Rectangular Single Crystals Research Articles

Nb-doped VO2 single crystal microtube arrays

Nb-doped VO2 microtube arrays were fabricated by a thermal oxidation process of V–Nb alloy foils in air. The Nb-doped VO2 microtubes are featured with the rod-like, hollow, and rectangular cross-sectional single crystal, where arrays aligned nearly vertically on the surface of a Nb-doped V2O5 substrate. Phase transition examinations demonstrate that Nb doping can decrease the metal-insulator transition temperatures of VO2 by approximately 15 °C/at. % Nb, which reaches the highest decrease efficiency for metal-insulator transition temperatures of Nb-doped VO2. The phase transition route and mechanism of Nb-doped VO2 microtube arrays are revealed.

Crystal and molecular structure of V-amylose complexed with ibuprofen

Rectangular V-amylose single crystals were prepared by adding racemic ibuprofen to hot dilute aqueous solutions of native and enzymatically-synthesized amylose. The lamellar thickness increased with increasing degree of polymerization of amylose and reached a plateau at about 7 nm, consistent with a chain-folding mechanism. The CP/MAS NMR spectrum as well as base-plane electron and powder X-ray diffraction patterns recorded from hydrated specimens were similar to those of V-amylose complexed with propan-2-ol. Amylose was crystallized in an orthorhombic unit cell with parameters a = 2.824 ± 0.001 nm, b = 2.966 ± 0.001 nm, and c = 0.800 ± 0.001 nm. A molecular model was proposed based on structural analogies with the Vpropan-2-ol complex and on assumptions on the stoichiometry of ibuprofen. The unit cell would contain four antiparallel 7-fold amylose single helices with ibuprofen molecules distributed inside and between the helices.

Growth of large unitary (K,Na)NbO3 single crystals using a seed-free solid-state crystal growth method by adjusting the B-site excess ratios and additional Bi2O3

We adjusted the B site excess ratio and the quantity of additional Bi2O3 in the KNN calcined powder in order to establish a process to fabricate, (K,Na)NbO3 (KNN)-based single crystals, with large and uniform configurations and relatively stable low porosity, using the seed-free solid-state crystal growth method. When the activity of abnormal grain growth during sample heating was controlled appropriately by adding 0.5 wt% Bi2O3 into the calcined powder with a B site excess ratio of 1%, extraordinarily large unitary rectangular single crystals were grown successfully. The individual crystals started to grow from the nucleus around the center of the green compacts formed by the calcined powder. Then they expanded until they became larger than 15 × 15 mm2 by heating at 1060 °C for 15 h. The relative density of the fabricated single crystals was higher than 99%.

An unexpected lanthanum chloride carbonate hydrate: synthesis, crystal structure, vibrational spectra and thermal degradation of LaCl[CO3]·3H2O

Abstract Microcrystalline powders of LaCl[CO3]·3H2O were precipitated and isolated from stoichiometric aqueous solutions of LaCl3·7H2O and Na2[CO3]·10H2O. This confirmed the surprising emergence of corresponding single crystals during a reaction of equimolar aqueous solutions of LaCl3·7H2O and Na[N3] under ambient conditions by the uptake of atmospheric CO2. According to the X-ray structure analysis of colorless, transparent and rectangular single-crystals, LaCl[CO3]·3H2O adopts an orthorhombic structure, space group Pbca with the unit-cell parameters a=856.82(5), b=1598.57(9) and c=967.68(6) pm for Z=8. The tenfold coordination polyhedron around La3+ consists of two monodentate as well as two bidentate [CO3]2− anions together with three oxygen atoms from coordinating water molecules and one chloride anion. According to DSC/TG studies and X-ray powder diffraction, LaCl[CO3]·3H2O loses first its water and then carbon dioxide under thermal treatment up to 900°C to leave LaOCl behind. Vibrational infrared and Raman spectra confirmed the presence of H2O and [CO3]2− in LaCl[CO3]·3H2O and its stepwise degradation at elevated temperatures.

Unusual crystals of poly(ε-caprolactone) by unusual crystallisation: The effects of rapid cooling and fast solvent loss on the morphology, crystal structure and melting

The lateral habit, unit cell structure and melting behaviour of single crystals of poly(ε-caprolactone) (PCL) prepared by the rapid expansion of a supercritical solution technique was studied by AFM at ambient and higher temperatures and by grazing-incident X-ray scattering using a synchrotron source. After dissolving PCL in a solution of supercritical CO2 and 0.1 vol.% chloroform, an extremely fast phase transfer from a supercritical to a gas-like state occurred during expansion into atmospheric conditions, leading to a temporary temperature drop to below −50 °C at the silica surface where the crystals were deposited. Single crystals of a hitherto unreported rectangular lateral habit were observed. Six-sided crystals were also observed, but they were fewer than the rectangular crystals and in addition the angles between the lateral faces were different from the theoretical angles between adjacent {110} faces and {110} and {100} faces. X-ray scattering indicated a polymorphic structure also including the orthorhombic (110) and (200) diffraction peaks. Distinct low angle peaks essentially along the c-axis indicated a stacking on a very fine scale (3.7–4.7 nm) within the crystals. The equatorial diffraction peaks indicated a less dense packing of the PCL stems. Rectangular single crystals with a height according to AFM of 11–27 nm melted between 40 and 45 °C, which is lower than the melting points (55 °C) recorded for the distorted six-sided crystals. The unusual conditions for crystallisation used gave the polymer molecules a severe limitation to rearrange from the initial random coil state. The facetted crystals consisted of a stack of 4 nm thick blocks; these blocks most probably constituted a regular variation in molecular packing, i.e. molecular order. The pronounced changes in the angles between adjacent faces from those observed in mature PCL crystals and the wide-angle X-ray scattering data indicated the presence of conformational disorder in the crystals.

Simulated morphological landscape of polymer single crystals by phase field model

The novel phase field model with the "polymer characteristic" was established based on a nonconserved spatiotemporal Ginzburg-Landau equation (TDGL model A). Especially, we relate the diffusion equation with the crystal growth faces of polymer single crystals. Namely, the diffusion equations are discretized according to the diffusion coefficient of every lattice site in various crystal growth faces and the shape of lattice is selected based on the real proportion of the unit cell dimensions. Spatiotemporal growth of syndiotactic polypropylene single crystals during isothermal crystallization has been investigated theoretically based on this phase field model. Two dimensional numerical calculations are performed to elucidate the faceted single crystal growth including square, rectangular, lozenge-shaped, and hexagonal single crystals. Our simulated patterns are in good agreement with the experimental morphologies, and the physical origin of polymer single crystal growth is discussed.

Rapid and reversible shape changes of molecular crystals on photoirradiation

The development of actuators based on materials that reversibly change shape and/or size in response to external stimuli has attracted interest for some time. A particularly intriguing possibility is offered by light-responsive materials, which allow remote operation without the need for direct contact to the actuator. The photo-response of these materials is based on the photoisomerization of constituent molecules (typically trans-cis isomerization of azobenzene chromophores), which gives rise to molecular motions and thereby deforms the bulk material. This effect has been used to create light-deformable polymer films and gels, but the response of these systems is relatively slow. Here we report that molecular crystals based on diarylethene chromophores and with sizes ranging from 10 to 100 micrometres exhibit rapid and reversible macroscopic changes in shape and size induced by ultraviolet and visible light. We find that on exposure to ultraviolet light, a single crystal of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene changes from a square shape to a lozenge shape, whereas a rectangular single crystal of 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene contracts by about 5-7 per cent. The deformed crystals are thermally stable, and switch back to their original state on irradiation with visible light. We find that our crystals respond in about 25 microseconds (that is, about five orders of magnitude faster than the response time of the azobenzene-based polymer systems) and that they can move microscopic objects, making them promising materials for possible light-driven actuator applications.

Preferential reorientation of twin variants in Ni–Mn–Ga single crystal

The evidence for the preferential orientation of Ni–Mn–Ga martensitic variants by compression or magnetic field is presented. The evolution of martensitic twinning configuration in Ni 50Mn 27.5Ga 22.5 single crystal was optically observed initially under the applied compressive stress and then followed by the magnetic field. The magnetic field induced strain (MFIS) was measured at different conditions. The MFIS was not observed in the original three-variant self-accommodated state. One percent MFIS was obtained after compression under 5 MPa, attributed to a disappearance of the self-accommodated structure replaced by a two-variant structure. During magnetization, the favorable variants expand at the expense of others. After several cycles of compression and successive magnetization, a near single variant state and MFIS of 5.5% were achieved in the rectangular single crystal. Forty-five degree macroscopic twin bands were observed during the MFIS testing on (0 1 0) surface of an oriented rectangular sample with {1 0 0} faces.

Chain Orientation and Defects in Lamellar Single Crystals of Syndiotactic Polypropylene Fractions

Sectorization is frequently observed in elongated rectangular single crystals of syndiotactic polypropylene (s-PP) grown from the melt in thin films. The crystals are bound laterally by {100} and {010} growth planes. The constituent sectors are readily observed because of a difference in thicknesses; specifically, the sectors bound by the {100} planes [the (100) growth sectors] are thicker than those bounded by the {010} planes [the (010) growth sectors]. Dark field (DF) transmission electron microscopy (TEM) was utilized to examine the chain orientation and lattice defects in the different growth sectors of s-PP single crystals. The (020) DF images exhibited pairs of bright streaks that are more or less perpendicular to the (100) planes and cross over the whole width of the (100) sectors. In the (200) DF images, the (100) sectors also exhibited similar but dimmer streaks than those in the (020) DF images. This suggests that the crystal c-axis orientation in the (100) crystal sectors undergoes a periodic change in inclination along both the longitudinal (parallel to the b-axis) and the transverse axis (parallel to the a-axis) directions of the single crystal. The ripples in the (100) sectors, previously observed in TEM, were also seen with atomic force microscopy as sinusoidal-like periodic height changes along the longitudinal axis direction at both room temperature and high temperatures. This periodic height change accounted for the pairs of bright streaks in the (020) DF images. The ripple formation was explained by lamellar thickening in the (010) sectors during crystal growth. This thickening process causes lateral contractions, which accumulate mainly along the longitudinal axis direction of the single crystal. On the other hand, the (020) DF images exhibited a relatively uniform brightness in the (010) sectors, while in the (200) DF images, several dark zones in the (010) sectors were more or less along the diagonal directions of the single crystal. This observation indicates that the crystal c-axis in these zones is slightly deviated from the (200) planarity due possibly to the lateral contraction in the (010) sectors. A regular Moiré pattern in the (010) sectors was observed in the (020) DF images, and no Moiré patterns were found in the (100) sectors. Again, this was presumably due to sinusoidal-like ripples, which substantially affect the crystal plane orientation with respect to the lamellar crystal normal. In the (200) DF images, only random Moiré fringes could be found and, in particular, when the (010) and the (100) sectors overlapped. However, regular Moiré fringes were observed continuously over both sectors in (220) DF images.

Single-crystal growth of La2-2xSr1+2xMn2O7 under pressure

Large single crystals of La2−2xSr1+2xMn2O7 (x=0.3 to 0.525) have been prepared under controlled atmospheric conditions. The crystals were grown by the floating-zone technique in an image furnace under a mixed oxygen/argon atmosphere pressurized to 6–8×105 Pa. Rectangular single crystals with sizes up to 50×9×4 mm3 have been obtained. The phase-purity, composition, and quality of the crystals were analyzed by X-ray diffraction and electron probe microanalysis. The magnetic behavior is found to be sensitive to the composition of the atmosphere during growth.

Plastic deformation of copper single crystals by simulation using X-ray Lang method and molecular dynamics

Copper rectangular parallelepiped single crystals with a notch have been extended or compressed. In a crystal with a notch, partial dislocations were created near the tip of the notch in elongation as we expected. It is hard to see where the dislocations were created just looking the displaced positions of atoms, but by a simulation of X-ray Lang topography, the creation of dislocations was easily observed. Before creation of dislocations, some indication was quite clearly shown. For compression dislocations did not start near the tip of the notch but started uniformly on the surface, the effect of notch was minor.

TENSILE DEFORMATION OF COPPER SINGLE CRYSTALS BY SIMULATION USING X-RAY LANG METHOD AND MOLECULAR DYNAMICS

Simulations were performed to model the response of a copper rectangular parallelepiped single crystal with a notch to tensile deformation. Partial dislocations were created near the tip of the notch as we expected. It is hard to see where the dislocations were created by just looking at the displaced positions of atoms, but the creation of dislocations was easily observed by a simulation of X-ray Lang method. Before creation of dislocations, some indication was quite clearly shown. Laue patterns during tensile deformation were also simulated.

Crystallization, Melting and Morphology of Syndiotactic Polypropylene Fractions. 4. In Situ Lamellar Single Crystal Growth and Melting in Different Sectors

Over a wide crystallization temperature (Tc) range, elongated rectangular lamellar single crystals of syndiotactic polypropylene (s-PP) grown from the melt of thin films exhibit sectorization along both diagonal directions to form the (010) [with the (010) free edge] and the (100) [with the (100) free edge] sectors. They can be observed by transmission electron and atomic force microscopy (TEM and AFM). Electron diffraction experiments show that the crystals in both sectors are form III orthorhombic structure, as recently proposed.1-4 The observation of this sectorization is due to the thickness difference between these two sectors with different fold planes. In situ observations of the single-crystal growth are monitored using an AFM coupled with a hot stage. The Tc-dependent crystal growth rates along both the (100) and (010) normal can be measured, and they are anisotropic. This leads to a large aspect ratio of the rectangular single crystal. AFM results also show that the thickness difference between ...

Single crystals of syndiotactic poly[propene‐co‐(1‐octene)] and syndiotactic polypropene crystallized in bulk

AbstractSingle crystals grown in bulk samples of a syndiotactic random copolymer of propene and 1‐octene with 4 wt.‐% 1‐octene are compared with single crystals of syndiotactic polypropene homopolymer. The random copolymer during isothermal crystallization at low supercoolings forms rectangular single crystals similar to syndiotactic polypropene. The single crystals of the random copolymer do not exhibit transverse cracks in contrast to neat syndiotactic polypropene. Both polymers have nearly the same linear thermal expansion coefficients along their crystallographic a‐ and b‐axis, calculated from temperature‐dependent wide‐angle X‐ray scattering (WAXS) measurements. Therefore, the absence of transverse cracks in single crystals of the random copolymer can only be explained by the relatively high disorder of the crystallite surface caused by the exclusion of 1‐octene segments which leads to a more isotropic thermal expansion coefficient in this region.

Erratum to: Thermal conductivity as a probe of unconventional superconducting gap

We have carried out a comparative study of the thermal conductivity of two rectangular single crystals: a niobium sample and a detwinned YBCO. Thermal conductivity was measured as a function of the relative orientation of the crystaline axes and a magnetic field applied parallel to the sample plane. Different angular variations, two-fold for Nb and four-fold for YBCO, were observed in the two case. We discuss the microscopic effects related to the gap structure which are at the origin of this qualitative difference in the behavior of thermal conductivity in these two superconductors.

Thermal conductivity as a probe of unconventional superconducting gap

We have carried out a comparative study of the thermal conductivity of two rectangular single crystals: a niobium sample and a detwinned YBCO. Thermal conductivity was measured as a function of the relative orientation of the crystaline axes and a magnetic field applied parallel to the sample plane. Different angular variations, two-fold for Nb and four-fold for YBCO, were observed in the two case. We discuss the microscopic effects related to the gap structure which are at the origin of this qualitative difference in the behavior of thermal conductivity in these two superconductors.

How is a fully penetrated state formed in an anisotropic superconductor?

The fully penetrated state of anisotropic superconductors which are extended in thec-axis direction is analyzed using a model to describe the current distributions. Based on magnetooptical investigations on a rectangular single crystal and on a cubic melt-processed YBa2Cu3O7−δ sample, the current distributions found in (a, b)-planes and (a, c)-planes with the external magnetic field perpendicular to the plane are discussed. The easy flux penetration along the CuO2 layers is shown to have important consequences for the analysis of angledependent magnetization or torque measurements inc-axis direction extended samples.

Current and field pattern in rectangular and inhomogeneous superconductors.

The penetration and exit of magnetic flux in type-II superconductors is investigated for the realistic situation where a transverse magnetic field is applied to a square or rectangular plate or film. In rectangular specimens the pattern of the sheet current and of the density of the perpendicular flux has some common features with the one-dimensional distributions in circular disks or long strips. Other features, however, are characteristic for the rectangular shape, e.g., the starlike pattern of the penetrating flux and, in the fully penetrated critical state, the discontinuity lines at which the current stream lines perform sharp bends and at which the perpendicular magnetic field ${\mathit{H}}_{\mathit{z}}$(x,y) exhibits sharp ridges. These typical features have to be calculated from a genuine two-dimensional theory. Such a theory based on a highly nonlinear current-voltage law is outlined. The field patterns obtained by this general theory are compared with patterns observed magneto-optically at the surface of square and rectangular single crystals or films of high-${\mathit{T}}_{\mathit{c}}$ superconductors with homogeneous and inhomogeneous critical-current distribution. It is shown that the analysis of the current-discontinuity lines is essential to understand the flux dynamics in superconductors. In samples with inhomogeneous critical current density ${\mathit{j}}_{\mathit{c}}$(r), a strong concentration of flux motion and electric field can occur along the lines where ${\mathit{j}}_{\mathit{c}}$ changes abruptly. This may trigger flux jumps.

Morphology and Thermal Properties of Fully Syndiotactic Polypropylene

The morphology of syndiotactic polypropylene (sPP) containing >99% racemic diads has been investigated following controlled crystallization from the melt in thin films. At high growth temperatures, large rectangular single crystals are obtained with preferred growth along the crystallographic b-axis, while at lower temperatures these branch out to yield dendritic, axialitic, and eventually spherulitic aggregates. These crystals exhibit two very unusual properties: transverse fractures and ripples. Both are quasi-periodic, the fractures with a spacing of 3-7 μm and the ripples generally 1 μm or less

Modified floating-zone growth of organic single crystals

For organic materials floating-zone crystal growth is superior to other melt growth processes in two significant respects: (1) the absence of crucible-induced mechanical damage and (2) minimum heating-induced chemical degradation. Due to the rather low surface tension of organic melts, however, floating-zone crystal growth under normal gravity has not been possible so far but microgravity is ideal for such a purpose. With the help of a modified floating-zone technique, organic single crystals of small cross-sections were test grown first under normal gravity. These small crystals were round and rectangular single crystals of benzil and salol, up to about 7 cm long and 6 mm in diameter or 9 mm × 3 mm in cross-section.