Berichte der Bunsengesellschaft für physikalische Chemie最新文献

Strong irradiation of NaCl crystals results in a formation of sodium colloids which exhibit metallic properties. In consequence, besides the ²³Na line of NaCl a second well-separated ²³Na line occurs in the NMR spectrum caused by the Knight shift of the conduction electrons. We present Knight shift and nuclear spin relaxation measurements performed on the “metallic” ²³Na line in e^–-irradiated NaCl single-crystals between about 4 and 390 K. Evaluation of the data leads to detailed information on the electronic properties, atomic self-diffusion, and melting behavior of the colloids. The results are discussed in view of corresponding data obtained in bulk sodium.

The study of solute point-defect interactions in crystals is performed on an atomic scale by employing radioactive solutes as local observers. Thereby, the presence of a point-defect next to the solute atom becomes observable via the altered electronic charge distribution about the solute, giving rise to a defect-specific electric field gradient. Using the perturbed γγ angular correlation technique, this electric field gradient is detected via the hyperfine interaction occurring at the site of the nucleus of the radioactive atom. The sensitivity to solute point-defect pairs is typically in the order of 10¹⁶cm^–3. Several properties of these pairs, such as their chemical nature and thermodynamical behaviour are discussed. The point-defects addressed in this paper are intrinsic, such as lattice vacancies, and extrinsic, such as substitutional and interstitial impurity atoms. Results for the elemental semiconductor Si and different II-VI compound semiconductors will be discussed.

Polymer-surfactant interactions in water-SDS-polyvinylpyrrolidone (PVP), water-SDS-polyoxyethylenglycol (PEO), water-(DTABr)-(PEO) and in water-SDS-polyoxypropylenglycol (PPO), systems have been investigated by ultrasonic relaxation, adiabatic compressibility and viscosity measurements.

Both the hydrodynamic and thermodynamic properties show significant changes at the critical association concentration in presence of polymer. As to the kinetic behaviour, investigation on systems containing PEO or PVP indicates the occurrence of a region where relaxation times are nearly constant. For the low molecular weight PPO polymer this effect has not been observed. Such differences have been ascribed to the minimum polymer chain length effect, required for the binding of surfactant aggregates on the polymer backbone. The dynamics of polymer-surfactant interactions has been discussed in terms of a kinetic model, previously proposed.

The sol gel process for the preparation of lead zirconate, lead titanate and lead zirconate titanate is studied by various experimental techniques, comprising solid state NMR methods, FT IR spectroscopy, DTA/TG and XRD. Two temperature ranges are examined that cover (i) the pyrolysis step from the solid gel to the amorphous oxide mixture and (ii) the calcination step from the amorphous state to the crystalline ceramics. ¹³C NMR and FT IR spectroscopy studies primarily are used to monitor the structural changes during the pyrolysis step. ²⁰⁷Pb NMR spectroscopy is employed to examine the development of the short range order at the lead nuclei over the whole temperature range. The latter studies clearly show that the observed changes in the short range order are accompanied by alterations in the long range order, as derived from the XRD data. In addition, ²⁰⁷Pb NMR experiments are performed on ceramic samples that were obtained via the sol gel or the mixed oxide route. It is demonstrated that these samples exhibit significant differences in the short range order, although the long range order was found to be the same, being independent of the actual preparation route.

The origin and nature of ESR signals of fluoride glasses (ZBLAN: ZrF₄/BaF₂/LaF₃/AlF₃/NaF) doped both with Fe³⁺ and Fe²⁺ ions was studied using the temperature and microwave power (P_MW) dependencies as well as a complete simulation of the X-band ESR spectra. For the first time, evidence is given for the existence of {Fe^II-OH-Fe^III} units (S' = 9/2) formed in the glasses involving traces of O₂ and H₂O present in the samples. Fluoride glassy matrices constrain the formation of interacting species and therefore nonuniform spatial distributions of the dopants. Reasons are seen (i) in the local structural chemistry (geometrical and charge compensation reasons), and (ii) in the melting and post-melting processes.

Low-energy electron-energy-loss spectra in the range of the lowest triplet state are presented for solid benzene, fluorobenzene, and p-difluorobenzene deposited at a temperature of 30 K. Vibronic structure within the triplet bands of fluorobenzene and p-difluorobenzene is observed for the first time. The spectrum of fluorobenzene closely resembles that of benzene, whereas p-difluorobenzene shows characteristic differences. The spectra of all three molecules are dominated by pure progressions of the totally symmetric ring-breathing vibrational mode v₁. In addition, less intense transitions to vibronic levels relating to the e_2g modes in benzene are observed and their assignment is given. The occurrence of e_2g modes in all three spectra underlines that the vibronic coupling pathways between the lowest and the second triplet state are similar in benzene and its derivatives. In p-difluorobenzene a splitting of the extremely low frequency mode that relates to the v₈ (e_2g) vibration in benzene is observed. This is in agreement with theoretical predictions of an asymmetric shape of the lowest triplet potential energy surface along a symmetry coordinate relating to v₈ in substituted benzenes.