Showing papers on "Crystal published in 1991"

Molecular recognition at crystal interfaces

Abstract: Nucleation, growth, and dissolution of crystals have been studied by stereochemical approach involving molecular recognition at interfaces. A methodology is described for using ``tailor-made99 additives designed to interact stereospecifically with crystal surfaces during growth and dissolution. This procedure was instrumental in controlling crystal morphology and in revising the concept of the structure and symmetry of solid solutions. Consequently, it was applied to the transformation of centrosymmetric single crystals into solid solutions with polar arrangement displaying second-harmonic generation and to the performance of asymmetric synthesis of guest molecules inside centrosymmetric host crystals. The method has led to a discovery of a new ``relay99 mechanism explaining the effect of solvent on crystal growth. Finally, it allowed for the design of auxiliary molecules that act as promoters or inhibitors of crystal nucleation that can be used to resolve enantiomers and crystallize desired polymorphs.

Cubic boron nitride: synthesis, physicochemical properties and applications

Abstract: A review, with 112 references, is given of the different ways to prepare cubic boron nitride (BN). The different crystal forms as well as the phase diagram of BN are described. The physical and chemical properties and the correlated industrial or potential applications are shown.

Intrinsic dielectric loss in crystals

Abstract: We review the current theory of intrinsic dielectric loss, that is the loss in a perfect crystal due to anharmonic interaction of a.c. electric field with the phonon system of the crystal. Both ordinary dielectrics and displacive ferroelectrics are considered. The theory predicts dependence of the loss on frequency ω and temperature T. This dependence is very sensitive to the symmetry of the crystal. For ordinary dielectrics, the results are presented and tabulated for all 32 symmetry groups, except for non-symmorphic groups. The existing experimental date are analysed and explained on the basis of the theory.

The Structure of the C60 Molecule: X-Ray Crystal Structure Determination of a Twin at 110 K

Abstract: Single-crystal x-ray diffraction methods were used to determine the crystal and molecular structure of C(60) buckminsterfullerene. At 110 kelvin C(60) is cubic, apparent Laue symmetry m3m, but it exhibits noncrystallographic systematic extinctions indicative of a twin in which I(hkl) and I(khl) are superimposed. In fact, C(60) crystallizes with four molecules in space group [See equation in the PDF file] of the cubic system (Laue symmetry m3) with lattice constant a = 14.052(5) angstroms (A) at 110 kelvin. The twin components are equal. A given component, which has crystallographically imposed symmetry [See equation in the PDF file] displays an ordered structure of a truncated icosahedron. The five independent C=C bonds that join C(6) rings average 1.355(9) A; the ten independent C-C bonds that join C(6) and C(5) rings average 1.467(21) A. The mean atom-to-atom diameter of the C(60) molecule is 7.065(3) A. The molecules are very tightly packed in the crystal structure, with intermolecular C...C distances as short as 3.131(7) A.

Dynamic light-scattering study of the glass transition in a colloidal suspension.

Abstract: This paper describes a light-scattering study of the glass transition in nonaqueous suspensions of sterically stabilized colloidal spheres. The observed phase behavior, fluid, crystal, and glass, is consistent with an essentially hard-sphere interaction between the particles. Metastable fluid states were obtained upon shear melting the crystalline phases by tumbling the samples. Their intermediate scattering functions, measured by dynamic light scattering, showed the emergence of a nondecaying component, implying structural arrest, at essentially the same concentration as that at which homogeneously nucleated crystallization was no longer observed. The overall forms of the intermediate scattering functions are consistent with the predictions of mode-coupling theories for the glass transition. Supplementary studies of the static structure factors indicated only short-ranged spatial order for particle concentrations ranging from the equilibrium fluid through the metastable fluid to the glass.

Dependence of slow crack growth in polyethylene on butyl branch density: Morphology and theory

Abstract: A theoretical equation has been developed to describe the rate of slow crack growth in an ethylen-hexene copolymer in terms of the spacing of the butyl branches, number of tie molecules, and the thickness of the lamellar crystal. The model of slow crack growth is based on the rate of disentanglement of the tie molecules. The rate of disentanglement varies inversely with the number of tie molecules and directly with the number of tie molecules that are not pinned by the branches

An Organic Radical Ferromagnet

Abstract: The γ-phase crystal of 2-(4′-nitrophenyl)-4,4,5,5-tetramethyl-4 5-dihydro-1H−imidazol-1-oxy-3-N-oxide is shown to become a bulk ferromagnet below about 0.65 K from the measurements of magnetization of magnetic susceptibility and heat capacity.

Stabilization of superionic α -Agl at room temperature in a glass matrix

Abstract: SINCE the discovery1 that the high-temperature phase of silver iodide (α-AgI) has an ionic conductivity comparable to that of the best liquid electrolytes, solid electrolytes have attracted wide interest. Possible applications of these materials range from solid-state batteries to electrochromic displays and sensors2. Although α-AgI displays conductivities of more than 10 S cm−1 (ref. 3), owing to the almost liquid-like mobility of Ag+ ions, the crystal transforms below 147 °C to the β-phase with a conductivity of only ∼10−5 S cm−1 at room temperature. Efforts to achieve good conductivities at lower temperatures have focused on the addition of a second component to AgI to form solid solutions or new compounds such as RbAg4I5 and Ag2HgI4 (refs 4–7). Here we report our success in depressing the α→β transformation temperature so as to stabilize α-AgI itself at room temperature. We use a melt-quenching technique to prepare crystallites of α-AgI frozen into a silver borate glass matrix. The quenched material showed diffraction peaks characteristic of α-AgI and displayed ionic conductivities of about 10−1S cm−1. Further development of these glass/crystal composites may make the high ionic conductivity of α-AgI available for room-temperature solid-state applications.

Calibration of the Electrochemical Quartz Crystal Microbalance

Abstract: The quartz crystal microbalance seems to be a very usefui tool in electrochemical studies, but, up to now, no attempts have been made to calibrate the microbalance under electrochemical conditions. The aim of this paper is to determine not only the average sensitivity for different active areas but also the differential sensitivity as a function of the radial position of a localized change of mass. In this way, for homogeneous mass perturbation, it is possible to calculate the change of mass (Am) from the frequency shift (AJ) for different active areas by taking into account the average sensitivity value. In the case of localized Am, the differential sensitivity value allows one to calculate hm from hfif the position of the event is known. The calibration procedure was made for an AT-6 MHz quartz crystal. Quartz crystals are important sensing devices in liquid phase. They can be used to monitor changes in electrode mass (1-3) or changes in the liquid properties (4, 5). In an electrochemical process, a shift of the value of the quartz crystal resonance frequency (fo) can be attributed to a change in mass (Am) of the electrode, provided that the true relationship between hf and Am is known. The problem of the sensitivity of the quartz crystal microbalance (QCM) has been already dealt with in the air (6-8). The differential sensitivity of the QCM has been calculated from the local damage produced in a thin polymer film by an ion beam sputtering. In the liquid phase the distribution of the vibration amplitude of the quartz crystal has been investigated by putting a tungsten wire probe in contact with the quartz crystal to provoke a change in frequency (9, 10). The results obtained in both liquid and gas phases are rather similar, in spite of the properties of liquid phase which extend the vibration of the quartz crystal even beyond the active region defined by the metal deposit. These works show the complexity of the system and that the boundary conditions used for the derivation of Sauerbrey's equation (11, 12) are not always fulfilled. The problem of the QCM calibration has been extensively studied in air. on the contrary, despite the wide use of the QCM in electrochemistry, the calibration procedure of this technique in electrochemical conditions is not depicted in the literature. The aim of this work is to calibrate the electrochemical quartz crystal microbalance (EQCM), not only in the case where hm is uniformly spread on the active electrode surface, but also for localized mass changes. The last case is very relevant to all electrochemical phenomena which produce a change of mass on a small area, e.g., in localized corrosion or gaseous bubble evolution. The general case where the mass change is continuously but nonuniformly distributed across the surface is not addressed in the paper.

Structural, bonding, dynamical, and electronic properties of liquid silicon: An ab initio molecular-dynamics study.

Abstract: We report an extensive first-principles molecular-dynamics study of metallic liquid silicon. Our description of the local order is in excellent agreement with x-ray- and neutron-diffraction experiments. The difference in internal energy between the simulated liquid phase and the crystal agrees well with the experimental enthalpy of melting. Analysis of the valence-electronic-charge density shows persistence of some covalent bonds in the melt. These bonds give rise in the power spectrum of the system dynamics to a well-identifiable feature associated with stretching vibrations. Unlike the case in the crystal, in the liquid the covalent bonds are continuously forming and breaking in response to atomic motion. The majority of bonds are broken on average, leading to fast diffusion and to metallic behavior of the melt. The calculated electronic conductivity shows good agreement with available experimental data.

Metastable phase transformations induced by ball-milling in the CuW system

Abstract: For the first time, based on X-ray diffraction, scanning electron microscopy/energy dispersion X-ray analyses, differential thermal analysis and differential scanning calorimetry experiments, the crystal to non-equilibrium phase transition induced by ball-milling has been evidenced in the CuW system. This system exhibits a total immiscibility in both solid and liquid states. Starting from elemental copper and tungsten powders, a partial solubility of copper into the cubic tungsten lattice as well as of tungsten into the f.c.c. copper lattice is induced by ball-milling. Such an enhancement of the solubility is revealed by an effect on the lattice parameter of both Cu(W) and W(Cu) crystalline solid solutions over the whole investigated composition range, i.e. Cu 5 W 95 to Cu 95 W 5 (wt.%). Such an experimental investigation of the crystal to amorphous phase transition induced by ball-milling in a system which exhibits a positive heat of mixing either in the solid or in the liquid states supports our previous results leading to the conclusion that the mechanisms of the phase transitions induced by ball-milling are different from those of the so-called classical solid-state amorphization.

Raman spectrum in PbTiO3 re-examined: dynamics of the soft phonon and the central peak

Abstract: Light scattering measurements are investigated as a function of temperature from 10 to 900 K in the tetragonal and cubic phases of a PbTiO3 crystal. Phonon frequencies, ionic effective charges, spontaneous polarization and contribution to the dielectric permittivity are deduced from these data. The results show that the soft phonon and relaxation mode are both needed to describe the dynamics of this material.

Quantum size microcrystals grown using organometallic vapor phase epitaxy

Abstract: Needle‐shaped quantum size microcrystals as thin as 10 nm have been selectively grown by employing reduced pressure organometallic vapor phase epitaxy using trimethylgallium and arsine as source materials. The microcrystals grown within a SiO2 window area have their growth axes along the [111] direction. Transmission electron diffraction analysis shows that the crystal structure of microcrystals is consistent with the zinc‐blende structure of GaAs. The mechanism for growing the needle‐shaped crystals is similar to a vapor‐liquid‐solid (VLS) equilibrium phase growth model. From photoluminescence measurements at 4.2 K, it is found that the microcrystals show a very distinct spectra for free exciton and neutral acceptor‐bound exciton recombinations, meaning good crystal quality.

Oriented crystallization of CaCo3 under compressed monolayers. Part 2.—Morphology, structure and growth of immature crystals

Abstract: Transmission electron microscopy and electron diffraction have been used to determine the structure, orientation and morphology of CaCO3 crystals at the early stages of growth under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H] and octadecylamine [CH3(CH2)17NH+3]. At [Ca]≈ 9 mmol dm–3, elongated plate-like calcite single crystals of narrow particle size distribution were nucleated under fully compressed stearate monolayers. The crystals were elongated along the c axis and oriented with a {1text-decoration:overline10} face parallel to the monolayer/solution interface. Subsequent development of the crystals resulted in rhombohedral outgrowth into the bulk solution. Calcite crystals nucleated under partially compressed monolayers showed the same crystal chemical properties but had a more uniform size population. Nucleation under octadecylamine monolayers resulted in disc-shaped vaterite single crystals of narrow particle size distribution. Two types of disc were observed: (a) circular/hexagonal crystals oriented with their c axis perpendicular to the monolayer, and (b) oval-shaped crystals oriented with their a axis perpendicular to the monolayer. Changes in monolayer surface pressure did not influence these properties. The preferred orientation of calcite can be rationalized in terms of geometric and stereochemical matching between the carboxylate headgroups and the ions in the {1text-decoration:overline10} crystal face. There is no geometric match for vaterite but a stereochemical relationship exists on stearate monolayers with respect to the (00.1) face. For octadecylamine monolayers, bidentate binding of HCO–3 may be important in determining the preferential orientation of nuclei formed at the organic surface.

Mechanism of diamond growth by chemical vapor deposition on diamond (100), (111), and (110) surfaces: Carbon‐13 studies

Abstract: The mechanism of diamond growth by hot‐filament chemical vapor deposition (CVD) was investigated on the (100), (111), and (110) crystal faces of natural diamond by competition studies using carbon‐13‐labeled methane and carbon‐12 acetylene. Homoepitaxial growth rates of 0.4, 0.5, and 1.3 μm/h were obtained for growth on the (100), (111), and (110) faces, respectively. The (100)‐ and (111)‐oriented films were smooth initially, while films grown on (110) substrates quickly became rough. The (111) films had graphitic inclusions, as evidenced by the Raman spectrum, while the (100) and (110) films were graphite‐free. The (111) films also exhibited substantial tensile stress, as indicated by a shift in the Raman peak and by spontaneous cracking in films grown thicker than 3.5 μm. The carbon‐13 mole fraction of mixed 13C/12C diamond films grown on the diamond substrates was determined from the shift of the first‐order Raman frequency, after correction for the shift due to stress. The carbon‐13 mole fractions of ...

Electrochemical synthesis and in situ Raman spectroscopy of thin films of titanium dioxide

Abstract: Raman spectra of thin films of the TiO 2 crystal modifications anatase and rutile are reported. The oxides are easily synthesized by electrochemical oxidation of titanium electrodes under controlled conditions. Evidence for the formation of a third crystal modification, possibly brookite, is presented

Method for simultaneous measurement of mass loading and fluid property changes using a quartz crystal microbalance

Abstract: A method, using a quartz crystal microbalance, to obtain simultaneous measurement of solid mass accumulation and changes in liquid density-viscosity product. The simultaneous real-time measurements of electrical parameters yields that changes in surface mass can be differentiated from changes in solution properties. Two methods to obtain the admittance/frequency data are employed.

Preparation of Tetragonal Barium Titanate Thin Film on Titanium Metal Substrate by Hydrothermal Method

Abstract: Tetragonal BaTiO3 thin films were prepared directly on Ti metal substrates in Ba(OH)2 solutions by a hydrothermal method at temperatures 400° to 800°C for 5 to 240 min. The film thickness estimated from weight gain of Ti plate was in the range from 0.5 to 2.5 μm, and it increased with increasing treatment temperature, treatment time, and Ba(OH)2 concentration. Rectangular crystals having {100} and {001} faces grew idiomorphically with approximate crystal size of 0.3 to 2.0 μm. The tetragonality of the BaTiO3 films became apparent when the average crystal size exceeded about 1 μm. Lattice parameters of the films were a= 3.994 A, c= 4.035 A, and c/a= 1.010. The films formed above 600°C had preferred orientation showing stronger XRD peaks of h00 and 00l than the other peaks.

Method for forming semiconductor thin film

Abstract: A method of forming a semiconductor thin film by crystallizing a thin film crystal from an amorphous thin film. A plurality of small regions which are preferentially made nuclei generation points are formed at predetermined positions in the amorphous thin film. Solid phase growth from single nuclei formed in the small regions is preferentially effected by heating to form a crystalline semiconductor thin film in which the grain boundary positions are adjusted to the desired positions. This crystalline semiconductor thin film is subjected to a heat treatment to reduce defects in crystal grains.

The structure of some simple amorphous network solids revisited

Abstract: The amorphographic relationships between the structures of a number of single component amorphous network solids are discussed New neutron diffraction data are introduced for several of the materials including vitreous P 2 O 5 which has not previously been studied by neutron diffraction These data suggest that network connectivity is more important than structural unit stereochemistry in determining intermediate range order Previous neutron data for amorphous Ge and vitreous SiO 2 are compared with bith random network and crystal based models and criteria are proposed for the minimum size of model required to accurately predict the shape of the first diffraction peak, which for a number of amorphous network solids is found to be Lorentzian It is concluded that new computer modelling techniques are capable of generating models in good agreement with modern diffraction data and should considerably enhance present understanding of the structure of amorphous solids

Isothermal thickening and thinning processes in low-molecular-weight poly(ethylene oxide) fractions. 1. From nonintegral-folding to integral-folding chain crystal transitions

Abstract: On the basis of SAXS and DSC experiments, a nonintegral-folding chain (NIF) crystal can be found in low molecular weight PEO fractions. This crystal grows first as a transient state, and integral-folding chain (IF) crystals form later through an isothermal thickening or thinning process. In a PEO fraction with MW=4250, it is observed that either isothermal thickening to the IF (n=0) crystal of thinning to the IF (n=1) crystal occurs depending upon the thermodynamic stability of the NIF crystal. At low crystallization temperatures, the main process is isothermal thinning; at intermediate temperatures, both thickening and thinning processes are observed

Pure and Nd3+‐, Pr3+‐Ion Doped Trigonal Acentric LaBGeO5 Single Crystals Nonlinear Optical Properties, Raman Scattering, Spectroscopy, Crystal‐Field Analysis, and Simulated Emission of Their Activators

Abstract: Complex spectral investigations are made on new Nd3+- and Pr3+-doped and pure nonlinear-laser trigonal LaBGeO5 crystals. Raman spectra and nonlinear optical properties are studied including second harmonic generation, absorption, and luminescence spectra, which allow to determine the energy of Stark levels of the activator ions, luminescence-intensity characteristics, and pulse stimulated-emission parameters of Nd3+ ions at two intermanifold 4F3/2 4I11/2 and 4F3/2 4I13/2 channels as well as of Pr3+ ions at the visible 3P0 3H6 transition. A preliminary theoretical analysis is made of the crystal field at activator ions using a new many-electron semiempirical method. All registered induced transitions are identified. LaBGeO5:Nd3+ is shown as a promising material for self-frequency doubled laser. [Russian Text Ignored.]