Band 53, Heft 3-4

Recently, the lengths of the two P-O bonds in the PO 4 tetrahedron were obtained by neutron diffraction of high real-space resolution. By use of the present X-ray diffraction experiments, the P-P distance belonging to pairs of corner-linked PO 4 units is determined. Using this length of (294 ± 2) pm and taking into account the P-0 bond distance to the bridging oxygen atom of 158 pm, a mean P-O-P angle of 137° ± 3° is calculated. The reverse Monte Carlo simulations fit the neutron and X-ray structure factors. The P-O-P angle distribution obtained this way possesses a mean angle of 141°. An interpretation of the first scattering peaks is presented by analysing the occupancy and the distances of various co-ordination shells by use of model configurations. The low occupancy of the first shells allows the application of the schematic hole model of Dixmier. The first X-ray diffraction peak at 13 nm -1 is related to the P-P 2nd shell, the shoulder at 20 nm -1 arises from the P-O 2nd shell. The most similar crystalline structure with vitreous P 2 O 5 is the orthorhombic P 2 O 5 , form II. But it has more effectively orientated terminal oxygen atoms and, thus, a higher packing than the glass.

Electric permittivity measurments as functions of temperature at constant dc electric field and as functions of the dc electric field at constant temperatures in the paraelectric phase have been performed for DMAGaS crystal. The changes of the permittivity maximum with the electric field intensity evidence the first-order character of the ferroelectric phase transition at T c1 . The field dependence of the permittivity ε(E) in the paraelectric phase is discussed, using the classical electric equation of state. A method for the determination of the corresponding coefficients is proposed. The obtained B and C coefficients, together with double hysteresis loop observed above T c1 prove the first-order character of the ferroelectric phase transition in DMAGaS crystal.

In this paper we analyse the sequences of the time intervals between heart-beats-the RR intervals-by means of AIP (artificial insymmetration patterns) diagrams. The sequences were produced by artificial heartbeat sequences generated numerically and compared with sequences obtained from real heart activity. We hope that the AIP diagrams method will prove useful for a rapid qualitative assessment of dynamics from nonlinear time series, and that it is able to distinguish various types of heart dynamics (regular and pathological), while other diagnostical methods fail.

We investigate electroconvection in nematic liquid crystals by means of optical microscopy. Time resolved optical images are used to study the director dynamics. For the first time we present instant images in the dielectric regime. A numerical simulation of the optical transmission patterns is performed on the basis of Fermat's principle. In the instant images of dielectric rolls, the periodicity of the observed optical pattern is equal to the wavelength λ 0 of the convection rolls. The well known low contrast 'stationary' optical texture observed in conventional experiments results from time averaging of these instant images; its wavelength is λ 0 /2.

Amorphous Si 37 C 32 N 31 and Si 37 C 29 N 34 ceramics were produced by pyrolysis of a polyhy-dromethylsilazane precursor. Their structure was investigated by X-ray and neutron diffraction. Wide angle diffraction showed that the Si-atoms are preferentially bonded to nitrogen atoms, but also bonding to carbon atoms was found. This suggests that the excess carbon atoms form an amorphous graphite-like phase. Small angle scattering revealed that the ceramics are inhomogeneous. The evolution of the phase separation during annealing was investigated and it was concluded that amorphous Si 3 N 4 precipitates grow in the Si-C-N materials. The results are compared with previous results for amorphous Si 24 C 43 N 33 produced from a polysilylcarbodiimide precursor [1 - 3].

The results of dielectric studies of 6BAP(F) (1-[4-(hexylbicyclo[2,2,2]octyl]-2-(3-fluoro-4-methoxy-phenyl)ethane) in the nematic and isotropic phases are presented. The substance has a negative dielectric anisotropy. By applying two experimental techniques, using a network analyzer and time domain spectrometer (TDS), the two main relaxation processes, connected with the molecular reorientation around the short and long axes, respectively, were resolved in the phases studied. The activation barriers hindering the motions were obtained. By extrapolation of the longitudinal and transverse relaxation times from the isotropic to the nematic phase the retardation factors, g ∥ and g ⊥ , and the nematic potential versus temperature could be calculated. These are discussed together with the order parameter S obtained from the refractive index, and are compared with the predictions of the mean-field theories.

The Maier-Saupe theory is employed in order to calculate order parameters S from the nematic potential q. It is found that one of the basic assumptions of the Maier-Saupe theory, q~S, is approximately fulfilled. The relation between q and S is analysed for various state changes. Previously reported findings for 7 PCH that q~S, not fulfilled along isochoric changes, can be explained by taking into account the pressure and temperature dependences of q. The procedure described in this paper allows to treat experimental data for the nematic potential in a unique way, without being affected by inadequacies of experimentally determined order parameters.

The dielectric spectra up to 72 GHz have been measured at 20 °C for solutions of glucose and sorbitol (max. 1 mol/1) in the mixed solvent water/tetrahydrofuran (THF) as a function of the THF content (max. volume fraction 0.4). There is no significant difference in the relaxation behaviour of corresponding glucose and sorbitol systems. The spectra of binary water/THF as well as hydroxy compound/water systems show that the respective minor components influence the structure and dynamics of water. The ternary systems hydroxy compound/water/THF, however, exhibit merely a superposition of those effects, thus there is no evidence for an interaction between the hydroxy compound and THF in aqueous environment.

The present paper details the way to determine the cation distribution in mixed oxides with transition ions from the diffraction and magnetic susceptibility data. This approach allows one to determine phase abundances and phase compositions by two combined procedures. By X-ray diffraction Rietveld profile refinement and magnetic susceptibility data analysis it is possible to estimate the ratio and the occupancy factor of paramagnetic ions in different oxidation states. A brief discussion of practical cases is reported. In the Li-Ni-O system the lithium cationic fraction of the ordered phase Li 2x Ni 2-2X O 2 increases monotonically with the total lithium fraction x t for 0.31 < x t < 0.46. In the Li-Mn-O system for 0.36 ≤ x t ≤ 0.53 the Li 2 MnO 3 fraction has been determined, and for the coexistent Li [Li y Mn 2-y ]O 4 spinel phase the dependence of y on x t has been evaluated.

The glass transition caused by a finite cooling rate is a continuous non-linear dissipative process whose description requires a clear distinction between equilibrium and non-equilibrium quantities. The so-called Davies or Prigogine-Defay relations (in form of an equation as well as in form of an inequality) are not relevant in such a process. The determining quantities of the glass transition are -from a macroscopic phenomenological point of view -the fluidity of the melt and the partial free enthalpy of the microscopic vacancies in the melt. All of the characteristics of the dynamics of the glass transition are essentially due to these two quantities.

A chemical reaction network can admit multiple positive steady states if and only if there exists a positive steady state having a zero eigenvalue with its eigenvector in the stoichiometric subspace. A zero eigenvalue analysis is proposed which provides a necessary and sufficient condition to determine the possibility of the existence of such a steady state. The condition forms a system of inequalities and equations. If a set of solutions for the system is found, then the network under study is able to admit multiple positive steady states for some positive rate constants. Otherwise, the network can exhibit at most one steady state, no matter what positive rate constants the system might have. The construction of a zero-eigenvalue positive steady state and a set of positive rate constants is also presented. The analysis is demonstrated by two examples.

Fundamental scientific reasons are advanced for preferring the formulation of the THE Rule offered in a Note of 1996 to that employed by Rooney in a Note of 1997. These illustrate that, whereas Transition State theory has been of considerable service to the development of chemical kinetics, it has not been of any utility in regard to issues of thermodynamics