Tilted Structures Research Articles

Transition to a new tetragonal phase of WO3: crystalstructure and distortion parameters

At 1100 K a new phase of WO3 with space group P4/ncc, a = 5.2885(5) Åand c = 7.8626(8) Å, was established, and its crystal structure was determined. The previously known tetragonal phase P4/nmm, which is shown to exist only above 1170 K, was also refined with a = 5.3031(4) Åand c = 3.9348(3) Åat 1200 K. The difference between the two perovskite-like structures relates to a tricritical non-ferroic transition characterized by a soft mode at the Z-point of the Brillouin zone, which induces antiphase rotations of the WO6 octahedra about [001]. The oxygen sublattice of the lower tetragonal structure is shown to possess the symmetry of the a0a0c- perovskite tilt structure F4/mmc (I4/mcm), which is violated by the off-centre antiferroelectric displacements of the tungsten atoms that lead to the observed space group P4/ncc.

Lamellar Mesophases Formed by Thermotropic Liquid Crystalline Ionic Polymer Systems without Mesogenic Units

Mixtures consisting of polyamine (branched polyethyleneimine (PEI) and polyallylamine (PAA)) and n-alkanoic acid (C n: n = the carbon number of alkanoic acid) were prepared and their thermal and liquid crystalline properties were examined. The occurrence of proton transfer from the carboxylic acid to the amine groups was found by IR measurement of the mixtures. The polyamines and C n exhibited no mesophase. Also, PEI/C n with n = 3, 4, 5, which are mixtures of PEI and C n, did not show a liquid crystalline phase. However, PEI/C n with n> 5 clearly displayed a lamellar fluid liquid crystal phase having a layer structure with low order inside the layer, due to ion–ion interactions. In the lamellar mesophase, PEI/C n showed a focal-conic fan texture which resembles the fan-shaped textures in smectic A and C phases. PEI/C n with 5 < n < 13 formed a bilayer structure in the lamellar mesophase. On the other hand, a tilted bilayer structure is proposed as a packing model of PEI/C n with 13 < n< 19. PAA/C18 consisting of polyallylamine and C18 also showed a lamellar fluid mesophase and formed a similar layer structure to PEI/C18.

Depression of superconductivity by Tilting of the 6-coordinated CuO 2 plane

Based on the concept of “interlayer epitaxy”, we considered a correlation between a Cu Cu length, a shape of a CuO 2 plane and a critical temperature (Tc) of cuprate oxide superconductors with a 6-coordinated CuO 2 plane. When the Cu Cu length of a layered cuprate was in a range of 3.760Å-3.805Åor 3.840Å3.880Å, the CuO 2 plane was flat and the cuprate showed superconductivity. When the Cu Cu length of the layered cuprate was between 3.805Åand 3.840Å, the CuO 2 plane changed into a tilt structure and at the same time the superconductor changed into a semiconductor. These results suggest that the shape of the CuO 2 plane including tilting is strongly correlated with depression of superconductivity.

Phase Identification in a Series of Liquid-Crystalline TPP Polyethers and Copolyethers Having Highly Ordered Mesophase Structures. 4. Phase Structures and Order Evolution in TPP(n = 12) Thin Films

A main-chain liquid-crystalline polyether has been synthesized from 1-(4-hydroxy-4'-biphenylyl)-2-(4-hydroxyphenyl)propane and 1,12-dibromododecane, TPP(n = 12). In our previous report, the phase structures and phase transitions in bulk TPP(n = 12) have been identified. In this study, thin film samples of TPP(n = 12) have been prepared with thicknesses ranging from 10 to 100 nm on amorphous carbon coated substrates. Transmission electron microscopy and electron diffraction observations show that the liquid-crystalline chain molecules possess a tilted homeotropic orientation within monodomains at a structural formation temperature of 180 °C in the smectic F (S F ) phase. This is due to the surface-induced alignment effect. With increasing isothermal time, the lateral size and thickness of the monodomain increase, indicating that the formation of monodomains is initiated by a uniformly oriented molecular aggregate and is conceptually similar to the nucleation-controlled process of polymer crystallization. The growth stage reveals that large-scale molecular motion is required in order to transfer the chain molecules from an isotropic orientation to a tilted homeotropic structure. Electron diffraction results on the TPP(n = 12) thin films confirm the phase structures identified in bulk materials. When the thin film samples are quenched from the isotropic melt to lower structural formation temperatures, the samples no longer show the tilted homeotropic molecular orientation. This indicates that the amorphous carbon surface-induced alignment process is sensitive to the type of phase in which the structural formation occurs. It is not possible to access the tilted homeotropic orientation when the molecules have been in an ordered structure higher than the S F phase. In the smectic crystal G phase, molecular mobility along the chain direction can still be expected, which is evidenced by an observation of significant morphological layer thickening during annealing in this phase. The regularity of morphological layers in the different ordered phases is also investigated.

Epitaxy and chainlength dependent strain in self-assembled monolayers

We use grazing incidence x-ray diffraction to systematically study the structure of an archetypal self-assembled monolayer as a function of the hydrocarbon chain length, n. The monolayers consists of n-alkyl thiol molecules, CH3(CH2)n−1SH (Cn, 10⩽n⩽30), self-assembled on single crystal Au(111) surfaces. At room temperature, the 2D structure is described by a C(4×2) unit mesh for all chain lengths. However, we demonstrate that there is a systematic dependence of the tilt structure (i.e., the tilt angle and tilt direction) of the hydrocarbon chains as a function of the chain length. Furthermore, we show that the monolayer structures are characterized by distinct “long’’ (n⩾16) and “short’’ (n⩽14) chain length regimes, as well as a smooth variation of the structural parameters within each regime. We associate these systematic structural changes with the conflicting requirements of epitaxy and molecular packing, and argue that the driving force is the changing intra-layer interaction strength (which is proportional to hydrocarbon chain length). We believe that these phenomena should be characteristic of the behavior of self-assembled monolayers, as well as the more general class of “soft/hard’’ interfaces.

Compositional Control of Ferroelectric Domain Structures in Eteroepitaxial PLZT Thin Films

ABSTRACTThe mechanism and control of ferroelectric domain formation in heteroepitaxial (Pb1−xLax)(ZryTi1-y)O3 (PLZT) thin films grown on MgO(001) substrates have been investigated as a function of composition and temperature. Pulsed laser deposition and RF magnetron sputtering techniques were used and optimized to fabricate epitaxial thin films with varying La and/or Zr concentrations. Periodic 9° domain structures were developed when the film transformed from the cubic phase to the tetragonal phase during cooling after deposition. As a result of the tetragonality of the films, the domain formation induced slightly tilted twin structures. All films were grown highly c axis oriented and the degree of c axis orientation was improved with increasing La or Zr concentration. Experimental observations of the 90° domain evolution in films have been carried out in-situ and ex-situ using conventional and synchrotron X-ray diffraction and demonstrated that the most important parameter affecting the domain structure and its abundance is the transformation strain at the Curie temperature, which can be varied systematically by changing the concentration of La and/or Zr in the PLZT system.

The high resolution video capture system on the alcator C-Mod tokamak

A new system for routine digitization of video images is presently operating on the Alcator C-Mod tokamak. The PC-based system features high resolution video capture, storage, and retrieval. The captured images are stored temporarily on the PC, but are eventually written to CD. Video is captured from one of five filtered RS-170 CCD cameras at 30 frames per second (fps) with 640×480 pixel resolution. In addition, the system can digitize the output from a filtered Kodak Ektapro EM Digital Camera which captures images at 1000 fps with 239×192 resolution. Present views of this set of cameras include a wide angle and a tangential view of the plasma, two high resolution views of gas puff capillaries embedded in the plasma facing components, and a view of ablating, high speed Li pellets. The system is being used to study (1) the structure and location of visible emissions (including MARFEs) from the main plasma and divertor, (2) asymmetries in gas puff plumes due to flows in the scrape-off layer (SOL), and (3) the tilt and cigar-shaped spatial structure of the Li pellet ablation cloud.

A neutron diffraction study of the magnetic ordering of TmMn 6Ge 6

The compound TmMn 6Ge 6 of HfFe 6Ge 6-type of structure ( P6 mmm ) orders antiferromagnetically below T N = 482 K and undergoes two magnetic transitions at lower temperatures. Neutron powder diffraction has shown that the high temperature region, T > 85 K, consists exclusively of the collinear antiferromagnetic ordering, q 2 = (0,0, 1 2 ), of the Mn sublattice along the c axis. In the low temperature region, T < T 1 = 35 K, both the Mn and the Tm sublattices participate in the magnetic ordering corresponding to a tilted spiral structure with the wave vector q 1 = (0,0, q z ) and q z = 1 4 . The intersublattice moment angle Mn-Tm-Mn, ± (π - ϕ{Mn}) = ± 142°, and and the angle of the spiral axis with the c axis θ s = 69° are slightly temperature dependent. For T > T t = 35 K the tilted spiral becomes destabilised and the two sublattices become weakly coupled. In the intermediate region 35–85 K the Tm spiral magnetic moment drops progressively to zero; the ordering of the Mn sublattice corresponds to a distorted skewed spiral with fluctuating Mn moment values and directions.

Modelling a nematic liquid crystal

Abstract The bulk phase liquid crystalline behaviour of a cyclic siloxane with a pentamethylcyclosiloxane core and biphenyl-4-allyloxybenzoate mesogens (BCS) was studied using molecular dynamics (MD) and wide angle X-ray analysis. This material exhibits partial crystallinity at room temperature and liquid crystalline behaviour above 120° C. For the MD simulations an ensemble of 27 molecules with 135 mesogenic units was simulated and a molecular mechanics force field was used to model the structural anisotropy of the siloxane molecules. Simulations were carried out both at room temperature and at an elevated temperature (425 K). Room temperature simulations showed that, contrary to our initial assumptions, the low energy molecular conformations were not cylindrical but splayed in shape. During the simulation a smectic-like, tilted layer structure was found to evolve for the cluster when full atom potentials were used, while no such development was observed when electrostatic interactions were neglected. The presence of a tilted layered structure was also suggested by the X-ray data. These results indicate that long range electrostatic interactions are significant for the molecular system under study. In order to calculate the orientational order parameter, the orientation of the molecular axis had to be determined. This was achieved by describing the mesogen shapes to be ellipsoidal and defining the principal axis of the ellipsoids to be the molecular directors. By sampling over 200 ps of simulation at 425 K, the time averaged order parameter (S) was calculated. The calculated S of 0.36 was comparable to the value of 0.4–0.45 found from the experimental data. Apart from providing insight into the relative importance of the various competing forces in the formation of the liquid crystalline phase, these simulations are also expected to be useful in predicting the mesophase behaviour of liquid crystalline systems.

The Effect of Melting Processes on the Development of a Tropical and a Midlatitude Squall Line

Several sensitivity tests are performed to assess the effect of melting processes on the development of a midlatitude continental squall line and a tropical oceanic squall line. It is found that melting processes play an important role in the structure of a midlatitude continental squall system. For the maritime tropical case, squall development is not as sensitive to the presence of melting, due to the dominance of warm rain processes. Melting processes exert an influence on midlatitude cloud system development through the conversion of ice particles to rain. The simulated convective system was found to be much weaker in the absence of evaporative cooling by rain. For a given vertical shear of horizontal wind, cooling by evaporation in the convective region was found to be essential for maintaining a long-lived cloud system. Diabatic cooling by melting played only a secondary role in this respect. In the absence of melting processes, the simulated mildlatitude squall system acquired the characteristics of unicell-type (erect and steady) convection rather than the observed multicellular (upsher tilt) structure. This suggests that the diabatic cooling by melting can have significant impact on the structure (dynamics) of a simulated midlatitude squall system. In addition, results from air parcel trajectory analyses indicate that jump-type downdrafts that originate either from the convective region or from above the melting level in the stratiform region are not simulated for convection that develops in the absence of melting. The horizontal momentum transport associated with the midlatitude squall system simulation were quite different in the presence and absence of melting. Significant horizontal momentum transport by convection was not observed in the absence of melting. However, an upper-level jet was simulated in the case where melting processes were active. It is also found that the horizontal perturbed pressure gradient force is comparable in magnitude yet almost always opposite in sign to the vertical transport effect by clouds.

Orientation of the COOH Headgroup in Stearic Acid Monolayers: Rigid Molecules Organization Calculations

The atom-atom potential calculations of stearic acid monolayers have been performed to determine orientations of the COOH headgroup. It is shown that the tilt structure of stearic acid monolayers may be due to a leaning of the COOH headgroup's plane away from vertical with the crossed conformation (see text)

Ｌａｎｇｍｕｉｒ‐Ｂｌｏｄｇｅｔｔ磁性膜の構造とスピン状態

In order to develop the magnetic thin film, we studied the structure and magnetic properties in the Langmuir-Blodgett (LB) multilayers of transition metal ion-fatty acid salt, using X-ray diffraction and electron paramagnetic resonance (EPR) methods. We found the various orientation as a function of multiplicity of layers in the LB films. In the film having more than four layers, the long molecular axis of fatty acid exhibits an apparent tilt structure; but in the film less than three layers, the molecular axis holds the perpendicular position against the substrate plane. Using the vector EPR measurements, we found the anomalous broadening of line shape due to the exchange interaction between successive layers. We concluded that the LB films of the transition metal ion-fatty acid salts may allow the highly ordered and the low-dimensional magnetic system.

Effects of structural phase transitions on Superconductivity and magnetism in substituted derivatives of La 2CuO 4

Structural (neutron and x-ray diffraction) and magnetic (neutron diffraction and magnetic susceptibility) measurements of materials derived from La 2CuO 4 by isovalent and aliovalent substitution in the La 3+ site have been performed. Such substitutions permit the tilts of the CuO 6 octahedra (about the (110) and (1 10) axes of the undistorted 14/mmm structure), and the hole concentration in the CuO layers, to be independently varied. The results of these measurements, in combination with those from other research groups, demonstrate that all four tilt structures, HTT (high temperature tetragonal, 14/mmm), LTO1 (low temperature orthorhombic 1, Bmab), LTO2 (low temperature orthorhombic 2, Pccn) and LTT (low temperature tetragonal, P4 2/ncm), support bulk superconductivity, with T c ( HTT) > T c ( LTO1) > T c ( LTO2) > T c ( LTT) at the same hole concentration. Furthermore, there is a strong suppression of superconductivity when the doped carrier concentration is 1/8 hole per Cu 2+ in the LTT and LTO2 phases of these materials. The reason for this T c suppression is not established. Finally, the magnetic structures adopted by the Cu 2+ spins in the insulating phases, determined by neutron diffraction, are also influenced by the tilts of the octahedra. This may reflect a structural dependence of interlayer and intralayer magnetic anisotropies.

Structural and vibrational properties of Ni(111)/NO adsorption complexes: a LCGTO-LDF model cluster investigation

The vibrational spectra and structural data for molecular adsorption of NO at the Ni(111) surface are analyzed with the help of first-principles LCGTO-LDF cluster models. The calculations support the interpretation of the vibrational bands observed below 1500 cm −1 as a manifestation of the nitrosyl species bound upright to the threefold hollow sites. Bent and tilted structures of the adsorbate are energetically less favoured. Slightly higher N-O vibrational frequencies are computed for molecules adsorbed in the twofold bridge position. Investigations of the lateral interactions are necessary to find out if the vibrational bands around 1580 cm −1 are related to the bridging NO species. The assignment of the RAIRS peak near 1680 cm −1 to vibrations of NO in the on-top position is not confirmed by the present study.

New display mode of ferroelectric liquid crystals with large tilt angle

Abstract New display modes of ferroelectric liquid crystals with large tilt angle are proposed. Average arrangement of liquid crystal is selected among three positions with positive and negative applied voltage and without applied voltage. There are four display modes by the combination of molecular directions and cell setups between crossed polarizers. Normally black setup of the homogeneously aligned cell with high pre-tilt aligning film and the configuration of antiparallel rubbing shows excellent optical characteristics. From transmittance vs. voltage characteristics and X-ray diffraction, the tilted layer structure is desirable to realize high contrast ratio. To explain these results, newly high pre-tilt induced interdigitated (HIPII) models are presented. Transmittance rise time and fall time to the applied rectangular voltage are 30 μ s and 2ms, respectively. Memory effect also has been studied under 1/256 duty pulse operation.

Nonlinear optical studies of polar polymeric Langmuir–Schaefer films

We report on the successful fabrication of X-deposited multilayer films using the Langmuir–Schaefer technique incorporating nonlinear optical chromophores. This technique was used on films of a polysiloxane copolymer containing both a mesogenic and a nonlinear optical side chain. Both x-ray and nonlinear optical studies of mono- and multilayer films reveal the head-to-tail deposition characteristic of X deposition. Both studies also indicate a highly polar noncentrosymmetric structure with nearly perpendicular orientation of the chromophore with respect to the film plane. The nonlinear optical studies also indicate a more tilted structure for the first layer and a second-harmonic coefficient of d⊥⊥=5.3±0.5×10−9 esu.

The magnetic field of Neptune

The planetary magnetic field of Neptune, with its large dipole tilt (approximately 47°) and complex structure, is similar to that of Uranus. Since Voyager's flyby trajectory brought the spacecraft much closer to this planet (closest approach = 1.18 R n) than its predecessors, and Uranus in particular, the rich complexity of the field is evidenced in the closest approach data as never before. The combination of a close flyby trajectory and a very complex field presents a formidable challenge to unique interpretation of the magnetic field observations. We present a model of this extraordinary field and discuss the limitations/uniqueness of our result.

Evidence of pressure-forced turbulent flow in a forest

Lagged cross-correlation analyses between streamwise velocity at several heights within and above a forest, and between streamwise velocity and surface pressure, provide evidence that turbulence in the sub-crown region of the forest is to a large extent driven by pressure perturbations. The analyses support earlier results based on examination of coherent structures observed in the same forest. The phase of the streamwise velocity signal exhibits an increasing delay with decreasing height, indicative of a downwind tilted structure, until the upper region of the forest is reached, at which point the effect is reversed. It is suggested that positive pressure perturbations ahead of advancing microfronts induce streamwise accelerations in the trunk space. This link between the pressure pattern and the wind field explains why velocity spectra in the trunk space are depleted in the higher frequencies, relative to levels above.

A molecular dynamics study of the packing structures in monolayers of partially fluorinated amphiphiles

We report the results of molecular dynamics simulations of liquid-supported monolayers of three partially fluorinated amphiphile molecules, namely CF3(CF2)9CH2COOH, CF3(CF2)6CH2(CF2)3COOH, and CF3(CF2)6(CH2)4COOH. These studies were undertaken to provide information on the interplay between molecular flexibility and the packing structure in a monolayer so as to better interpret the results of recent experiments. The qualitative aspects of the predictions of the simulations are consistent with the recent experimental data for monolayers of CF3(CF2)9CH2COOH [S. W. Barton, A. Goudot, O. Boulassa, F. Rondelez, B. Lin, F. Novak, A. Acero, and S. A. Rice, J. Chem. Phys. 96, xxx (1992)]. In particular, the observed breakup of the homogeneous ordered monolayer into ordered islands with the same collective tilt of the molecules is correctly predicted, and the fact that the collective tilt angle is small is correctly predicted. However, the experimental and theoretical values of the tilt angles are not in quantitative agreement, which we attribute to the inadequacy of the atom–atom potentials used in the simulations. In general, for monolayers of CF3(CF2)9CH2COOH we find that the collective tilt angle predicted is a sensitive function of the area per molecule and is smaller than in monolayers of alkane alcohols and alkane acids. The results of the simulations of monolayers of other partially fluorinated species suggest that the difference in size between the fluorocarbon segments and the smaller head groups or flexible ‘‘spacer’’ CH2 segments can generate subtle changes in the packing structure of a monolayer and the relative stabilities of the untilted and tilted structures.

1D pseudomorphism and strain relief by coherent tilt

We report the first detailed studies of the occurrence and subsequent strain relief of one-dimensional pseudomorphism in single crystal thin films. Of special interest are the coherent tilt and coherent surface structure that evolve within specific growth windows.