Volume 50, Issue 9

We present simple description of a system with two bilinearly coupled order parameters in external field based on an exact mean-field solution of the Landau Hamiltonian. It reproduces the qualitative form of the "field-temperature" phase diagram given by a molecular-field model and by more sophisticated theories and experiments on metamagnets. The solution gives the same critical exponents as the molecular-field theory, but it is not restricted to the magnetic systems only and it is easier to handle, since it formulates the results in explicit analytical form. The susceptibility in this model does not diverge at the second order transition line (far from a higher order critical point separating the second and first order transition lines), but jumps down from the lower temperature wing to the higher temperature one. The jump amplitude is proportional to the square of the field in small fields and diverges in large fields close to the higher order critical point.

A continuum theory of a biaxial nematic phase in liquid crystals is presented. The liquid crystalline ordering is described by a field of three unit vectors. Because these are macroscopic directors, the present approach is a generalization of the Ericksen-Leslie theory to biaxial nematics. The applied tools from nonequilibrium thermodynamics are classical Linear Irreversible Thermodynamics and Gyarmati's variational principle.

In this paper we study the nonlinear Rayleigh-Taylor instability in the case of magnetic fluids in the presence of heat and mass transfer. We find that there is a normal field instability in the linear case. The behaviour of the bubbles in the nonlinear analysis is the same as if they were leaving the surface when the liquid is superheated. The criterion for "Explosive Instability" is also examined.

Von Laue reformulated Ewald's dynamical theory of X-ray diffraction in crystals by solving Maxwell's equations for an electric susceptibility which has the periodicity of the crystals lattice. Absorption was in the original paper not included. Kohler showed that in this case the electric susceptibility is directly proportional to the electron charge density within the crystal. Von Laue assumed that the divergence of the electric field vector E is equal to the electron charge density which is excited by the electromagnetic field, and this is in accordance with Lorentz' classical electron theory. Under this assumption E is in contrast to D not a transverse wave. This is the reason that von Laue derived the fundamental equations in terms of D, and in order to obtain a simple wave equation he used the approximation 4πP = ϰD. Here P is the vector of the electric polarization and ϰ is 4π times the electric susceptibility. This approximation is well justified, and von Laue's formulation is used in most publication on this subject. However some authors derived the fundamental equations in terms of fusing the correct relation 4πP = ϰE . In most publications they assumed that the longitudinal component of the vector E is small and neglected it. This point and the comparison with von Laue's formulation was discussed by Miyake and Ohtsuki. We show here that the divergence of E can be put zero if inelastic processes are excluded. This follows from Kohler's and Moliere's papers on the wave mechanical foundation of the dynamical theory of X-ray diffraction. However if higher perturbation terms are taken into account then the divergence of E differs from zero. In multiple beam cases the fundamental equations are much simpler if they are expressed in terms of E. In the region of anomalous dispersion, and here in particular in the region where the X-ray energy is close and above and crystal absorption edge, the wave equations should be expressed in terms of the electric field vector E.

This paper introduces a method of calculating theorectical alpha coefficients for the X-ray fluorescence analysis of major and minor components in iron-rich samples. We choose a group of hypothetical standard samples whose average concentrations are those of the actual samples. The theoretical X-ray fluorescence relative intensities of the given components are calculated using the fundamental parameter NRLXRF program. We derived formulas from the Lachance-Traill equation and used these to calculate the basic, hybrid and modified alpha coefficients which are used respectively for the analysis of elements in compact specimens, oxides in compact specimens and oxides in diluted specimens. In order to use the theoretical alpha coefficients on-line, we also discuss the calculation of the alpha coefficients used in the D.J model.

The pressure-dependence of the dielectric anisotropy, Δε = ε 0|| - ε 0⊥ , of two nematogens, n-pentyl-and n-hexyl-cyanobiphenyl (5CB and 6CB) has been measured at constant temperature. For this purpose a high pressure chamber made of non-magnetic materials was constructed, allowing for the orientation of the samples by a magnetic field. The pressure dependencies of Δε T were analyzed in the frame of the Maier-Meier equations for the principal permittivity components in the nematic phase. It was found that the relation between Δε and the order parameter S is not properly described by these equations. The Kirkwood correlation factor calculated on the basis of the present results and the Maier-Meier equations is smaller than 0.5, whereas it increases to a more realistic value (~0.7) if the electronic polarization is incorporated in the equations.

The molten alloys Au 28.5 Mn 71.5 and Au 68 Mn 32 are investigated with the energy dispersive X-ray diffraction method which works rather fast so that the evaporation loss of Mn from the molten alloys is kept low. From the observed prepeak follows that both melts are compound-forming but the gold rich melt Au 68 Mn 32 shows only 50% of the short range order existent within the Au 28.5 Mn 71.5 melt. Total structure factors and total pair correlation functions are discussed.

The annealing history of glasses in the system (20-40) BaO-40TiO 2 (40-20)SiO2 has been mon­itored by 29 Si MAS NMR and FTIR spectroscopy from the dried gel to the final glassy state. Assignment of chemical shifts to specific building units has been facilitated by comparative studies of the limiting less complex binary silicate systems. From the NMR spectra at various compositions appreciable condensation to a three-dimensional network is inferred already at low drying temper­atures. Processes like the pyrolysis of acetate rests of the starting materials and modification of the silicate structure due to Ba 2+ cations have been mirrored in the spectra of both methods for samples annealed at intermediate temperatures. The microstructure of the final gel glass of the ternary system has been determined spectroscopically to consist of silicate and titanate species typical of crystalline fresnoite, whereas remaining amounts of TiO 2 and SiO 2 develop a separate network each. Only small amounts of Si-O-Ti linkages have been recognized in the vibrational absorptions; they vanish at higher annealing stages.

NMR measurements of the oil mass flow have been carried out in all-oil flow as well as in oil-nitrogen two-phase flow. For the calibration a simplified method was used which circumvents spin-lattice relaxation calculations due to the short polarization time of the protons at rather high flow velocities. With this method an overall accuracy of the oil-flow in oil-nitrogen two-phase flow of about ±5% was achieved. Some proposals for an improvement of the NMR apparatus are made, which would render the evaluations simpler and more accurate.

This paper reports the double resonance signal observed by combination of a cw CO 2 laser with a pulsed uv ionisation technique (REMPI). The infrared laser was used to pump a single ro-vibrational transition of the ν 1 band of CF 3 I, and the resulting change in the two-photon, vuv spectrum was observed. Signals were detected under conditions of mass selectivity using a time of flight mass spectrometer. Previously unpublished data from infrared-microwave double resonance experiments are also included.

Alkanoid systems are chemical graphs which can be embedded in the diamond lattice. They correspond to the staggered conformers of alkanes. Non-overlapping and overlapping systems are defined. The previously deduced algebraic solution for the numbers of unbranched alkanoids is revisited and reformulated in terms of generating functions. Computerized enumerations for non-overlapping alkanoids, both unbranched and branched, are reported. Some of the smallest overlap­ ping unbranched alkanoids are generated by a method called combinatorial constructions.

The formation of tyrosine isomers by γ-radiolysis of neutral aqueous phenylalanine solutions was found to be strongly dependent on oxygen concentration and temperature. Changing the dose rate did not influence the degradation process. In the presence of 0.25 x 10 -3 mol dm -3 oxygen at room temperature the yields of o-tyrosine as well as of m- and p-tyrosine drop from G(o-Tyr) = 0.5 and G(m-Tyr) = G(p-Tyr) = 0.4 at a dose of 0.3 kGy to 0.18 and 0.16 at 2.5 kGy, respectively. In solutions containing 1.25 x 10 -3 mol dm -3 oxygen the initial yields remain unchanged but decrease at 2.5 kGy only to G(o-Tyr) = 0.3 and G(m-Tyr) = G(p-Tyr) = 0.20. Under the latter reaction conditions also 3,4-dihydroxyphenylalanine was found. Samples irradiated in frozen state did not show remarkable radiolysis of phenylalanine and tyrosine formation. In the range between 5 and 20°C no essential influence of temperature on the phenylalanine radiolysis and tyrosine yields was observable. The obtained results are important for methods using the tyrosine yields as markers for the detection of irradiated food. Storage conditions and irradiation temperature play an essential role on radiation induced changes of food.

The temperature dependences of the NMR spin-lattice and spin-spin relaxation times and the NMR spectra of 15 N in TI 15 NO 2 were observed in the plastic phase (282.4 K < T < 459 K) and the low-temperature phase. From the analysis of 15 N NMR data, we attributed the motion reported in dielectric and TI NMR studies in the low-temperature phase to the 180°-flip of NO 2 - ions which takes place with an activation energy of 14 kJ mol -1 . In the plastic phase, we detected isotropic NO 2 - reorientation with an activation energy of 13.5 kJ mol -1 and a new anionic motion with 10 kJ mol -1 .

The local structure of CH 3 NH 3 I around the I - ion in Phase α' and δ was studied by iodine K-edge EXAFS. The crystal structure in Phase α' was redetermined by single crystal X-ray diffraction. The Debye-Waller factor σ (2) derived from EXAFS spectra shows an anomalous behavior around the order-disorder transition temperature from Phase δ to α'. The phase transition has been interpreted as the disordering process which involves not only the H atoms but also the C atoms in the cation.

The resonance Raman spectra of ( n Bu 4 N)[Fe(CN) 2 TPP] (( n Bu 4 : tetra(n-butyl)ammonium; TPP: m-tetraphenylporphyrin), K 3 [Fe(CN) 6 ] and [Fe(C 5 H 5 ) 2 ]BF 4 have been investigated. A molecular electronic Raman (ER) effect at 545 cm -1 is observed for ( n Bu 4 N)[Fe(CN) 2 TPP]. The transition occurs between lower “Γ 7 ” and upper “Γ 8 ” level of the spin-orbit split ground state assuming pseudo-octahedral symmetry. In contrast to earlier studies no ER effect is detected for K 3 [Fe(CN) 6 ] and [Fe(C 5 H 5 ) 2 ]BF 4 . For Raman shifts < 800 cm -1 only one strong Raman line is observed for [Fe(C 5 H 5 ) 2 ]BF 4 at 311 cm -1 , assigned to the iron ring stretch designated by ν4 (a 1g in D 5d ). Due to a phase transition of K 3 [Fe(CN) 6 ], two additional strong vibrational lines at 329 and 352 cm-1 appear in the Raman spectrum taken at 10 K. Their intensities show A "term" behaviour of the resonance Raman effect with overtones and combinations for both lines. These are components of the vibronic combinations with the ν 6 , ν 7 , and ν 8 modes (t 1u in O h ) in the IR spectra, too.

GHCA undergoes four phase transitions at T c1 = 402 K, T c2 = 373 K, T c3 = 162 K, and T c4 = 146 K. Below T c3 it possesses pyroelectric properties with the spontaneous polarization vector (Ps) in the ac plane and the maximum along the c axis equal to 8 μC/m2. Dielectric dispersion studies of GHCA show that the main dielectric dispersion connected probably with collective motions of chlorine ions is above 1GHz. For the phase transition at T c2 to a superionic phase the thermal dilatation and electric conductivity were measured. The anomalies of the electric conductivity at T c2 and T c1 were observed with large values of σ(10 -3 S/m) above T c3 . The guanidinium cations above T c2 , besides reorientational motions, undergo slow self diffusion.

The phase diagrams of the binary mixtures of mesogenic potassium iso-butyrate with non-mesogenic lithium-, sodium-, and caesium iso-butyrate have been investigated by differential thermal analysis and hot stage polarization microscopy. The temperature and concentration ranges of liquid crystal formation have been established. Sodium and caesium iso-butyrate have been found to possess latent mesogenic properties.