Table of contents

Measurements of the dielectric permittivity are reported for a solid solution of antiferroelectric betaine phosphate and ferroelectric betaine phosphite at frequencies . The freezing phenomena in reveal the characteristics of a transition into a dipolar glass state. The activation energy was found to be K and an external bias field lowers this value to 513 K.

Dynamic size effects in a lattice gas model with two-vacancy-assisted hopping are studied by Monte Carlo simulation. A characteristic length of cooperativity is derived from the slowing down of the relaxation of the site-occupation autocorrelation function and of the orientation autocorrelation function of non-spherical particles in confining geometries. Good agreement is found with the cooperativity length obtained previously by measuring the size dependence of the fraction of permanently blocked particles.

Nematic liquid-crystal wetting at a solid interface presents particularly interesting features when the nematic director orientation at the solid interface is antagonistic to that favoured by the emerging nematic - isotropic interface. We have used the Landau - de Gennes theory of an inhomogeneous liquid crystal to make a quantitative study of this phenomenon, for the case when the solid surface favours homeotropic anchoring but the nematic - isotropic surface favours planar anchoring. By generalizing the theory of Sheng to allow spatial variation of the director, we find a richer surface phase diagram exhibiting a prewetting line or boundary transition shifted from that discussed by Sheng, as well as a transition between two nematic wetting phases, at which the wetting layer director profile changes from a homeotropic to a distorted texture.

Differential scanning calorimetric (DSC) studies have been carried out in an attempt to explain the composition dependence of the thermal crystallization behaviour of glasses. It is seen that these glasses exhibit a single glass transition and single-stage crystallization on heating. The glass transition temperature, crystallization temperature and the activation energy for crystallization are each found to exhibit an anomaly at around the composition x = 5. The anomalies observed in the thermal properties at x = 5 can be associated with rigidity percolation.

Kinematic viscosities and densities of in aqueous solutions, from pure water to very concentrated solutions, were determined at different temperatures. Shear viscosity data for this asymmetrical 3:1 electrolyte were calculated. For the low concentrations , the Jones - Dole coefficients A and B were determined for this rare-earth salt by fitting the quadratic behaviour of the square root of the molar concentration with the least-squares method. At concentrations above 1 mol , the logarithm of viscosity can be fitted by a third-order polynomial, where the coefficients exhibit a linear dependence on the reciprocal of the absolute temperature. This phenomenon is due to the strong ion - solvent correlation.

The practical importance of the leakage phenomenon of a liquid into a vessel is emphasized. The physics of the same is theoretically elucidated by incorporating the Young - Dupre relation, the Gibbs inequality, Poiseuille's flow, and the conditions for hydrostatic equilibrium.

We have used a differential effective-medium (DEM) theory to model the dielectric properties of sintered glass beads, impregnated with salty water, in the kHz and MHz ranges. The starting point of the theory is a lattice of touching spheres in pore liquid. The sintering is modelled by adding solid material and using the DEM method to calculate the dielectric response. Dipolar interactions in the system are treated by using effective depolarization factors, obtained from the dependence of the formation factor on porosity. The frequency dependence of the formation factor is in good agreement with experimental data. For the real part of the dielectric permittivity, discrepancies arise at low frequencies. This is probably due to percolation effects.

We report in this paper the measurement of the Kerr constant for six mixtures of polar liquids: acetone - quinoline, acetone - isopropyl benzene, acetone - nitromethane, acetone - water, acetone - acetic acid and water - t-butyl alcohol. The Kerr constant was measured at the wavelength of 632.8 nm using a pulsed electric field technique. A deviation from the pure additivity of the Kerr constant was observed for all the mixtures. This non-additive behaviour is discussed in terms of the refractive index, dielectric constant and molar volume of the components. We also include the fitting of our data to an equation for mixtures of polar liquids derived from a modified mean field theory, in which the interaction between the permanent dipole moments of the two liquids of the system is included. We also report for the first time the measurement of the constant B, at 632.8 nm, for four pure liquids: quinoline, t-butyl alcohol, isopropyl benzene and nitromethane, using the technique of short-pulse electric field.

The effect of water thickness on the mean curvature modulus of an inverted bilayer has been investigated. Our system is a lamellar phase made up of a series of water films each surrounded by two ionic surfactant monolayers and separated with hydrophobic solvent. The elastic constant has been estimated using the excess-area method and it is shown to decrease when the thickness of the water layer increases. This result cannot be interpreted in terms of electrostatic effects - an increase of is expected instead - but may arise from a coupling between short-range surfactant interactions and curvature strains.

The ordered structure of at 70 - 75 K has been studied by time-resolved neutron powder diffraction. The ordered superlattice has a structure with approximately four full D sites and one vacant D site in successive (420) planes, which is consistent with previous observations. Several superlattice lines not reported in previous single-crystal studies were observed. Quantitative information about the ordered structure was obtained by careful Rietveld analysis of the data, including anisotropic line broadening and diffuse peak background corrections. The structure refinement confirms a relaxation of D atoms towards the vacant planes and reveals a related second-order displacement of the D atoms in the octahedral sites towards a preferred vacant nearest-neighbour site. The thermal vibration along the c-axis is anisotropic as a result of the preferential relaxation.

Aluminium samples in the form of single crystals were treated by uniaxial tensile strain, so that dislocations were practically the sole type of defect produced. A positron lifetime study of these samples, carried out at different dislocation densities, and in a wide temperature range, gives information about the mechanism of positron trapping, including trapping rates, binding energy and the development of defect density with deformation and applied stress.

We study scaling properties of the localized eigenstates of the random dimer model in which pairs of local site energies are assigned at random in a one-dimensional disordered tight-binding model. We use both the transfer matrix method and the direct diagonalization of the Hamiltonian in order to find how the localization length of a finite sample scales with the localization length of the infinite system. We derive the scaling law for the localization length and show it to be related to scaling behaviour typical of uncorrelated band random matrix, Anderson and Lloyd models.

Starting from an expression for the double-differential cross section for electron Compton scattering in a crystal for a two-beam case, valid for a small Ewald sphere, the contributions of M-shell and L-shell electrons in silicon to the mixed dynamic form factors are calculated. The shape and intensity of the interference effects that superimpose on the Compton profile are determined. It is shown that the appearance of interference terms renders the cross section asymmetric and dependent on the momentum transfer also in the usual momentum representation. A procedure for an experimental investigation of the interference terms is proposed.

We study the ground state of the one-band SU(N) Hubbard model on a square lattice in the leading order of a 1/N expansion using a Baym - Kadanoff expansion and the X-operator formulation. We find that the homogeneous paramagnetic solution can become unstable against phase separation due to next-nearest-neighbour hopping.

Gd EPR measurements on the heavy-fermion system are reported. In agreement with NMR results, we observe a heavy-fermion behaviour for and the opening of a spin gap below . Our data reveal an almost constant dynamical susceptibility for , followed by a Curie - Weiss-like decrease for higher temperatures. The temperature dependence of the magnetic relaxation rate is calculated and compared with that derived from neutron scattering experiments.

The thermoelectric power above room temperature of the unconventional lead vanadate glass is reported. It has been observed that the thermoelectric power shows weak variation with temperature and its magnitude decreases with increase in content in the glass compositions. Heikes' formula for the thermoelectric power can give an adequate explanation for the experimental data. Thermoelectric power data also give information for the formation of small polarons in these glasses.

In this paper we report the crystal structure and superconductivity of (Ln= Pr, Sm, Eu, Gd and Dy). The lattice parameters of these compounds decrease nearly linearly with the decrease of the average ionic radius of the rare-earth element (Ln, Ce), and the superconducting transition temperatures of these compounds increase as the ionic radius of the rare-earth element Ln decreases from Pr to Gd. has the highest -values K, K) which, to our knowledge, are also the highest -values reported for any 1222 phase. The superconductivity of the system is affected by many factors: (i) the coupling effect between planes; (ii) the charge-carrier concentration in the planes; and (iii) the degree of buckling of planes.

The ternary compound , which has the monoclinic structure, has ben successfully prepared from a starting composition of by annealing at for 3 d. The Gd:Fe:Ti ratio for 3:29, determined by energy dispersive x-ray analysis (EDAX), is 9.1:85.7:5.2. Thermo-gravimetric analysis gives a magnetic ordering temperature of 524(3) K for . The lattice parameters, determined by refinement of powder x-ray diffraction data, are a = 10.6409(16) Å, b = 8.5658(9) Å, c = 9.7375(15) Å and . Mössbauer measurements on yield an average hyperfine field of 24.4(2) T at 293 K, which corresponds to an average Fe magnetic moment of about .

The PLZT 8/65/35 compound, commonly known for relaxor behaviour, as well as PLZT 8/65/35 + Me compositions elements (Mn, Fe, Co or Cu)) is examined, a number of different techniques being used:

(1) dielectric measurements of complex permittivity at frequencies from to Hz in the range of temperatures from 0 to ;

(2) Raman scattering (RS), EPR and x-ray diffraction.

The behaviour of is interpreted assuming the existence of three different polarization mechanisms, the partial contributions to the dielectric constant of which depend on the dopant concentration and the ranges of frequencies, temperatures or field intensities. Structural criteria are proposed to evaluate the contributions from each of the mechanisms.

The tetragonal-to-cubic transition in ferroelectric (BCT) has been studied using a single-crystal neutron diffraction technique. The intensities of the Bragg profiles are observed to increase anomalously with temperature above are then to drop sharply before reaching the structural transition temperature . The intensity variations and are sensitive to sample heating rate with at slow heating rates. Structural refinements indicate that the observed intensity variations are due to changes in the domain structure and the thermal parameters B with the latter becoming anomalously large several degrees Celsius below compared with those above and at room temperature, indicating the onset of significant pre-transition disorder for Ti ions along the polar c axis. The observed linewidths of the diffraction profiles reflect contributions from the anisotropic domain sizes and the internal strains and an added contribution from the coexistence of tetragonal and cubic phases below . The results suggest that the fluctuations in the Ti positions existing well below could be responsible for the broadening of dielectric response observed for BCT.