X-ray Birefringence Research Articles

Disentangling x-ray dichroism and birefringence via high-purity polarimetry

High-brilliance synchrotron radiation sources have opened new avenues for x-ray polarization analysis that go far beyond conventional polarimetry in the optical domain. With linear x-ray polarizers in a crossed setting, polarization extinction ratios down to 10 − 10 can be achieved. This renders the method sensitive to probe the tiniest optical anisotropies that would occur, for example, in strong-field quantum electrodynamics due to vacuum birefringence and dichroism. Here we show that high-purity polarimetry can be employed to reveal electronic anisotropies in condensed matter systems with utmost sensitivity and spectral resolution. Taking CuO and L a 2 C u O 4 as benchmark systems, we present a full characterization of the polarization changes across the Cu K-absorption edge and their separation into dichroic and birefringent contributions. At diffraction-limited synchrotron radiation sources and x-ray lasers, where polarization extinction ratios of 10 − 12 can be achieved, our method has the potential to assess birefringence and dichroism of the quantum vacuum in extreme electromagnetic fields.

Effect of xylenol orange on the crystalline perfection, optical, piezoelectric and NLO behavior of unidirectionally grown imidazolium L-Tartrate single crystal

Pure and 0.1 mol% Xylenol orange (XO) doped Imidazolium L-tartrate (IMLT) nonlinear optical (NLO) single crystals were grown using Sankaranarayanan–Ramasamy (SR) method of uniaxial solution crystallization technique. The pure IMLT crystal is 100 mm in length and 15 mm in diameter. The doped IMLT crystal is 25 mm in length and 10 mm in diameter. UV–Vis-NIR spectrum shows that the pure and XO doped IMLT crystals' cut off wavelength is 235 nm. High-resolution X-ray diffraction (HRXRD) measurement and birefringence interferometry confirm that the SR method grown crystal has good crystalline perfection and optical homogeneity. The wavelength dependent refractive index and piezoelectric d33 coefficient were measured. The difference refractive index (Δn) of (100) plane in parallel and perpendicular directions shows a strong dependence on wavelength. The dielectric constant, hardness and SHG conversion efficiency were also increased as a result of XO dye doping in IMLT crystal.

Nanoscale crystal grain characterization via linear polarization X-ray ptychography.

X-ray linear dichroism and X-ray birefringence microscopy are yet to be fully utilized as instruments in the microstructural characterization of crystalline materials. Here, we demonstrate analyser-free X-ray linear dichroism microscopy using spectroscopic hard X-ray ptychography. First experiments enabled a spectroscopic and microstructural characterisation of polycrystalline vanadium pentoxide on the nanoscale, outside of diffraction-contrast based methods.

Spatially resolved mapping of phase transitions in liquid-crystalline materials by X-ray birefringence imaging.

The X-ray Birefringence Imaging (XBI) technique, first reported in 2014, is a sensitive method for spatially resolved mapping of the local orientational properties of anisotropic materials. We report the first application of the XBI technique to characterize molecular orientational ordering in a liquid crystalline material, demonstrating significant potential for exploiting XBI measurements to advance structural understanding of liquid crystal phases.

Structure and properties of regenerated cellulose fibers from aqueous NaOH/thiourea/urea solution

Regenerated cellulose fibers were successfully prepared through dissolving cotton linters in NaOH/thiourea/urea aqueous solution at −2 °C by a twin-screw extruder and wet-spinning process at varying precipitation and drawing conditions. The dissolution process of an optimized 7 wt% cellulose was controlled by polarizing microscopy and resulted in a transparent and stable cellulose spinning dope. Rheological investigations showed a classical shear thinning behavior of the cellulose/NaOH/thiourea/urea solution and a good stability towards gelation. Moreover, the mechanical properties, microstructures and morphology of the regenerated cellulose fibers were studied extensively by single fiber tensile testing, X-ray diffraction, synchrotron X-ray investigations, birefringence measurements and field-emission scanning electron microscopy. Resulting fibers demonstrated a smooth surface and circular cross-section with homogeneous morphological structure as compared with commercial viscose rayon. At optimized jet stretch ratio, acidic coagulation composition and temperature, the structural features and tensile properties depend first of all on the drawing ratio. In particular the crystallinity and orientation of the novel fibers rise with increasing draw ratio up to a maximum followed by a reduction due to over-drawing and oriented crystallites disruption. The microvoids in the fiber as analysed with SAXS were smaller and more elongated with increasing drawing ratio. Moreover, a higher tensile strength (2.22 cN/dtex) was obtained in the regenerated fiber than that of the viscose rayon (2.13 cN/dtex), indicating higher crystallinity and orientation, as well as more elongated and orientated microvoid in the regenerated fiber. All in all, the novel extruder-based method is beneficial with regard to the dissolution temperature and a simplified production process. Taking into account the reasonable fiber properties from the lab-trials, the suggested dissolution and spinning route may offer some prospects as an alternative cellulose processing route.

Crystallographic and optical study of PbHfO3 crystals.

The symmetry of the intermediate high-temperature phase of PbHfO3 has been determined unambiguously to be orthorhombic using a combination of high-resolution X-ray diffraction and birefringence imaging microscopy measurements of crystal plates. While lattice parameter measurements as a function of temperature in the intermediate phase are consistent with either orthorhombic or tetragonal symmetry, domain orientations observed in birefringence imaging microscopy measurements utilizing the Metripol system are only consistent with orthorhombic symmetry with the unit cell in the rhombic orientation of the pseudocubic unit cell.

Radial mosaic internal structure of rounded diamond crystals from alluvial placers of Siberian platform

The specific gray to almost black diamonds of rounded morphology are especially typical in alluvial placers of the northeastern part of the Siberian platform. The results of study of internal structure of these diamonds are presented. X-ray topography and birefringence patterns of polished plates of studied diamonds show their radial mosaic structure. Diamonds consists of slightly misorientated (up to 20′) subindividuals which are combined to mosaic wedge-shaped sectors. Electron back-scatter diffraction technique has demonstrated that subindividuals are often combined in the single large blocks (subgrains). The whole crystals commonly consist of several large subgrains misoriented up to 5° to one another. The total nitrogen content of these diamonds vary in the range 900–3300 ppm and nitrogen aggregation state (NB/(NB + NA)*100) from 25 to 64 %. Rounded diamond crystals of variety V are suggested to have been formed at the high growth rate caused by the high oversaturation of carbon in the crystallization medium. It may result in the splitting of growing crystal and their radial mosaic structure as a sequence. High content of structural nitrogen defects and the great number of mechanical impurities – various mineral and fluid inclusions may also favor to generation of this structure.

New features from transparent thin films of EuTiO3

ABSTRACTThe almost multiferroic perovskite EuTiO3 (ETO) has been prepared as films on substrates of SrTiO3. For all prepared film thicknesses highly transparent insulating films with atomically flat surfaces and excellent orientation have been grown. They were characterized by X-ray diffraction, magnetic susceptibility and birefringence measurements and found to exhibit bulk properties, namely an antiferromagnetic transition at TN = 5.1 K and a structural transition at TS = 282 K. The latter could only be identified due to the high transparency of the samples since the optical band gap is of the order of 4.5 eV and larger than observed before for any bulk and thin film samples.

Improvement of dielectric properties of cyanoethyl cellulose via esterification and film stretching

This study offers a concept of material functionalization for cellulose derivatives via chemical modification followed by orientation control. Namely, we improved the dielectric properties of cyanoethyl cellulose (CyEC) by a combination of esterification and uniaxial heat-stretching. To this end, we prepared CyEC ester derivatives (Cm-CyEC: m = 3, 4, 5, 8, 12 and 18; m is the carbon number of the side chain) with almost no residual hydroxyl groups from CyEC (degree of cyanoethyl substitution = 2.54) in a homogeneous system. The introduction of higher fatty acids (m ≥ 8) improved the thermal processability of the derivatives, because of diminishing crystallinity. Cast films were stretched at temperatures close to the glass transition temperature (T g) (152–166 °C), and different manners of orientation between the original CyEC and C18-CyEC were confirmed by IR dichroism, wide-angle X-ray diffraction and birefringence measurements. The dielectric constant for as-cast films monotonically decreased with an increase in m. The constants of the drawn CyEC films also decreased with increasing percent elongation (γ), but those of the C18-CyEC films increased with increasing γ. Such contradictory behaviour may be attributed to a local segmental orientation, which greatly increased the sum of the dipole moments of carbonyl and cyano moieties for C18-CyEC. Consequently, the dielectric constant of the stretched film of C18-CyEC increased up to ~23 (at 102 Hz), a 2.5-fold increase beyond the maximum dielectric constant of the unstretched C18-CyEC film, while the dielectric loss remained ≤0.05 at 102–105 Hz and 25 °C.

Structure and mechanical properties of multi-hollowed fibers produced by CO2 laser irradiated neck drawing

Multi-hollowed fibers with a hollowness ratio of over 20% were successfully produced using a neck-drawing technique accompanied by CO2 laser irradiation on unoriented and amorphous polytrimethylene terephthalate (PTT) fibers with diameters of approximately 104 µm. The structure and mechanical properties of the multi-hollowed fibers were characterized using scanning electron microscopy, wide angle X-ray diffraction (WAXD), birefringence measurements, and tensile tests. Crystal orientation factors in the direction normal to the (010) and (002) planes, which were obtained from WAXD patterns, were lower in comparison to values obtained in previous studies on fibers that did not feature these hollows [9] because of the development of these internal hollows. The PTT fibers in this study also displayed high toughness, about 465 MPa (3.5 cN/dtex) strength and about 40% elongation, and had excellent elastic recovery (>95%) after 10 stretch cycles. POLYM. ENG. SCI., 2015. © 2015 Society of Plastics Engineers

Linear and third order nonlinear optical properties of LiRbB4O7 single crystal

Lithium rubidium borate (LiRbB4O7) single crystal has been grown by the Czochralski method. Crystalline perfection and optical homogeneity of the grown LiRbB4O7 crystal are analyzed by high resolution X-ray diffraction and birefringence interferometric technique, respectively. Third order nonlinear optical parameters of LiRbB4O7 crystal are determined by Z-scan experimental technique. The nonlinear refractive index (n2) and third-order nonlinear optical susceptibility (χ3) are estimated to be −4.935 × 10−11 cm2/W and 2.719 × 10−7 esu, respectively. The measured (n2) value reveals the self-focusing nature of LiRbB4O7 crystal.

Structural and optical properties of LiKB4O7 single crystals grown by Czochralski technique

One of the alkali metal borates, lithium potassium borate (LiKB4O7) single crystal, was grown following two different micro step pulling movements employing the modified crystal puller. The influence of two different micro step pulling movements on the crystalline nature, optical properties and micro morphology of the grown LiKB4O7 crystal was investigated by high resolution X-ray diffraction (HRXRD) analysis and birefringence interferometry and chemical etching techniques, respectively. HRXRD studies revealed that the crystalline perfection of the grown crystals is reasonably good. Interferometric images showed that the crystal grown under higher micro step pulling movement has very less number of scattering centers. The etching studies revealed that the crystal grown under higher micro steps pulling movement contains relatively low level dislocation density.

Theoretical analysis of the background intensity distribution in X-ray Birefringence Imaging using synchrotron bending-magnet radiation

In the recently developed technique of X-ray Birefringence Imaging, molecular orientational order in anisotropic materials is studied by exploiting the birefringence of linearly polarized X-rays with energy close to an absorption edge of an element in the material. In the experimental setup, a vertically deflecting high-resolution double-crystal monochromator is used upstream from the sample to select the appropriate photon energy, and a horizontally deflecting X-ray polarization analyzer, consisting of a perfect single crystal with a Bragg reflection at Bragg angle of approximately 45°, is placed downstream from the sample to measure the resulting rotation of the X-ray polarization. However, if the experiment is performed on a synchrotron bending-magnet beamline, then the elliptical polarization of the X-rays out of the electron orbit plane affects the shape of the output beam. Also, because the monochromator introduces a correlation between vertical position and photon energy to the X-ray beam, the polarization analyzer does not select the entire beam, but instead selects a diagonal stripe, the slope of which depends on the Bragg angles of the monochromator and the polarization analyzer. In the present work, the final background intensity distribution is calculated analytically because the phase space sampling methods normally used in ray traces are too inefficient for this setup. X-ray Birefringence Imaging data measured at the Diamond Light Source beamline B16 agree well with the theory developed here.

X-ray Birefringence Imaging of Materials with Anisotropic Molecular Dynamics.

The X-ray birefringence imaging (XBI) technique, reported very recently, is a sensitive tool for spatially resolved mapping of the local orientational properties of anisotropic materials. In this paper, we report the first XBI measurements on materials that undergo anisotropic molecular dynamics. Using incident linearly polarized X-rays with energy close to the Br K-edge, the X-ray birefringence is dictated by the orientational properties of the C-Br bonds in the material. We focus on two materials (urea inclusion compounds containing 1,8-dibromooctane and 1,10-dibromodecane guest molecules) for which the reorientational dynamics of the brominated guest molecules (and hence the reorientational dynamics of the C-Br bonds) are already well characterized by other experimental techniques. The XBI results demonstrate clearly that, for the anisotropic molecular dynamics in these materials, the effective X-ray optic axis for the X-ray birefringence phenomenon is the time-averaged resultant of the orientational distribution of the C-Br bonds.

Structural and optical study of the phase transitions in Lead Hafnate, PbHfO3

Lead Hafnate undergoes two structural phase transitions as a function of temperature. The first occurs at about 1630C, consisting of a transition from an antiferroelectric orthorhombic Pbam structure [1] to another antiferroelectric orthorhombic phase with an as-yet undetermined space group. The second is to a paraelectric cubic Pm3m structure at 2090C. Dielectric spectroscopy measurements on a single crystal have shown a distinct temperature hysteresis at the orthorhombic to orthorhombic transition [2]. Recently, dielectric measurements on a ceramic sample have shown a much larger temperature hysteresis and following x-ray diffraction measurements, it is suggested that the second orthorhombic phase is in space group A2mm and undergoes another transition to a tetragonal P4mm structure before the cubic transition [3]. We report on the results of an investigation of a PbHfO3crystal using a combination of high-resolution x-ray diffraction and birefringence imaging measurements with the Metripol system. These measurements have been performed as a function of temperature from the room-temperature orthorhombic structure to the high-temperature cubic structure. The results are discussed both in the context of the published work and fundamental understanding of the origin of antiferroelectricity.

X-ray birefringence imaging.

The polarizing optical microscope has been used since the 19th century to study the structural anisotropy of materials, based on the phenomenon of optical birefringence. In contrast, the phenomenon of x-ray birefringence has been demonstrated only recently and has been shown to be a sensitive probe of the orientational properties of individual molecules and/or bonds in anisotropic solids. Here, we report a technique-x-ray birefringence imaging (XBI)-that enables spatially resolved mapping of x-ray birefringence of materials, representing the x-ray analog of the polarizing optical microscope. Our results demonstrate the utility and potential of XBI as a sensitive technique for imaging the local orientational properties of anisotropic materials, including characterization of changes in molecular orientational ordering associated with solid-state phase transitions and identification of the size, spatial distribution, and temperature dependence of domain structures.

X-ray Birefringence in highly Anisotropic Materials

Birefringence is the dependence of a material's refractive index on the direction of linear polarization. It induces a phase shift between two perpendicular polarization directions and thus couples linear and circular polarization states. Birefringence in x-ray absorption is as common as linear dichroism but is rarely discussed in the literature. We outline a mathematical framework for describing experiments on birefringence and illustrate the importance of the phenomenon with three examples.

X-ray Birefringence: A New Strategy for Determining Molecular Orientation in Materials.

While the phenomenon of birefringence is well-established in the case of visible radiation and is exploited in many fields (e.g., through the use of the polarizing optical microscope), the analogous phenomenon for X-rays has been a virtually neglected topic. Here, we demonstrate the scope and potential for exploiting X-ray birefringence to determine the orientational properties of specific types of bonds in solids. Specifically, orientational characteristics of C-Br bonds in the bromocyclohexane/thiourea inclusion compound are elucidated from X-ray birefringence measurements at energies close to the bromine K-edge, revealing inter alia the changes in the orientational distribution of the C-Br bonds associated with a low-temperature order-disorder phase transition. From fitting a theoretical model to the experimental data, reliable quantitative information on the orientational properties of the C-Br bonds is determined. The experimental strategy reported here represents the basis of a new approach for gaining insights into the orientational properties of molecules in anisotropic materials.

Interaction of microwave radiation with polyester yarn to enhance its textile properties

The interaction of microwave radiation with Partially Oriented Polyester yarn (PET-POY) was studied with a view to improve its structure and textile related properties. The PET-POY filaments were exposed to Microwave (MW) radiations of frequency 2450 MHz for different durations of time from 15 to 105 sec. The changes in structure and morphology were investigated by using the techniques of X-ray diffraction, birefringence and DSC. The changes in the textile related properties like tensile strength, shrinkage and dye uptake were evaluated. The microwave treatment enhanced the structural properties of PET-POY samples. Significant increase in crystallinity, crystal size and crystalline orientation were noted. In addition a great improvement in total orientation, as measured by birefringence, was found for the treated samples. DSC results showed that crystal distribution became narrower and crystallization rate increased for microwave treated samples as compared to control sample. The tensile strength, shrinkage and dye uptake also showed significant increase, which is very helpful for the textile processes.

The structure–property relationships of artificial silk fabricated by dry-spinning process

Regenerated silk fibroin (RSF) fibers were dry-spun from RSF aqueous solution and then post-treated in ethanol aqueous solution. In order to prepare artificial silk which are tougher and stronger than their natural counterpart, the structure–property relationships of the RSF fibers and natural silkworm silks were investigated by using synchrotron radiation X-ray microdiffraction technology, birefringence measurements and Raman spectroscopy. The as-spun RSF fibers with poor mechanical properties exhibited a strong diffraction peak of the [021] lattice plane and a weak diffraction peak of the [020]/[200] lattice plane. However, both the natural silk and the post-treated RSF fibers with exceptional mechanical properties showed weak diffraction peaks of the [021] lattice plane and strong diffraction peaks of the [020]/[200] lattice plane. Nevertheless, the two crystalline peaks are attributed to the silk II structure of silk fibroin. By deconvoluting the one-dimensional wide-angle X-ray microdiffraction pattern, the crystallinity and the degree of crystalline orientation were obtained. The as-spun fibers showed low crystallinity and low crystalline orientation, but the microstructure of the RSF fibers could be improved greatly and even become similar to that of degummed cocoon silk by post-treatment. When the as-spun RSF fibers were first drawn 3 to 4 times with a draw rate of 0.9 mm s−1 in ethanol aqueous solution and then immersed in the same solution for another hour, the breaking strain and breaking energy of the post-treated fibers were significantly greater than those of degummed cocoon silk.