Development and Applications of X-ray Birefringence Imaging
Lead Research Organisation:
CARDIFF UNIVERSITY
Department Name: Chemistry
Abstract
The PhD research project will focus on the new technique of X-ray Birefringence Imaging (first reported in 2014), with the aim of advancing fundamental aspects, developing improved experimental methodology and exploring wide-ranging applications of the technique.
The X-ray Birefringence Imaging technique allows X-ray birefringence of materials to be measured in a spatially resolved manner (spatial resolution ca. 10 microns), representing, in many respects, the X-ray analogue of the polarizing optical microscope. However, while optical birefringence depends on the overall symmetry properties of a material, X-ray birefringence (when studied using an X-ray energy corresponding to the absorption edge of an element in the material) is sensitive to the local orientational properties of individual molecules and/or bonds.
Since the first report in 2014, X-ray Birefringence Imaging has been shown to be a sensitive technique for characterizing changes in molecular orientational ordering associated with solid-state phase transitions, including the identification of domain structures in which different regions of a material comprise orientationally distinct arrangements of the molecules. As X-ray birefringence is sensitive to local molecular orientational properties, there is no requirement for the sample to be crystalline, and X-ray Birefringence Imaging may be applied to any material with an anisotropic distribution of molecular orientations (including liquid or amorphous phases). The technique also has particular utility in cases (e.g. partially ordered materials, multiply twinned crystals or materials with complex domain structures) for which the application of diffraction-based techniques is limited. Furthermore, X-ray Birefringence Imaging data can be recorded rapidly, leading to exciting prospects for exploiting the time-resolved capabilities of the technique, such as probing time-dependent changes in molecular orientational distributions.
The PhD project will apply X-ray Birefringence Imaging to a diverse range of materials and structural phenomena, with the aim of demonstrating the utility of the technique for exploring molecular orientations in solids and other anisotropic materials, including: (i) molecular orientational distributions in liquid crystalline materials, (ii) domain structures in ferroelectric materials, (iii) orientational properties of molecules at interfaces, (iv) alignment of molecules within fluid phases in response to applied fields, and (v) propagation of domain boundaries associated with phase transitions in solids (leading to insights on mechanistic aspects of phase transitions).
In another aspect of the project, we aim to establish the theoretical basis for computing the X-ray optic axis of any molecule at a specific X-ray absorption edge. To date, studies of X-ray Birefringence have focused on brominated organic materials at the Br K-edge, for which the X-ray optic axis is defined by the C-Br bond vector. However, when the X-ray absorbing atom has a more complicated bonding environment, it is not straightforward to assign the X-ray optic axis simply from inspection of the molecular structure. In such cases, computational protocols must be developed to allow the X-ray optic axis to be calculated. The project will include computational developments in this direction, which represent an essential prerequisite before X-ray Birefringence Imaging can be generalized to study any type of material in the future.
The overall aim of the project is to consolidate X-ray Birefringence Imaging as an experimental technique that will be used widely in different fields in the future. At present, the technique is still in its infancy, and the next few years represent an exciting opportunity to drive the rapid evolution of a new technique at a very early stage of its development.
The X-ray Birefringence Imaging technique allows X-ray birefringence of materials to be measured in a spatially resolved manner (spatial resolution ca. 10 microns), representing, in many respects, the X-ray analogue of the polarizing optical microscope. However, while optical birefringence depends on the overall symmetry properties of a material, X-ray birefringence (when studied using an X-ray energy corresponding to the absorption edge of an element in the material) is sensitive to the local orientational properties of individual molecules and/or bonds.
Since the first report in 2014, X-ray Birefringence Imaging has been shown to be a sensitive technique for characterizing changes in molecular orientational ordering associated with solid-state phase transitions, including the identification of domain structures in which different regions of a material comprise orientationally distinct arrangements of the molecules. As X-ray birefringence is sensitive to local molecular orientational properties, there is no requirement for the sample to be crystalline, and X-ray Birefringence Imaging may be applied to any material with an anisotropic distribution of molecular orientations (including liquid or amorphous phases). The technique also has particular utility in cases (e.g. partially ordered materials, multiply twinned crystals or materials with complex domain structures) for which the application of diffraction-based techniques is limited. Furthermore, X-ray Birefringence Imaging data can be recorded rapidly, leading to exciting prospects for exploiting the time-resolved capabilities of the technique, such as probing time-dependent changes in molecular orientational distributions.
The PhD project will apply X-ray Birefringence Imaging to a diverse range of materials and structural phenomena, with the aim of demonstrating the utility of the technique for exploring molecular orientations in solids and other anisotropic materials, including: (i) molecular orientational distributions in liquid crystalline materials, (ii) domain structures in ferroelectric materials, (iii) orientational properties of molecules at interfaces, (iv) alignment of molecules within fluid phases in response to applied fields, and (v) propagation of domain boundaries associated with phase transitions in solids (leading to insights on mechanistic aspects of phase transitions).
In another aspect of the project, we aim to establish the theoretical basis for computing the X-ray optic axis of any molecule at a specific X-ray absorption edge. To date, studies of X-ray Birefringence have focused on brominated organic materials at the Br K-edge, for which the X-ray optic axis is defined by the C-Br bond vector. However, when the X-ray absorbing atom has a more complicated bonding environment, it is not straightforward to assign the X-ray optic axis simply from inspection of the molecular structure. In such cases, computational protocols must be developed to allow the X-ray optic axis to be calculated. The project will include computational developments in this direction, which represent an essential prerequisite before X-ray Birefringence Imaging can be generalized to study any type of material in the future.
The overall aim of the project is to consolidate X-ray Birefringence Imaging as an experimental technique that will be used widely in different fields in the future. At present, the technique is still in its infancy, and the next few years represent an exciting opportunity to drive the rapid evolution of a new technique at a very early stage of its development.
People |
ORCID iD |
| Kenneth Harris (Primary Supervisor) | |
| Rhian Patterson (Student) |
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509449/1 | 01/10/2016 | 30/09/2021 | |||
| 1767012 | Studentship | EP/N509449/1 | 01/07/2016 | 31/12/2019 | Rhian Patterson |
| Description | The phenomenon of X-ray birefringence relates to the way in which refractive index depends on the orientation of an anisotropic material relative to the direction of linear polarization of an incident X-ray beam. The ability of X-ray birefringence to yield insights into molecular orientational properties was first exploited by our group for accurate determination of bond orientations and for establishing changes in molecular orientational distributions associated with order-disorder phase transitions in solids. However, these early X-ray birefringence studies used a narrowly focused incident X-ray beam and did not provide spatially resolved mapping of X-ray birefringence across the material. In 2014, we developed a new technique called X-ray Birefringence Imaging (XBI) to allow X-ray birefringence data to be measured in a spatially resolved manner. In many respects, XBI is the X-ray analogue of the polarizing optical microscope, and is a sensitive technique for imaging the local orientational properties of anisotropic materials, allowing orientationally distinct domain structures to be identified and yielding information on the size, spatial distribution, temperature dependence and orientational relationships of such domains. XBI experiments are carried out on beamline B16 at Diamond Light Source. The XBI technique has developed significantly during the PhD project. Previous work focused solely on the bromine K-edge (13.4 keV), and we have now established that the copper K-edge (8.97 keV) is also highly suitable for this technique. Expansion to the copper K-edge meant that a single of crystal of copper dibromide could be imaged at two absorption edges. Further work on B16 has established that the iron K-edge may also be imaged, although we have not yet recorded images of a suitable material. Improvements to I16 at DLS means that the X-ray dichroism of a thiourea sample at the sulphur K-edge could be recorded. The sample showed strong dichroism which in turn means that the sample will also have birefringence as they rely on the same physical properties. However, currently the setup on I16 does not allow for X-ray birefringence imaging. In the PhD project, XBI has been applied to study molecular orientational distributions in a wide range of materials, including those that undergo interesting phase transitions. For example, 1,2,4,5-tetrabromobenzene is a thermosalient material, with a phase transition at 45 °C. This structural transformation causes the crystals to jump or break, we also noticed some unusual domains where XBI behaviour was present in only specific regions of the crystal. Furthermore, tetrabromobenzene commonly forms twins who's X-ray optic axis vector is significantly different, meaning that while one crystal shows bright in intensity the other will be dark. XBI gives insights into changes in molecular orientation preceding, during and after the phase transition. Some of the materials studied have interesting physical properties, including 1,4-dibromo-2,3,5,6-tetramethylbenzene (DBTMB). Single crystals of DBTMB can undergo plastic bending. Details of the molecular orientations as a function of position within the bent crystals has been imaged using XBI, revealing a continuous change in molecular orientations rather than domain formation. XBI has also been used to determine molecular orientational ordering in liquid crystalline phases, allowing the degree of molecular orientational ordering to be probed as a function of temperature, including the study of materials that exist in a number of different phases upon cooling (e.g. isotropic liquid - nematic - smectic A - smectic B - crystal). We have designed and constructed a sample cell for these XBI experiments, in which the liquid crystal phases are aligned in an external magnetic field, with the XBI measurements carried out as a function of the orientation of the magnetic field relative to the direction of polarization of the incident X-ray beam. From the XBI measurements, the degree of molecular alignment in the liquid crystalline phases can be determined quantitatively as a function of temperature. We have also recently initiated XBI studies to explore the alignment of molecules in liquid phases subjected to an applied electric field. |
| Exploitation Route | The expansion of the technique to the copper K-edge has widened significantly the range of potential materials that can be imaged using XBI. In addition to demonstrating successful XBI experiments at both the copper K-edge and the bromine K-edge, in principle all elements between copper and bromine in the periodic table should be accessible for XBI experiments on B16. The copper K-edge is of particular interest as it opens up the opportunity to image a range of important materials such as biological materials (e.g. haemocyanin, a protein that transports oxygen in some invertebrates, analogous to haemoglobin in vertebrates) and copper-based electronic materials. |
| Sectors | Chemicals,Electronics,Healthcare,Pharmaceuticals and Medical Biotechnology |
| URL | https://pubs.rsc.org/en/content/articlepdf/2019/sc/c8sc05285a |
| Description | Electron Diffraction and AIRSS |
| Organisation | University of Cambridge |
| Country | United Kingdom |
| Sector | Academic/University |
| PI Contribution | Attempting to solve the structure of an ammonium cyanate co-crystal using X-ray powder diffraction. Two viable crystal structures have been determined, similar but with different space groups. X-ray powder diffraction suggests one space group over the other, but validation of the structure using other techniques would be beneficial. |
| Collaborator Contribution | Electron diffraction on the powder was attempted to give a unit cell, as of yet there has been no success. Due in part to the sample being rather impure, with little to distinguish between pure and impure phase. AIRSS (Ab-intio random structure searching) is also going to be used to try and validate the crystal structure. |
| Impact | Crystal structure of ammonium cyanate cocrystal determined using X-ray powder diffraction, electron diffraction and AIRSS |
| Start Year | 2019 |
| Description | Cardiff Chemistry Conference |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Professional Practitioners |
| Results and Impact | A talk about XBI intended to be understandable by a professional audience not necessarily engaged in research in a similar field. A few people appeared interested in the potential application of XBI to their own work. |
| Year(s) Of Engagement Activity | 2019 |
| Description | Conference; UKSR50 50 years of Synchrotron Radiation in the UK and its global impact |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Professional Practitioners |
| Results and Impact | A poster of about X-ray Birefringence Imaging (XBI) of a crystal of copper acetyl acetonate. The poster was presented at a conference (UKSR50 50 years of Synchrotron Radiation in the UK and its global impact). It was awarded the Wiley Publishers poster prize at the conference. |
| Year(s) Of Engagement Activity | 2018 |