Panos Photinos

Dynamical Bragg diffraction from crystalline colloidal arrays

Polystyrene spheres with attached functional groups that ionize in solution repel one another; at sufficiently high sphere concentrations the spheres self‐assemble into a crystalline lattice with lattice constants large enough to diffract visible light. We have experimentally and theoretically examined diffraction phenomena from colloidal crystals of polystyrene spheres of diameters between 69 and 127 nm in water. We relate the diffraction bandwidths to the sphere scattering powers in the context of the dynamical diffraction theory and demonstrate the importance of the dynamical theory for predicting the observed diffraction angles, intensities, and bandwidths. We also discuss the mechanism contributing to the diffuse scattering and show the significance of the coherent scattering by lattice phonons.

Fresnoite: A new ferroelectric mineral

Fresnoite, Ba2TiOSi2O7, has been predicted structurally to be a new ferroelectric. Detection of both ac and dc dielectric hysteresis verifies the prediction. The spontaneous polarization Ps estimated from the hysteresis at 1.2 MV m−1 ac is ∼0.2 C m−2 at 295 K, comparable with the minimum Ps observed in one-dimensional ferroelectrics. A reproducible calorimetric anomaly with entropy change 0.19(3) J mol−1 K−1 at 433(2) K in polycrystalline material coincides with a small dielectric and pyroelectric anomaly previously reported in single crystals; an entropy change ∼0.2 J mol−1 K−1 at 810(5) K also accompanies a dielectric anomaly observable in ceramic samples at 805(5) K. Both calorimetric anomalies are ∼60 K wide. Observation of dielectric hysteresis at 875 K shows that neither anomaly corresponds to the Curie temperature; both are likely associated with small changes in atomic position, not with symmetry changes. Melting onset in fresnoite is 1703(1) K with undercooling as deep as 435 K.

Measurements of density and expansion coefficient for the nematic, lamellar smectic, and isotropic phase of micellar systems

Measurements of the density and expansion coefficient as a function of temperature are presented for two micellar systems, namely, decylammonium chloride/NH4Cl/water, and cesium perfluorooctanoate/water. The measurements cover the lamellar smectic, the nematic, and the isotropic micellar solution. The measurements show that away from the phase transitions, the density decreases smoothly with increasing temperature. At the nematic‐to‐isotropic transition the density shows steplike features, indicating a change in the micellar structure at the transition, with denser packing of the amphiphiles in the micelles of the isotropic phase. A similar change is observed in the lamellar to nematic transition, however, the detailed behavior at this transition showed some dependence on the history of the sample. The expansion coefficients decrease at the transitions. Considerable pretransitional behavior is observed about the lamellar‐to‐nematic transition.

Measurements of the conductivity and relaxation times for the micellar nematic phase of the system ammonium perfluorononanoate/H2O

Measurements of the electric conductivity for two mixtures of ammonium perfluorononanoate and H2O are presented. The molar ratios for the two mixtures are 0.0159 (sample S2) and 0.0112 (sample S3). The mixtures show a nematic (NL) range about 8 °C wide, between a lamellar smectic and an isotropic micellar solution. The conduction is predominantly due to the NH4+ ions, and it is estimated that about 50% of the NH+4 ions have free mobility. At the same reduced temperature in the NL phase, the lower concentration mixture is more anisotropic, indicating larger micelles for the lower concentration. The reorientation of the director of the NL phase in a magnetic field is also measured. Near the NL to isotropic transition the relaxation times at 10 kG are 30 s for S2 and 80 s for S3. The divergence behavior of the relaxation times at the NL to lamellar transition is different for the two samples. Also, at corresponding temperatures, the relaxation time for the lower concentration is longer than the high concentr...

Micellar structures in lyotropic liquid crystals and phase transitions

The formation of micellar nematics is discussed with emphasis on the transitions between nematic phases and nematic-smectic transitions. Phase diagrams for MTAB/l-decanol/D,O systems show a direct transition between uniaxial nematics. Electrical conductivity and birefringence measurements on a mixture of sodium decylsulfate. 1-decanol, D,O demonstrate, on the other hand, the existence of a biaxial nemantic range that separates the Uniaxial nematics. On a mixture of cesium perflouroctanoate and H2O the electrical conductivity and rotational viscosity are used to discuss the relevant features of nematic-lamellar-smectic transitions. The formation of elongated ribbon-like micelles at the nematic-smectic transition is suggested. Transitions between different nematic phases in the MTAB system may be connected with a structural change from long micelles with a fairly circular cross section to similar micelles with a more elliptical cross section.

Electric conductivity measurements on the nematic states of a micellar solution of potassium laurate/1‐decanol/D2O

We present electric conductivity measurements on a mixture of 25.15 potassium laurate/6.33 1‐decanol/68.52 D2O by weight. The mixture shows two isotropic phases, two uniaxial nematics, and a biaxial nematic phase. The variation of the conductivities with temperature shows the phase transitions, which were also optically verified. The average conductivity does not change at the phase transitions. It shows a temperature dependence that is almost fully due to the effect of the viscosity of the aqueous phase on the ion mobility. The anisotropy of the conductivity in the uniaxial nematic phase is relatively weak. An analysis of the conductivity in terms of micellar structures suggests relatively small micelles of ellipsoidal shape. The reorientation of NC in magnetic fields of 10 kG has relaxation times of 20–40 s. The rotational viscosity is in the order of 10 P.

Phase transitions in K2Cr2O7 and structural redeterminations of phase II

Crystals of phase II K2Cr2O7, potassium dichromate, space group P1 , grown from aqueous solution undergo a first-order transition to phase I, space group reportedly P21/n, at a phase-transition temperature, TPT, of 544 (2) K on first heating; the corresponding transition on cooling is at 502 (2) K. The endotherm on subsequent heatings occurs reproducibly at TPT = 531 (2) K. Mass loss between ca 531 and 544 K, identified as included water, is rapid and continues more slowly to higher temperatures for a total loss of ca 0.20%. The higher TPT on first heating is associated with the increasing pressure of superheated water occupying inclusion defects. The latent diagonal glide plane in phase II allows the structure of phase I to be inferred. The triclinic structure at 296 K has been independently redetermined. Normal probability analysis shows high consistency between the resulting and previous atomic coordinates, but with uncertainties reduced by a factor of ca 2. The earlier uncertainties are systematically underestimated by a comparable factor. The structure of phase IIb, space group A2/a on transposing axes, was determined at ca 300 K by Krivovichev et al. [Acta Cryst. (2000), C56, 629-630]. The first-order transition between phases I and II arises from the ca 60 degrees relative rotation of terminal O atoms in each tetrahedron as the n glide plane is gained or lost. A transition between phases IIb and I, also of first order, is likely but not between phases II and IIb. An intermediate phase may exist between phases IIb and I.

Critical properties of the uniaxial–biaxial transition in micellar nematic phases

The critical properties of the second order uniaxial–biaxial nematic transition (NL–Nbx) of the potassium laurate/1‐decanol/D2O system were studied on surface and magnetic field aligned films of 0.1 and 0.5 mm thickness. The biaxial order was measured as a function of temperature and field. We find deviations from mean field behavior in a 20 mK range below the transition NL and in a temperature range of at least the same extent above the transition Nbx. The values obtained for the critical exponents of order parameter (β) and susceptibility (γ) in the uniaxial range are in good agreement with the values calculated for the xy model. A reliable value for the susceptibility exponent in the biaxial phase could not be obtained because the field range for which the effect is proportional to H2 is very small and the low field limit of the susceptibility could not be determined. From high field measurements we obtain a value of 4.0 as the lower limit for the exponent of the critical isotherm (δ). The result confi...

Calculations on the electric conductivity of a lyotropic mesophase with perforated lamellae

We calculate the electric conductivity of a system consisting of parallel layers of insulator, embedded into a uniform conductor. Each layer is perforated by holes, which are arranged on a two‐dimensional hexagonal lattice. We investigate various stacking patterns of the layers, in volume fractions of 40% to 50%. The results are used to interpret conductivity measurements on the lamellar lyotropic mesophase. We find that on the basis of conductivity anisotropy, a layered structure of disk‐shaped aggregates, and a structure of perforated layers are not readily distinguishable. The analysis of experimental data indicates that the amphiphile layers in a lamellar phase can be perforated to about 20% of the area. Also, the interpretation of the conductivity data permits evaluation of the periodicity of the layered structure.

Calculation of the electric conductivity of the hexagonal lyotropic mesophase

The electric conductivity of the hexagonal lyotropic mesophase is evaluated numerically, as a function of the volume fraction. It is assumed that the conductivity is solely due to the mobile ions in the aqueous phase, and that the ion concentration and mobility are uniform throughout the aqueous phase. The conductivities are then evaluated for the principal directions by integrating Laplace’s equation numerically. The calculated values for the anisotropy in the conductivity are compared with experimental findings on flow‐induced conductivity anisotropy.