Berichte der Bunsengesellschaft für physikalische Chemie最新文献

In this work we discuss the application of tight-binding molecular dynamics to the study of migration of intrinsic point defects in c-Si. In particular, we investigate self-diffusion and formation of vacancy clusters. Furthermore, by means of Hartree-Fock calculations, we present a quantitative picture for the chemical bond in silicon when point defects are present.

Hydroxyl ion is a common impurity in insulating crystals: by interacting with other impurities, it gives rise to new complexes. The OH-stretching frequency is a very sensitive probe of the hydroxyl environment. High resolution (0.04 cm^_1) FTIR spectroscopy in the temperature range 9–300 K was applied to study the OH-cation impurity interaction in alkali halides, fluoroperovskites, and sillenites, suitable for optoelectronic applications. Proper thermal treatments and isotopic substitutions allowed to assign the stretching mode absorption lines to the defects in which OH^– is embedded and to supply possible models for them. Anharmonicity effect of the OH^– -stretching modes, monitored by weak overtone lines, were well described in the framework of the Morse model. Electric anharmonicity was also detected. The phonon coupling of the OH^–-stretching mode in different defects was studied by analysing the temperature dependence of the line-position and -width: in most cases the single-phonon coupling model accounted for the experimental data and supplied the coupled phonon frequencies.

The standard molar enthalpies of formation Δ_fH⁰_m, (1 or cr) at the temperature 298.15 K were measured by using combustion calorimetry for 4-ethylphenol, 2-iso-propylphenol, 2-iso-propyl-5-methylphenol, 2-sec-butyl-phenol, 4-iso-propylphenol, 4-sec-butylpnenol, and 4-cyclo-hexylphenol. The enthalpy of formation Δ_fH⁰_m, (1) of 3-iso-propylphenol was obtained from the results of calorimetric and equilibrium studies. The standard molar enthalpies of sublimation (or vaporization) of these compounds, and also of phenol, were obtained from the temperature dependence of the vapour pressure measured in a flow system. Molar enthalpies of fusion Δ¹_crH⁰_m of the solid compounds were measured by d.s.c. Resulting values of Δ_fH⁰_m (g) were obtained at the temperature 298.15 K and used to derive strain enthalpies of alkylphenols. The intra-molecular interactions of the substituents were discussed in terms of deviations of Δ_fH⁰_m (g) from the group additivity rules. Ortho-, para-, and meta-interactions of sec-alkyl substituent with OH-group in the gaseous phase was found to be equal 3.9 kJ·mol⁻¹. These values provided a further improvement on the group-contribution methodology for estimation of thermodynamic properties of organic compounds.

Longitudinal ¹H, ¹³C and ¹⁵N nmr relaxation rates were measured in the liquid phase for N-methylformamide (NMF) over the temperature range 228–328 K and for N-methylacetamide (NMA) over the range 300–378 K. Rotational correlation times were determined for the formyl C-H and the N-H bonds. The motion of both molecules is anisotopic, particularly that of N-methylacetamide. The activation energies are 5.6 kJ/mol for C-H and 19.7 kJ/mol for N-H in NMF and 20.4–22.5 kJ/mol for N-H in NMA. Quadrupole coupling constants were determined by relaxation measurements for the nuclei ¹⁴N and ²H. The quadrupole coupling constant for the amide deuteron in NMF is temperature dependent and varies from 193 to 206 kHz. For NMA this quadrupole coupling constant falls into the range 178 to 238 kHz. The quadrupole coupling constant for the formyl ²H in NMF (151–169 kHz) is temperature independet while for the ¹⁴N (NMF: 1.85–2.40 MHz; NMA: 1.65–2.45 MHz) it is temperature dependent.

Atomistic simulation calculations are used to predict the solution mechanisms and the defect cluster geometries of: M²⁺ dopant cations in Y₂O₃, M³⁺ dopant cations in CoO and M²⁺ dopant cations in SrTiO₃. The interatomic potential parameters were derived by simultaneously fitting the properties of a range of mixed cation materials. The results suggest that although both solution enthalpies and cluster binding energies do scale with ionic radius, the relationships can be quite complex, materials specific and will not necessarily exhibit simple minima when the radius of the host cation equals the dopant cation, as described in previous studies.

High pressure-high temperature procedure and equipment is described to determine the specific volume of ammonia as a function of pressure and temperature. An autoclave consisting of a nickel-based superalloy and a suitable pressure-sensitive separator between ammonia and the pressure-transmitting fluid is used. Ten volume isotherms between 298 and 723 K and at pressures from 10 to 950 MPa have been determined. Previous literature data extend to 950 MPa but only up to 473 or to 723 K at pressures below 500 MPa. Polynomials with up to eleven terms are used to represent each experimental volume isotherm with a maximum average deviation of 0.16%. A table with smoothed density values is presented for the whole range of conditions. A comparison with data calculated earlier by Haar and Gallagher (1978) up to 500 MPa showed deviations between 0.06 and 1.1% with an average of 0.4%. The present isothermal volume data are reasonably well described by Tait equations. Parameters are given.

In cubic Y₂O₃ (10–25 mol%) or CaO (14 and 17 mol%) stabilized ZrO₂, oxygen vacancies are created as charge-compensating defects. Mechanical loss (internal friction) measurements are performed on single crystals of cubic zirconia with various orientation at frequencies of f ≈︁ 1 Hz and 1 kHz, to study the local crystallographic structure of the defects. The mechanical spectra show a composite loss maximum at 400–600 K consisting of two overlapping submaxima: I and I_A in ZrO₂-Y₂O₃; I' and I'_A in ZrO₂-CaO. Submaxima I (I') are assigned to defect pairs of oxygen vacancies and impurity atoms forming elastic (and electric) dipoles. These are oriented parallel to <111>, with the vacancy on nearest neighbour sites (trigonal symmetry). Submaxima I_A(I'_A) are attributed to relaxation of vacancies within Y- (or Ca-) clusters with various sizes and configurations corresponding to lower defect symmetry. The electrical conductivity (4 probe d.c. technique) decreases for concentrations ≥10mol% Y₂O₃, whereas the activation enthalpy increases with Y₂O₃ content. A high temperature peak C in ZrO₂-CaO around 1400 K (0.1 Hz) is probably due to local atomic jumps of cations.

Thermodynamic properties of the so-called All-Vanadium battery used as energy storage system and other vanadium redox systems related to the All-Vanadium battery have been studied. An electrochemical cell has been constructed to measure the equilibrium cell voltages as function of the degree of charging in the temperature range from 278 K to 323 K. The first system studied is the disproportion reaction of VO²⁺ ions with the two half cell reactions VO²⁺ + H₂O ⇄ VO⁺₂ + 2H⁺ + e⁻ and VO²⁺ + 2H⁺ + e⁻ ⇄ V³⁺ + H₂O. The second system is the All-Vanadium battery reaction with the two half cell reactions VO²⁺ + H₂O ⇄ VO⁺₂ + 2H⁺ + e⁻ and V³⁺ + e⁻ ⇄ V²⁺. The third system is the disproportion reaction of V³⁺ ions with the half cell reactions V³⁺ + H₂O ⇄ VO²⁺ + 2H⁺ + e⁻ and V³⁺ + e⁻ ⇄ V²⁺. The molar reaction Gibbs energy, the molar reaction enthalpy, and the molar reaction entropy of each system has been obtained from these data. Additionally the molar reaction enthalpy of the two disproportion reactions and of the reaction V²⁺ + 2VO²⁺ + 2H⁺ → 3VO²⁺ + H₂O has been measured directly by titration calorimetry. The results agree with those obtained from the electrochemical data for the disproportion reactions. No direct calorimetric measurements of the molar reaction enthalpy of the All-Vanadium battery reaction is possible, but the sum of the molar reaction enthalpies of the two disproportion reactions gives the molar reaction enthalpy of the All-Vanadium battery reaction. Comparison with the electrochemically determined molar reaction enthalpy of the All-Vanadium battery reaction shows good agreement indicating satisfying thermodynamic consistency of the whole procedure.

Combination of comprehensive investigations of the spin-lattice relaxation rate of proton and low temperature ¹⁵N-CP/MAS NMR spectrum provides unique information of proton dynamics in two interacting NHO hydrogen bonds of solid N,N′-di(2-hydroxy-1-naphthylmethylene)-p-phenylenediamine (DNP). It was evidenced from the ¹H-NMR relaxation measurement that tunneling mechanism operates for the proton transfer in the hydrogen bonds. The tunneling phenomenon is closely related to the very small energy differences among the four tautomeric states accompanied with the proton transfer in the two NHO hydrogen bonds. The very small values of the energy difference, in spite of the chemically asymmetric NHO hydrogen bond, were revealed by the ¹⁵N-CP/MAS NMR spectrum. This is a unique character of solid DNP. It was also suggested from the derived energy scheme of the four tautomers and activation energies of the proton transfer that an interaction exists between the two NHO hydrogen bonds linked by π-electronic molecular frame. This means that the information of one NHO hydrogen bond, i.e. OH-form or NH-form, propagates to the other hydrogen bond and the proton transfer in the first hydrogen bond induces the change of the potential function for the proton transfer in the second hydrogen bond.