Conoscopic image of a biaxial nematic phase in a sodium decyl sulfate – decanol – D2O mixture

ABSTRACT

The nematic phases of a lyotropic system NadS/ decanol/ heavy water are investigated using optical conoscopy and image processing. The phase diagram obtained from these lyotropic materials predicts the occurrence of a direct phase transition, which does not present the biaxial nematic phase, between the discotic (N_D) and calamitic (N_C) nematic phases. A biaxial nematic (N_B) phase is optically characterized and confirmed through conoscopic image, inside the biaxial range, between the two uniaxial nematic phases. Also, their respective transition points are determined by means of image processing. The N_B phase observed here is discussed as part of the nature of the micellar configuration of lyotropic materials which exhibit uniaxial nematic phases.     

Cover Picture: Cryst. Res. Technol. 11/2009

The cover picture shows a lyotropic nematic texture of a liqid crystal near the biaxial nematic – calamitic nematic phase transition. (see pages 1255–1258) (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An optical microscopy study of the biaxial ‐ calamitic nematic lyotropic phase transition

Lyotropic nematic textures are investigated, using optical microscopy, near the reentrant isotropic (I_RE) ‐ discotic nematic (N_D^‐) ‐ biaxial nematic (N_B) ‐ calamitic nematic (N_C⁺) ‐ isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and D₂O. The N_B, intermediate phase between the two uniaxial nematic ones, is characterized by optical birefringence measurements. In this way, using a polarizing microscope and a colour CCD digital camera, the N_B – N_C⁺ phase transition is identified as well as the domain of the N_B phase and N_D^‐ – N_B transition point in accordance with optical birefringence data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of Salt on the Orientational Order in Decylammonium Chloride/Ammonium Chloride/Water Systems

The effect of NH₄Cl on the temperature dependence of deuterium oxide splitting (water order) and proton line width (surfactant order) in decylammonium chloride (DACl), D₂O systems is reported. The molar ratio of DACl to D₂O was held constant at .085 and the molar ratio of NH₄Cl to D₂O was varied from .012 to .029. We find for the above concentration ranges that in the nematic range the relative temperature dependence of the D₂O splitting is little affected by changes on salt concentration. The slopes of the proton line widths as a function of temperature decrease with increasing salt concentration. The results are compared with earlier data on salt effects on order obtained with the disodium cromoglycate system.     

Chemical lap polishing of PbTe and Pb1−xSnxTe crystals

Chemical lap polishing of PbTe and Pb_1−xSn_x Te crystals is investigated. A mixture of potassium hexacyanoferrate (III), sodium hydroxide, glycerol and water is used as a polish. The effect of various parameters of the process on the quality of the surface and on the rate of material removal is studied. The quality of the surface is strongly is strongly affected by the composition of the polish. The roughness increases with rising sodium hydroxide concentration but decreases with rising glycerol concentration. If the molar ratio of sodium hydroxide and potassium hexacyanoferrate (III) exceeds the value 0.5 etch figures are formed. The rate of material removal largely depends on the velocity of the relative movement between specimen and cloth, the rate of supply of polishing solution and on the concentration of sodium hydroxide.     

Measurement of the Refractive Index and Estimate of the Order Parameter in Oriented Liquid Crystals

The refractive indices of a cholesteric liquid crystal, cholesteryl dodecyl carbonate (CDC), and a smectic liquid crystal, p-n-octoxybenzilidene-p-n-butylaniline (OOBBA), have been measured as a function of temperature in the transition temperature regions. The measurements were made on oriented liquid crystals with an Abbe refractometer. By combining the data on ordinary and extraordinary refractive indices, the relative order parameter, α_aS/α, is plotted as a function of temperature. The behavior of the order parameter near the cholesteric to isotropic transition of CDC is similar to a nematic liquid crystal, whereas, the order parameter in OOBBA shows only a slight temperature variation like many solids.     

Chiral SmC* liquid crystals obtained by mixing of SmC and cholesteric phase

The binary mixtures of the substance providing cholesteric phase exhibit a chiral SmC* phase for certain concentration and temperature range. This phase was verified to be ferroelectric. The temperature dependences of the spontaneous polarization and the coercive field were determined for three concentrations. The pitch of the helical structure is approximately indirectly proportional to the molar concentration of the cholesteric substance and varies from 3 μm to about 25 μm in the concentration range from 20 mol% to 5 mol% of the cholesteric substance. For the lower concentrations the sample is unwound in planar samples due to the surface anchoring. Two possible unwound planar configurations can be switched by electric field.     

Local and nanoscopic structure of potassium triflate-doped siloxane–polyoxyethylene ormolytes

Siloxane–poly(oxyethylene) hybrids obtained by the sol–gel process and containing short polymer chain have been doped with potassium triflate (KCF₃SO₃). The local structure of these hybrids was investigated by X-ray absorption spectroscopy near the potassium K-edge. Small angle X-ray scattering was used to determine the structure at the nanometer scale. Results revealed that at low and medium potassium concentration (n = [O]/[K] ⩾ 8, where n represents the molar ratio of ether-type oxygen atoms per alkaline cation) the cations interact mainly with the polymer chains, while at larger doping level (n < 8) the formation of a polyehter:KCF₃SO₃ complex is observed. The nanoscopic structure of the hybrids is also affected by doping. By increasing the doping level, decreasing trends in the electronic density contrast between siloxane nanoparticles and polyether matrix and in the siloxane interparticle distance are observed. At high doping level the small angle X-ray scattering patterns are strongly modified, showing the disappearance of the correlation peak and the formation of a potassium-containing nanophase.     

Study of microstructure in Agx(As0.33Se0.67)100−x chalcogenide glasses

Glasses with general formula Ag_x(As_0.33Se_0.67)_100−x were studied by means of modulated differential scanning calorimetry (MDSC), conductivity and permittivity measurements and atomic force acoustic microscopy (AFAM). In the whole range of doping (x = 2–12 at.% Ag) a phase separation was supposed. The proportional amount of the ion-conductive Ag rich phase increased with the Ag content. A rapid increase (almost four orders of magnitude) in the conductivity was observed in the Ag concentration range 4–8 at.%, which could be associated with the interconnection of this ion-conductive phase. The transition from a hole conductivity (at low Ag concentration) to a mixed ionic-hole conductivity at higher Ag concentration was studied by permittivity measurements.     

Pitch Behavior of the Cholesteric Helix at the Phase Transition Point Cholesteric/Smectic A up to 2500 Bars. Study of Some Cholesteryl n-Alkanoates

Experimental evidence is given, for the first time, for a finite pitch of the cholesteric helix at the phase transition point cholesteric/smectic A (Ch/S_A). This finite pitch changes into an infinite one at a given point on the phase equilibrium line Ch/S_A. Therefore this point is called the pitch infinity point (PIP).

With increasing n-alkyl chain length of the investigated cholesteryl n-alkanoates the pressure coordinate of the PIP is shifted to higher values up to a chain length of 13 C atoms. For the alkanoates with 13,15 and 17 C atoms the same PIP pressure of about 1000 bars was determined.

The Ch/S_A phase equilibrium line of the cholesteryl esters could at best be followed up to 2600 bars. The respective transition enthalpies were measured at normal pressure and the corresponding changes in volume calculated by means of the Clausius-Clapeyron equation.     

Cholesteric Pitch and Blue Phases in a Chiral-Racemic Mixture

The cholesteric and “blue” phases (BP) of a mixture of the chiral and racemic forms of the same compound were studied as a function of composition and temperature. It was found that the clearing point and the cholesteric-S_A transition temperature was independent of composition. The pitch of the cholesteric at a given temperature was inversely proportional to the fraction of chiral material, as was the lattice parameter of BPII. The lattice parameter of BPI did not follow the above rule. No second-order BPI-BPIIb transition was seen.

It is concluded that this mixture is thermodynamically ideal^(6,7) in that the intermolecular interactions between two molecules are independent of their relative chirality. Thus, this system comes close to the theorist's ideal of one in which only the chirality is varied.     

Electronic structure investigation of V1−xNbxO2 mixed oxide by XES and XPS

The mixed oxide system V_1−xNb_xO₂ (x ≦ 0.12) has been studied by X-ray and photo-emission spectroscopy as a function of the composition. The metal L valence band spectra (VL_III and NbLβ_2.15) with increasing Nb concentration change their width and relative band intensity substantially pointing to charge flow and hybridization effects. The XPS valence band for x ≧ 0.04 indicates a total d band shift of 0.4 eV relative to smaller Nb concentrations. The measured core level spectra at higher Nb doping x ≈︁ 0.10 support the expected existence of V³⁺-Nb⁵⁺ pairs. A comparison of different X-ray spectra and the XPS valence band spectrum for x = 0.08 exhibits a more stronger V3d localization relative to Nb4d, favouring charge transfer and bonding variations with Nb doping. Our spectroscopial data confirm the established ideas concerning the electronic structure of rutile-like compounds and its change at the phase transition point.     

Liquid-phase epitaxy of Potassium Niobate

The possibility of potassium niobate films growing by means of liquid phase epitaxy technique was investigated. The physical and chemical analyses of K₂O-V₂O₅-Nb₂O₅ system were carried out. Corundum wafers with {1¯210} and {0001} orientations served as substrates. The grown films were 10–30 μm thick. The grown heterostructures had a sharp, phase boundary line substrate-film. Epitaxial layer perfection was investigated by X-ray and electronographic methods.     

High Pressure Investigations of the Phase Transitions in Cholesteryl Pentanoate and Cholesteryl Hexanoate

At high pressures too cholesteryl n-penta-noate (CH-5) and cholesteryl n-hexanoate (CH-6) exhibit only a cholesteric liquid crystalline phase. Our light reflection measurements up t o 2600 bars yield no indication of a pressure induced smectic phase which was suggested by Tikhomirova and Ginzberg' for CH-5 at about 850 bars and 120°C. The Obtained pressure (P_t)-temperature (T_t) phase diagrams show the expected widening of the cholesteric range with P_t and T_t. In the case of CH-5 the marked crystallization curve besides a melting curve could be explained by the transformation of the cholesteric into a metastable crystalline solid phase.     

Phase Diagram Behaviors for Rod/Plate Liquid Crystal Mixtures

We present a simple theory of rod/plate liquid-crystal mixtures in which the angle-dependent pair interactions are assumed to be of second-rank form. Calculations are reported for varying relative anisotropies of the “rods” and “plates”. Temperature vs. mole-fraction phase diagrams show successive isotropic (I) → uniaxial ( U) and U → biaxial ( B) transitions which are first- and second-order, respectively. For each pair of species there exists a special composition ( x _rod*) for which cooling of the isotropic phase leads directly (and continuously) to a biaxial liquid. As X _rod approaches X _rod* from either side, the first-orderness of the I → U transition (i.e. discontinuities in volume, order parameter, etc.) becomes vanishingly small. Furthermore the transition temperature of the rod-(plate-) solvent is found to be depressed when “doped” by not-too-anisotropic plates (rods) and elevated when doped by sufficiently anisotropic plates (rods). These behaviors are explained in terms of simple excluded volume considerations and compared with recent experimental data on rod/plate mixtures.     

Phase Transition Studies in Mixed Polymesomorphic Liquid Crystals with Cholesteric and Smectic A Phase

Cholesteric mesomorphism was induced in a polymesomorphic liquid crystal with nematic and smectic A phases by mixing it with a low pitch cholesteric. A variety of mixtures for a wide range of concentrations were studied. It was observed that the addition of cholesteric liquid crystal disrupts the smectic phase of the parent polymesomorphic liquid crystal. The measured valuses of ultrasound velocity and specific volume and their related parameters were found to exhibit anomalies near each of the phase transitions. The mean interaction parameter $ \bar \alpha $ was estimated for different mixtures and was found to decrease with increasing concentration of cholesteric liquid crystal in the mixture. It was observed that smectic A-cholesteric transition becomes second-order at a particular concentration in agreement with the theoretical predictions.     

Synthesis,properties, and structure of potassium titanyl phosphate single crystals doped with hafnium

Single crystals of potassium titanyl phosphate doped with hafnium are grown by spontaneous flux crystallization. Their physical properties are studied, and the structure of three KTi_{1 − x} Hf _x OPO₄ crystals (x = 0.01, 0.03, and 0.12) is determined. In the crystals studied, hafnium mostly occupies the second titanium position. The doping of KTP crystals with hafnium results in an elongation of K-O bonds in the potassium polyhedra and, as a consequence, in a considerable (by approximately 180°C) decrease in the temperature of ferroelectric phase transition. The magnitude of anomalous permittivity substantially decreases. The electrical conduction in the specimens studied decreases by approximately half an order of magnitude in the low-temperature region but remains almost unchanged in the high-temperature region. Even at minor concentrations, the presence of a hafnium additive in the specimens considerably (by 35%) enhances the intensity of the second harmonic generation of laser radiation.     

Dielectric Properties of Beta Barium Borate and Potassium Pentaborate Single Crystals

Single crystals of beta barium borate and potassium pentaborate (commonly known as β-BaB₂O₄ or BBO and KB₅O₈. 4 H₂O or KB5) were grown by high and low temperature solution growth methods, respectively. The dielectric constant and loss tangent were determined as a function of temperature. Frequency response of the dielectric constant and tan δ have been studied over a frequency range of 0.1 −100 kHz; it is found that both the dielectric constant and tan δ decrease with increase in frequency for both crystals. The dielectric constant and tan δ are found to increase with increase in temperature for both the crystals. Further, the dielectric constant and tan δ measured on different orientations of the grown crystals reveal the anisotropic nature of the crystals.     

New chalcone based liquid crystals with allylidene amino linking unit: Synthesis and characterization

In this study, the synthesis, structural characterization and mesomorphic properties of newly thirteen calamitic shaped compounds derived from allylidene amino chalcone and 4-n-alkoxy benzoyloxy benzoic acid. Comp.H₁ to H₃ exhibited nonliquid crystalline nature, while comp.H₄ to comp.H₁₈ displayed enantiotropical smectic C phase. Phase transition temperatures of present synthesised compounds were determined by optical polarising microscopy (POM), differential scanning calorimetric (DSC) and powder X-ray diffraction (PXRD) techniques. Thermal stabilities of smectic to isotropic phase are 150.0 °C and temperature range of mesophase is in decreasing order from comp.H₃ to comp.H₁₈ respectively. The presences of SmC phase are the type of broken fan and needle type in present synthesized series. It is also shown that presence of chalcone amino allylidene central linking group favors a calamitic liquid crystalline behaviour in molecules with lower member to higher member aliphatic side chain in alkoxy group (-OR).     

On the kinetics of nitrogen incorporation in GaP LPE layers using NH3 vapour doping

The partial substitution of N for P in GaP has a considerable effect on the luminescence quantum efficiency of GaP epitactic layers. The incorporation of N in GaP liquid phase epitactic layers using NH₃ vapour doping has been studied at different H₂ partial pressures in argon gas. It is shown that the incorporation of N in different ambients is conveniently described by just one parameter. The reaction of NH₃ with Ga is found to be an equilibrium process. The N concentration in the layer is directly proportional to the GaN concentration in the melt. From the correlation of the dissociation of NH₃ with the formation of GaN it is concluded that the NH₃ pressure, as measured in the exit gas, is not representative of the NH₃ pressure at the melt. The dissociation of NH₃ is found to be strongly retarded by H₂ owing to adsorption of H₂ on the surface of reactor materials which catalyse the dissociation.