First Principles Study of Ferroelectricity in Halide Perovskites for Future Photoferroics

Perovskite and perovskite-like materials have garnered warranted attention over the previous decade due to their potential use in high efficiency photovoltaic devices. Their flexibility both chemically and sterically gives materials' scientists and engi- neers the opportunity to tune the properties of such materials to suit the application that it is needed for. In this thesis we use ab initio methods to describe the funda- mental distortions that appear in a series of structures and their associated optoelec- tronic properties. In the first results chapter we complete a rigorous exploration of a variety of chemistries where we explore the various distortions that can appear in these materials. I show that the system exhibits a very complex interplay between pri- marily the breathing distortion and Jahn-Teller distortion and subsequently all other distortions that appear at lower tolerance factors. We also explore the appearance of a polar instability. In the second results chapter we take the understanding of these distortions and physical aspects of the gold double perovskites to discuss the opto- electronic properties of these materials. We find that the Jahn-Teller distortion opens up the gap in a metallic to insulator phase transition, we examine the effects of each of the distortions on the size of the gap and the effective masses of the structures and we see that the introduction of tilt distortions opens up the gap even more and increases the effective masses of the charge carriers. Examination of pressure presented us with the unusual discovery that hydrostatic pressure suppresses the appearance of the tilt distortions in this perovskite structure due to the cooperative nature between the tilt and Jahn-Teller distortions(something that is suppressed with pressure). With the knowledge of the effects of pressure we predicted a polar phase in the Rb2Au2(Br, I)6 structures with the introduction of a small hydrostatic pressure. We explored the pos- sibility of an enhanced dielectric constant through tuning of lattice parameters with pressure and was able to show an enhanced dielectric constant at a region where the phonon frequency was sufficiently softened. In the final part of the thesis we dis- cussed the application of hybrid improper ferroelectricity. We showed through sym- metry analysis that a polar distortion appears in the low-symmetry A3B2X7(A = Cs, Rb, K; B = Pb, Sn, Ge; X = I, Br, Cl) Ruddlesden-Popper structures via a trilinear coupling term with two non-polar distortions. We explored the effect of chemical substitution on the size of the gap and the effects each change in the chemistry had on the size of the distortions, we suggested that the Cs3Ge2I7 system is a proper ferroelectric (does not follow the improper method) and finally we explored the effects of Spin-Orbit coupling and the type of exchange-correlation functional that was used on the ac- curacy of our results. Overall we believe that the work presented in this review puts forward some exciting and interesting discussions of charge-separation in perovskite photovoltaics. As we look forward in this review we make some suggestions of other structures which would be more suitable for a possible experimental analysis

Wales: challenges and opportunities for post-Brexit environmental governance

Brexit represents a major change to environmental governance in Wales and the United Kingdom (UK), raising both opportunities and challenges. Welsh stakeholders are worried that English interests will predominate in the design of environmental governance after Brexit and are also concerned about the prospect of greater instability and weaker environmental protections. Crucially, the key planks of the UK government’s ‘Green Brexit’ strategy—the 25 Year Environment Plan (25 YEP) and Defra’s environmental governance and principles consultation—do not cover governance in the devolved nations. This gap in coverage raises the prospect of policy divergence and inconsistent implementation and enforcement across the UK. Most importantly, there is a strong fear in Wales that Welsh environmental policy ambition will be thwarted by Brexit and deregulatory pressure emanating from England

Northern Ireland: challenges and opportunities for post-Brexit environmental governance

Brexit represents a major change to environmental governance in Northern Ireland and the UK. Yet it is occurring at a time when Northern Ireland has no government, curtailing its ability to engage in both local and UK-wide preparations. Northern Irish stakeholders are worried that tensions between England and Scotland are dominating Brexit preparations, hampering discussions of UK-wide cooperation, as well as of the specific needs of Northern Ireland. They are concerned pre-existing environmental governance issues in the region (such as the lack of an independent environmental agency or the prevalence of cross-border environmental crime) will remain unaddressed, and that current North/South cooperation on environmental issues will be negatively impacted by the Brexit deal. Crucially, the key planks of the UK government’s ‘Green Brexit’ strategy (such as the commitments laid out in the 25 Year Environment Plan and the Environmental Principles and Governance consultation) do not cover the devolved nations. This raises the prospect of further policy divergence and inconsistent implementation and enforcement across the UK, and for Northern Ireland’s environment to continue deteriorating

Environmental policy in a devolved United Kingdom: challenges and opportunities after Brexit

The European Union (EU) has had a profound effect on UK environmental policy and governance. The EU provides treaty-based principles to underpin and inform new policy development and a well-developed system of monitoring and enforcement to ensure it is implemented. The EU’s system of environmental governance provides a set of structures that establishes minimum common standards across the UK. These structures have allowed the devolved nations to develop their own policies, some with a higher level of ambition than the UK’s. EU membership, therefore, provides a common framework that enables both transboundary cooperation and local policy innovation. Brexit consequently represents a major change to environmental governance in the United Kingdom (UK), raising significant opportunities and challenges. Environmental policy in the UK is devolved, but UK devolution is asymmetrical: England has no formal representation or parliament. For environmental policy this means that the Department for Environment, Food and Rural Affairs (Defra) often acts on behalf of England. This model of devolution raises concerns amongst stakeholders in the devolved nations that Defra approaches policy with an English mind-set, which suggests that English interests will dominate after Brexit. There are also concerns that Brexit will lead to greater instability and weaken environmental protections. In addition, Brexit has prompted a constitutional dispute between the Scottish and UK governments, which may jeopardise future environmental governance. These tensions have created uncertainty, making Brexit preparations highly challenging for both government and civil society actors. Crucially, the key planks of the UK government’s ‘Green Brexit’ strategy— the 25 Year Environment Plan (25 YEP) and Defra’s consultation on environmental governance and principles—do not cover governance in the devolved nations. This gap in coverage raises the prospect of policy divergence and inconsistent implementation and enforcement across the UK. More importantly, there is a strong fear in Scotland and Wales that their environmental policy ambitions could be thwarted by Brexit and deregulatory pressures emanating from England. Meanwhile Northern Ireland, which has a history of weak environmental governance and sits alongside the politically sensitive border with Ireland, has no government and therefore no voice in the Brexit negotiations. As a result, it is poorly represented in the discussions over the future shape of UK-wide environmental policy and governance

Wales: challenges and opportunities for post-Brexit environmental governance

Brexit represents a major change to environmental governance in Wales and the United Kingdom (UK), raising both opportunities and challenges. Welsh stakeholders are worried that English interests will predominate in the design of environmental governance after Brexit and are also concerned about the prospect of greater instability and weaker environmental protections. Crucially, the key planks of the UK government’s ‘Green Brexit’ strategy—the 25 Year Environment Plan (25 YEP) and Defra’s environmental governance and principles consultation—do not cover governance in the devolved nations. This gap in coverage raises the prospect of policy divergence and inconsistent implementation and enforcement across the UK. Most importantly, there is a strong fear in Wales that Welsh environmental policy ambition will be thwarted by Brexit and deregulatory pressure emanating from England

Scotland: challenges and opportunities for post-Brexit environmental governance

Brexit represents a major change to environmental governance in Scotland and the United Kingdom (UK), raising both opportunities and challenges. It has prompted a constitutional dispute between the Scottish and UK governments, which may jeopardise future environmental governance. The current constitutional impasse has created even more uncertainty, making Brexit preparations highly challenging for government and civil society actors. Scottish stakeholders are worried that English interests will predominate in the design of environmental governance after Brexit and are also concerned about the prospect of greater instability and weaker environmental protections. Crucially, the key planks of the UK government’s ‘Green Brexit’ strategy—the 25 Year Environment Plan and the Department for Environment, Food and Rural Affairs’ (Defra) environmental governance and principles consultation—do not cover governance in the devolved nations. This gap in coverage raises the prospect of policy divergence and inconsistent implementation and enforcement across the UK. Most importantly, there is a strong fear in Scotland that Scottish environmental policy ambition will be thwarted by Brexit and deregulatory pressure from England

<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties

Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7. Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release. Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue. Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7. Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data

Gaia Early Data Release 3 Acceleration of the Solar System from Gaia astrometry

Context. Gaia Early Data Release 3 (Gaia EDR3) provides accurate astrometry for about 1.6 million compact (QSO-like) extragalactic sources, 1.2 million of which have the best-quality five-parameter astrometric solutions. Aims. The proper motions of QSO-like sources are used to reveal a systematic pattern due to the acceleration of the solar systembarycentre with respect to the rest frame of the Universe. Apart from being an important scientific result by itself, the acceleration measured in this way is a good quality indicator of the Gaia astrometric solution. Methods. Theeffect of the acceleration was obtained as a part of the general expansion of the vector field of proper motions in vector spherical harmonics (VSH). Various versions of the VSH fit and various subsets of the sources were tried and compared to get the most consistent result and a realistic estimate of its uncertainty. Additional tests with the Gaia astrometric solution were used to get a better idea of the possible systematic errors in the estimate. Results. Our best estimate of the acceleration based on Gaia EDR3 is (2.32 +/- 0.16) x 10(-10) m s(-2) (or 7.33 +/- 0.51 km s(-1) Myr-1) towards alpha = 269.1 degrees +/- 5.4 degrees, delta = -31.6 degrees +/- 4.1 degrees, corresponding to a proper motion amplitude of 5.05 +/- 0.35 mu as yr(-1). This is in good agreement with the acceleration expected from current models of the Galactic gravitational potential. We expect that future Gaia data releases will provide estimates of the acceleration with uncertainties substantially below 0.1 mu as yr(-1).Peer reviewe