A New Association Model for Nematogenic Systems—Its Significance for Liquid Crystal Materials Research

The importance of molecular association is considered in relation to the physical properties of a number of nematogens. It is proposed that association leads to the formation of polar and apolar dimers, which influence the dielectric and electro-optic properties of nematic liquid crystals. Dielectric and light scattering measurements of polar and apolar correlation factors are reported for a series of dipolar mesogens, and interpreted in terms of a model for molecular association. Both the solute molecular structure and nature of the solvent influence the degree of association, and the good device properties of mixtures of positive and negative nematogens is tentatively attributed to the strong tendency of dipolar solutes to associate in negative nematic solvents.     

Nonstoichiometric fluorides—Solid electrolytes for electrochemical devices: A review

The solid electrolytes with fluorine-ion conductivity that were revealed during the analysis of the phase diagrams of the MF _m -RF _n systems within the program of search for new multicomponent fluoride crystalline materials carried out at the Shubnikov Institute of Crystallography, Russian Academy of Sciences, are described. The most widespread and promising materials are the nonstoichiometric phases with fluorite (CaF₂) and tysonite (LaF₃) structures, which are formed in the MF₂-RF₃ systems (M = Ca, Sr, Ba, Cd, or Pb; R = Sc, Y, or La-Lu). These phases have superionic fluorine conductivity due to the anion sublattice disorder. The ionic conductivity of crystals of both structure types has been studied and the limits of its change with composition and temperature are determined. Nonstoichiometric fluorides are used as solid electrolytes in chemical sensors, fluorine sources, and batteries. The prospects of the use of fluorine-ion conductors in solid-state electrochemical devices, principles of their operation, and the problems of optimization of their composition are discussed.     

Synthesis and structural evidence for new diimidazo-[1,5-a:1′,5′-d] pyrazinediones

The synthesis of derivatives of 5H, 10H-diimidazo [1,5-a:l,5-d] pyrazine-5,10-dione is described. The mechanism of the reaction is discussed and the chemical, spectral and X-ray data giving the evidence of the structure of these compounds are presented. X-ray studies were carried out on compound6c (C₂₄H₂₂N₆O₂) witha=9.250(3),b=11.879(3),c=10.250(3),Z=2 and space groupP2₁/a. The structure was solved using direct methods and refined by weighted full-matrix least squares. The refinement, based on 990 reflections withI2.5(I), converged to a finalR of 0.041 andR _w=0.036.     

Conformational Behavior and Influence of Organic Solvents on a Strong Polar Group Nematogen at Room Temperature—A Computational Model

The conformational behavior and influence of organic solvents on a nematogen, 4’-n-alkyl-4-cyanobiphenyl, with strong polar group propyl (3CB) that is of commercial and application interest has been studied with respect to the translational and orientational motions. The atomic net charge and dipole moment components at each atomic center have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh-Schrodinger Perturbation theory with the multicentered-multipole expansion method has been employed to evaluate the long-range interactions, and a “6-exp.” potential function has been assumed for the short-range interactions. The minimum energy configurations obtained during the different modes of interactions have been taken as input to calculate the configurational probability using the Maxwell–Boltzmann formula in nonpolar organic solvents, i.e., carbon tetrachloride (CCl₄), and chloroform (CHCl₃) at room temperature 300 K. It has been observed that the molecules show the interesting property in the organic solvents. The interaction energies of dimer complexes have been taken into consideration in order to investigate the most energetically stable configuration. An attempt has been made to develop an interesting computational model for nematogen at molecular level.     

Extrinsic doping of CdxHg1−xTe— A review

Extrinsic doping by elements which are stable to subsequent processing will become increasingly important in future infra-red device structures based on Cd_xHg_1−xTe. This paper reviews the incorporation and activation of dopants in the most widely used bulk and epitaxial growth methods. Stoichiometry at the growth temperature is shown to be the critical factor affecting dopant activation. Various factors, including stoichiometry, can affect the as-grown electrical properties and the importance of determining the type of conduction in the as-grown state, if successful extrinsic doping is to be accomplished, is stressed. Data on dopant segregation behaviour, in growth from liquids, acceptor ionization energies and carrier lifetimes are also presented and their importance is discussed.     

Structure of poorly crystalline MoS2 — A modeling study

Current interest in amorphous and poorly crystallized phases of the layered transition metal dichalcogenides arises primarily from their unique catalytic and electrochemical properties not present in the corresponding crystalline phases. Computer calcultions of the scattered X-ray intensity from model structures of poorly crystalline (px-) MoS₂ are compared here with experimental patterns from materials prepared at different temperatures. The results are used to identify the salient features of the observed diffraction patterns of px-MoS₂ in which the measured diffuse scattered intensity may be clearly attributed to the stacking and rotational disorder of the layers as well as the limited number of these layers. A procedure for the interpretation of the X-ray diffraction pattern of px-MoS₂ has thereby been developed in which the entire scattering pattern may be calculated without separation into Bragg peaks (of varying breadth and shape) and diffuse background.     

Bridgman growth of CdxHg1−xTe— A review

Recent developments in the bulk Bridgman growth method for Cd_xHg_1−xTe are reviewed. Both melt mixing and heat flow control techniques have been applied in attempts to produce more uniform material in terms of composition. In the U.K. work has concentrated on application of the Accelerated Crucible Rotation Technique (ACRT) to achieve the required uniformity improvements. Elsewhere, various means to control isotherm shape have been used with the same aim. The ultimate use of the material is in infra-red detectors and Bridgman grown Cd_xHg_1−xTe has produced these successfully for both photoconductive and photovoltaic applications.     

A facile sol–gel synthesis of crack-free photopolymerizable silica for holographic recording

A series of sol–gel based photopolymerizable silica glass for holographic storage has been synthesized via a facile chemical design in an effort to overcome the cracking problem incurred during the irradiation stage. The holographic composite material includes a low molecular weight polydimethylsiloxane (PDMS) with end-capped hydroxyl groups, an epoxide-containing coupling agent, a photopolymerizable acrylate monomer, and the sol–gel-derived silica matrix. Inclusion of PDMS provides improved compression stress and strain and toughness over the original unmodified samples, without deterioration on the diffraction efficiency (η). A plateau value of η ~ 40% can be found under a beam power of 5.02 mW. Correlation between the molecular weight of acrylate polymer and η of the holographic composite material is described and the discrepancies are analyzed. The premature saturation of polymerization in the holographic system is attributed to the retarded free radical polymerization within the silica matrix. Minor variation of η with photoinitiator concentration is observed, indicating that only moderate molecular weight is required for a reasonably high η. Based on the chemical and physical interactions within the material system, a crack-free mechanism is elucidated. Finally, the adverse effect of the residual photoinitiator on the holographic media is addressed.     

The crystal and molecular structure of one of the possible conformers of “the smallest piece of ice”—A solid-state isolation of a hydrogen bonded water hexamer trapped within channels of a crystal of a dirhodium molecule

Reexamination of our study of [Rh₂(HNOCCH₃)₄(2H₂O)] 3H₂O (Ahsan, M.Q.; Bernal, I.; Bear, J.L. Inorg. Chem. 1986, 26 260) showed it to be interesting not just because the dirhodium molecule is an antineoplastic but because it contains a hexameric cluster of waters trapped in Rh–Rh lattice cavities. It may well provide an interesting model for the smallest piece of ice (Nauta, K.; Miller, R.E. Science 2000, 287, 293).     

Regular domain structure in a lithium niobate crystal—Period stabilization

High stability of the period and homogeneity of a regular domain structure was attained in Nd: Mg: LiNbO₃ crystals grown from melt with an excess of lithium oxide by the Czochralski method along the normal to the close-packed $\{ 01\bar 12\} $ face.     

A decomposition study of a eutectoid Cu?Al alloy

A study of the eutectoid decomposition of a Cu-11.8wt.%Al alloy was carried out by means of differential scanning calorimeter and an electron microscope. The activation energies of eutectoid and peritectoid reactions were determined.     

Advancement trends in dye-doped polymer dispersed liquid crystals—a survey review

At present there is significant interest in the doping of various types of dyes into polymer dispersed liquid crystals (PDLCs) in display films. Till to date, various types of dyes have been employed to improve the electro-optical properties of PDLC films in various situations by different research groups. Some of the advantageous features of dye-doped PDLC predict that such dye-doping into the PDLC films resolve the scattering effect, improve the films absorbance, enhance the degree of alignment, stability, and the electro-optical properties of the film such as reflectance, contrast ratio and better transmittance. The current article contains a short review on the present and past studies conducted on the dye-doped polymers dispersed liquid crystals films. The fabrication techniques and their various applications in the different display areas are reviewed, along with the new key findings are discussed.     

Bismuth a new dopant for GaN films grown by molecular beam epitaxy—surfactant effects,formation of GaN1−xBix alloys and co-doping with arsenic

We have investigated the influence of an additional bismuth flux during growth on the properties of GaN films prepared by plasma-assisted molecular beam epitaxy (MBE). A wide range of bismuth fluxes have been used, the highest Bi flux was larger than the flux of Ga. We have demonstrated that using a Bi flux during the growth of GaN by MBE at a temperature ∼800°C improves the surface morphology of the films and decreases the deep emission. We have demonstrated for the first time the growth of GaN_1−xBi_x alloys by MBE, the Bi concentration was small and increased with decreasing growth temperature. We have studied the influence of an additional bismuth flux on the growth of As-doped GaN layers and observed an increase of blue emission from the layers at some optimum range of Bi fluxes. All the results allow us to conclude that Bi may be a potential new dopant for the growth of GaN by MBE.     

Dynamical Properties of a Disordered Crystal: Adamantane—C10D16

Inelastic neutron scattering measurements on C₁₀D₁₆ show the existence of underdamped collective librational modes with temperature-independent frequency widths.

A preliminary dynamical model, with no adjustable parameters, shows that the observed widths are consistent with the existing quasi-static disorder.

Direct measurements of the orientational pair correlation function by means of elastic neutron scattering agree with the prediction of a simple Ising model.

Intensity measurements between room-temperature and the ordering temperature (T _c = 208.6K) indicate that the orientational correlations develop very slowly as T → T _c.     

Structure determination of 3β-acetoxy-cholest-5-ene-7-one—A steroid

The crystal structure of 3β-acetoxy-cholest-5-ene-7-one (C₂₉H₄₆O₃) has been determined by X-ray diffraction methods. The compound crystallizes in the monoclinic crystal system (space group P2₁) with the unit cell parameters a = 9.632(1) Å, b = 12.280(1) Å, c = 23.099(2) Å, β = 99.52(1)°, and Z = 4. The structure has been solved by direct methods and refined to an R-value of 0.065 for 3927 observed reflections [F ₀ > 4σ(F ₀)]. Two crystallographically independent molecules (I and II) in the asymmetric unit have been observed. In both molecules, rings A and C of the steroid nucleus exist in a chair conformation. Ring B of molecule I adopts a 5α,6β half-chair conformation, and ring B of molecule II shows a 6α sofa conformation. Ring D adopts a 13α,14β half-chair conformation in molecule I and a 13α,14β half-chair conformation in molecule II. The crystal structure is stabilized by the intramolecular and intermolecular C-H?O interactions.