: Gibbs energy, enthalpy, and entropy of solution and of mixing were obtained from these data by using the van’t Hoff and Gibbs equations. The solubility was maximal in 0.95 mass fraction of 1,4-dioxane and very low in pure water at all the temperatures. A non-linear plot of ΔH_soln ° vs. ΔG_soln ° with negative slope from pure water up to 0.60 mass fraction of 1,4-dioxane and positive beyond this up to 0.95 mass fraction of 1,4-dioxane was obtained. Accordingly, the driving mechanism for IMC solubility in water-rich mixtures is the entropy, probably due to water-structure loss around the drug non-polar moieties by 1,4-dioxane, whereas, above 0.60 mass fraction of 1,4-dioxane the driving mechanism is the enthalpy, probably due to IMC solvation increase by the co-solvent molecules. The preferential solvation of IMC by the components of the solvent was estimated by means of the quasi-lattice quasi-chemical method, whereas the inverse Kirkwood-Buff integral method could not be applied because of divergence of the integrals in intermediate compositions.     

Tetraalkylammonium Ions in Aqueous and Non-aqueous Solutions

Property data for tetraalkylammonium cations, [H(CH₂) _n ]₄N⁺, are reviewed. They pertain to the isolated cations and their transfer from the gas phase into aqueous solutions. Various properties of these cations in aqueous and non-aqueous solutions and data for their transfer between these are also reviewed. Emphasis is placed on the dependence of data on the length n of the alkyl chains rather than on the absolute values. Most of the data are available only for the first four members of the series. The properties of the isolated ions increase linearly with the chain length. Molar enthalpies of formation of the gaseous and aqueous cations, and absolute standard molar enthalpies of hydration, are derived. Standard molar entropies of dissolution of several salts in water are obtained from their solubilities and enthalpies of solution. The molar entropies of the crystalline iodides of the first four members of the series then give the standard partial molar entropies of the aqueous cations and their molar entropies of hydration. The standard partial molar volumes in aqueous and non-aqueous solutions are quite linear with n and in non-aqueous solutions the molar volume hardly depends on the nature of the solvent. On transfer from water to non-aqueous solvents the volume of Me₄N⁺ suffers some shrinkage, that of Et₄N⁺ appears to be unaffected, but from Pr₄N⁺ onwards an increasing expansion takes place. This unexpected result is tentatively explained by hydrophobic intra-molecular association of pairs of alkyl chains in aqueous solutions, resulting in a tightening of the structure. The transfer of the R₄N⁺ cations from water into non-aqueous solvents is governed by a large positive entropy change, outweighing the smaller positive enthalpy change. The transport properties of the aqueous R₄N⁺ cations are non-linear with n. A major impediment to movement is thus the sticking of the water molecules to the ice-like hydrophobic hydration sheaths of the larger cations. The number of water molecules affected by the hydrophobic cations is open to widely differing estimates resulting from various approaches, and constitute an open issue.