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College Senior Scholarship for Ben

last modified Nov 25, 2019 02:05 PM
PhD student Ben Shires has been awarded a Fitzwilliam College Senior Scholarship.

Ben, a second-year PhD student in the Materials Theory Group, received the award from Fitzwilliam College in November. The Scholarship is awarded annually by Fitz to its highest performing PhD students, and is provided in recognition of excellent intellectual achievements and excellent work. The award includes £350 to help towards academic-related costs during the year. Well done Ben!

Ben's talk at the Machine Learning Discussion Group

PhD student Ben Shires will be presenting his work on SHEAP - Stochastic Hyperspace Embedding And Projection - at the Machine Learning Discussion Group in the Cavendish Laboratory.
When Nov 25, 2019
from 04:30 PM to 05:00 PM
Where Mott Seminar Room, Cavendish Laboratory, University of Cambridge
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Abstract. We present SHEAP - Stochastic Hyperspace Embedding And Projection – a tool designed to process the structural data obtained from a materials structure search, in order to produce a visualisation of the energy surface being sampled. We have drawn inspiration from state-of-the-art algorithms for dimensionality reduction of high-dimensional data, such as t-SNE and UMAP . We illustrate the power of SHEAP through its application to the model energy landscapes defined by systems of particles interacting via a simple Lennard-Jones pair potential.

Talks.cam link here

In-browser cloud-hosted AIRSS examples available

last modified Aug 08, 2019 02:46 PM
In-browser cloud-hosted AIRSS examples available

Screenshot of the interactive AIRSS command prompt, accessible at http://bit.ly/2Yybv3O.

Materials Theory Group member and PhD student Bonan Zhu has created an interactive online environment for the ab initio random structure searching (AIRSS) code.

The environment uses binder and allows a user to access and run selected examples from the AIRSS code in-browser.

You can access the in-browser interface, and try it yourself here: https://mybinder.org/v2/gl/airss%2Fairss-demo/master?urlpath=terminals/1

Note that it may take a few minutes to build the AIRSS image and deploy it. Once you have command line access, standard bash commands like ls, more and cd can be used to navigate the example directories. Each example contains a README file explaining how to run it.

The AIRSS code implements the random-structure-searching approach to structure prediction, as developed by Pickard and Needs. As well as the online examples above, you can also download and build the full AIRSS code, which is available under the GPL 2.0 licence.

binder uses sponsorship from Google Cloud and OVH.

Helium–water compounds exhibit multiple superionic states

last modified Jul 26, 2019 03:14 PM
Helium–water compounds exhibit multiple superionic states

Molecular dynamics trajectories showing the superionic motion of hydrogen and helium atoms (green and dark blue paths), while the oxygen sub-lattice (light blue spheres) remains fixed.

Superionic materials are simultaneously liquids and solids. In a multicomponent compound, the different constituents could melt independently, at different temperatures. In a collaboration between the University of Cambridge and Nanjing University, a computational study has discovered that helium-water compounds under pressure are expected to exhibit multiple superionic states, with the hydrogen and helium melting independently while the oxygen atoms remain in fixed, regular positions.

First principles structure prediction techniques were used to find that water and unreactive helium can form stable compounds at surprisingly low pressures. Molecular dynamics simulations were then performed on these new structures, at increasing temperatures. It was found that the helium atoms started to wander freely through the water ice framework, exhibiting diffusive behaviour: the hallmark of superionicity. At even higher temperatures, the hydrogen sub-lattice also melted, giving rise to a multiply superionic phase.

The work is to appear in a forthcoming issue of Nature Physics.

Additional news coverage about the study can be found on phys.org: Study unveils new superionic states of helium-water compounds

 

Multiple superionic states in helium–water compounds

Cong Liu, Hao Gao, Yong Wang, Richard J. Needs, Chris J. Pickard, Jian Sun, Hui-Tian Wang and Dingyu Xing

Nature Physics (July 2019)

DOI: 10.1038/s41567-019-0568-7

Research Assistant/Associate (Fixed Term) - Exploiting XFEL for High Energy Density and Materials Science

last modified Jul 26, 2019 10:32 AM

Research Assistant/Associate (Fixed Term) - Exploiting XFEL for High Energy Density and Materials Science

Department/Location: Department of Materials Science and Metallurgy, West Cambridge Site

Salary: £26,243-£30,395 or £32,236-£39,609

Reference: LJ20045

Closing date: 31 August 2019

Applications are invited for a postdoctoral research position in the Materials Theory Group led by Professor Pickard at the Department of Materials Science and Metallurgy, University of Cambridge.

The successful candidate will hold (or be close to obtaining) a PhD in condensed matter or materials physics, materials science (or similar) and have significant experience in density functional based first principles methods (CASTEP/VASP/QE).

You will have a proven track record in the development and application of first principles methods to the study of materials, and preference will be given to candidates with experience in high pressure research or a proven track record in a closely related research field.

The postholder for this computational project will be expected to be comfortable in a UNIX-like environment, developing tools for the high throughput control of computations and analysis of results. Structure prediction will be performed using the AIRSS package, but prior experience in structure prediction is not required. An interest in modern approaches to data analysis and management would be an advantage, as would a theoretical appreciation of novel phenomena.

The ability to work in close collaboration with experimental groups / project partners in this research area is essential. The candidate must have the ability to manage their own workload and research data, and to publish scientific results in a timely manner.

The project will run in collaboration with the Universities of Edinburgh, York and Oxford and the postholder will be expected to attend and contribute to project meetings.

Project Details

At ambient conditions, the light alkali metals Li, Na and K are nearly free electron (NFE) metals. But rather than becoming more free-electron like when compressed, these metals undergo transitions to unusual and complex structural and electronic forms as a result of density-driven changes in the interactions of the ions and electrons. While such behaviour is expected in all high-density matter, the physics is most evident in the alkali metals due to their NFE behaviour at ambient conditions, and their very high compressibility. They thus offer a unique insight into the behaviour of all other metals at very high densities. We aim to exploit the Edinburgh/ York/ Cambridge/ Oxford team's expertise in experimental and computational high-pressure physics to create solid and fluid simple metals (both alkali metals and others) at unprecedented densities using dynamic compression, and then determine their structural behaviour using X-ray diffraction techniques at X-ray free electron lasers (XFELs). We will then use electronic structure and quantum molecular-dynamics calculations to predict structural behaviour, understand the physics behind the observed behaviour, and thereby develop new understanding and improved predictive capabilities in the behaviour of matter at ultra-high densities. Creating these materials at pressures up to 10 Mbars is exceptionally challenging, and will utilise the DiPOLE laser to be installed at European-XFEL in 2020.

Job application timeline and length of tenure

This is a fixed-term position, and the funds for this post are available for 2 years in the first instance.

Applications close 31 August 2019. To apply, follow the online application link at https://www.jobs.cam.ac.uk/job/22507/. Interviews will be conducted in early September and the anticipated starting date is October 2019.

Please quote reference LJ20045 on your application and in any correspondence about this vacancy.

The University actively supports equality, diversity and inclusion and encourages applications from all sections of society.

The University has a responsibility to ensure that all employees are eligible to live and work in the UK.

Enquiries

For informal enquiries please contact Prof. C. Pickard, cjp20@cam.ac.uk.

Introducing GOSH: the geometry optimisation of structures from hyperspace

last modified Feb 07, 2019 04:30 PM
Introducing GOSH: the geometry optimisation of structures from hyperspace

Snapshot from a hyperspatial optimisation of a graphene sheet. Carbon atoms (spheres) are coloured according to their position in the extra dimension (red/blue colouring).

Techniques which predict crystal structure are a mainstay of modern materials science. A key ingredient in these techniques is the optimisation of candidate structures, which then reveal local energy minima. Optimisations are typically carried out in ordinary 2-dimensional space (for 'flat' materials like graphene), or in 3-dimensional space (for bulk materials). In the case of methods like ab initio random structure searching (AIRSS), a sufficiently dense sampling of local minima then reveals the global energy minimum - and likely crystal structure - of a material.

However, it is well known that local optimisations can run the risk of becoming 'trapped' in sub-optimal high energy configurations. This usually occurs because of energy barriers which prevent a structure from rearranging into a more optimal configuration.

Writing in Physical Review B, Chris Pickard (University of Cambridge, Tohoku University) shows that adding extra dimensions to local optimisations can greatly enhance their efficacy. These additional dimensions allow structures to circumvent energy barriers that would otherwise be too large to overcome in ordinary 2- or 3-dimensional space. This technique - GOSH (geometry optimisation of structures from hyperspace) - gives significantly improved efficiency in structure prediction techniques that use stochastic sampling.

The article was published online on 6 February 2019 as an Editor's Suggestion, with an accompanying Synopsis in Physics.

 

Hyperspatial optimization of structures

Chris J. Pickard

Editor's suggestion in Physical Review B, 99, 054102 (2019)

DOI: 10.1103/PhysRevB.99.054102

Bonan was awarded the best poster prize at the MMM Hub Conference 2018

last modified Sep 12, 2018 12:15 PM

The MMM Hub is a national high performance computing hub for materials and molecular modeling. Bonan attended its annual conference (link) 5th-7th Sept 2018 and presented his poster "Interfaces in vertically aligned nanocomposites for enhanced ionic conduction - A computational study".  He was awarded the best poster prize!

Iron and helium get friendly under pressure

last modified Aug 08, 2018 12:34 PM
Iron and helium get friendly under pressure

The FeHe compound, predicted to form at conditions to be found in the centre of Jupiter. At high temperatures, the helium sub-lattice melts before the iron, and becomes a super-ionic phase.

Helium is the second most abundant element in the universe, but as the most noble gas it is extremely chemically inert. It strongly resists forming compounds. Iron is also highly abundant. With its excellent material properties, especially when alloyed with other elements, iron has played a central role in human development. It also forms an important part of planets and other astronomical bodies. The Earth’s core is largely iron. Iron is also expected to be found in the centre of large exoplanets, and white dwarf stars.

 

A recent theoretical study, conducted by researchers at the Universities of Cambridge and Edinburgh, asked - might iron and helium react under some conditions? The surprising answer is yes, and at pressures to be found within the Solar System - in the core of Jupiter and possibly Saturn. By computationally searching through randomly generated compositions and structures, and optimising them to their lowest quantum mechanical enthalpy, compounds of iron and helium were discovered which are much more stable than iron or helium separately. Future models of the planets and stars should treat helium as a compound forming element.

 

Helium-Iron Compounds at Terapascal Pressures

Bartomeu Monserrat, Miguel Martinez-Canales, Richard J. Needs, and Chris J. Pickard

Physical Review Letters, 121, 015301 – Published 3 July 2018

DOI:https://doi.org/10.1103/PhysRevLett.121.015301

New release of the AIRSS package

last modified Jul 03, 2018 11:27 AM

Ab initio Random Structure Searching (AIRSS) is a very simple, yet powerful and highly parallel, approach to structure prediction. The concept was introduced in 2006 and its philosophy more extensively discussed in 2011.

AIRSS has been used in a number of landmark studies in structure prediction, from the structure of SiH4 under pressure to providing the theoretical structures which are used to understand dense hydrogen (and anticipating the mixed Phase IV), incommensurate phases in aluminium under terapascal pressures, and ionic phases of ammonia.

The AIRSS package is tightly integrated with the CASTEP first principles total energy code. However, it is relatively straightforward to modify the scripts to use alternative codes to obtain the core functionality, and examples are provided.

In addition to numerous bug fixes, the new version has an improved build system, more examples and supports the VASP code.

The AIRSS package is released under the GPL2 licence. 

Click here to find our more, and download version 0.9.1

An icy treasure map

last modified Jun 27, 2018 05:27 PM

The phase diagram water ice is extremely complex, and has implications ranging from cloud formation to ice skating. Experimentally, 18 crystalline ice phases can be formed under various conditions, and many others have been proposed theoretically. A collaboration between the University of Cambridge and the École Polytechnique Fédérale de Lausanne has combined a high throughput computational search with modern materials informatics techniques to map the crystal structure space of ice, and identify 34 possible new phases.

 

The known stable phases of ice form tetrahedral (four-fold) hydrogen bonded networks of water (H2O) molecules. Databases of tetrahedral networks which have been generated for silica (SiO2) and contain millions of candidates, were used at a starting point to build thousands of possible ice structures. After removing those that were likely to have very high energy, full quantum mechanical density functional theory (DFT) structural optimisations are computed. 

 

The optimised structures (almost 16,000) live in a very highly dimensioned configuration space. To help humans to comprehend this data, the configuration space was mapped onto two dimensions, using “sketch map” - an algorithm which places similar structures near to each other on the map.

 

Mapping uncharted territory in ice from zeolite networks to ice structures

Edgar A. Engel, Andrea Anelli, Michele Ceriotti, Chris J. Pickard & Richard J. Needs

Nature Communications, 9:2173, 2018

DOI: 10.1038/s41467-018-04618-6 

 

Figure Caption: Nearly 16,000 DFT relaxed ice crystal structures are plotted using the “sketch map” algorithm, which places similar structures near to each other. A “generalised convex hull”  identifies 34 structural diverse structures of ice that might be accessible under the appropriate thermodynamic conditions.

Research Assistant/Associate (Fixed Term) - Algorithms to navigate material structure space

last modified Jun 19, 2018 04:50 PM

Research Assistant/Associate (Fixed Term) - Algorithms to navigate material structure space

Department/Location: Department of Materials Science and Metallurgy, West Cambridge Site

Salary: £25,728-£38,833

Reference: LJ15801

Closing date: 22 July 2018

University of Cambridge, Department of Materials Sciences & Metallurgy, United Kingdom and Tohoku University, Advanced Institute for Materials Research (AIMR), Japan

Algorithms to navigate material structure space

A post-doctoral fellowship is offered to take part in a project to explore the development of novel algorithms to manage and extract meaning from large quantities of computational materials data. As computational materials discovery moves beyond the consideration of existing materials data to a broad search of configurational space, the quantity of data generated is overwhelming. Structure prediction techniques (such as Ab Initio Random Structure Searching - Pickard and Needs, JPCM, 23 (5), 053201, 2011) produce tens of million local minima. Each of these local minima requires a full, first principles, structural optimisation, and the consideration of several hundred configurations. There is already a need to manage and explore billions of structures.

Profs Akagi and Pickard have explored the application of persistent homology (Hiraoka et al.,PNAS, 113, 7035-7040, 2016) and complex network theory (Ahnert, Grant and Pickard, npj Computational Materials, 2017) to the analysis of materials structure data. It is expected the project will critically examine these approaches, propose and develop alternative algorithms.

The role consists in the following duties:
- to develop, implement, and critically assess novel algorithms to manage and extract meaning from large quantities of materials structure data
- to apply these novel algorithms to challenging materials problems
- to analyse and appropriately manage and disseminate generated data
- to promptly write up results for publication
- to visit and host visitors to/from Tohoku University as required, and to prepare joint workshops.

This post is suitable for a candidate who has or is about to obtain a PhD in the computational aspects of Condensed Matter Physics, Materials Science or Chemistry, with a strong mathematical background.

This is a joint project between the Materials Theory Group, Department of Materials Science and Metallurgy, University of Cambridge, and the Advanced Institute for Materials Research (AIMR), Tohoku University. The project is central to the AIMR's mission to integrate mathematics and the materials sciences, and while the position will be based at the University of Cambridge, the candidate will be expected to play an active role in the development and promotion of the AIMR-Cambridge partnership. Example activities include the preparation of joint workshops, and extended research visits to Tohoku University.

Fixed-term: The funds for this post are available for 3 years in the first instance, subject to probation.

Starting date: position available immediately.

To apply online for this vacancy and to view further information about the role, please visit: http://www.jobs.cam.ac.uk/job/17766.

Learning energies from random searches as you go

last modified Apr 17, 2018 11:52 AM

First principles structure searches, where initially random atomic configurations are repeatedly relaxed to low energy arrangements according to quantum mechanical forces, have led to the discovery of many fascinating structures and phenomena (from the formation of xenon oxides under pressure, to high temperature superconductors). However, the determination of the quantum mechanical forces is computationally challenging, even using efficient density functional theory (DFT) based codes such as CASTEP. The search for complex structures becomes extremely costly - in terms of time, money and energy.

It is sometimes possible to construct simple interatomic potentials that describe the interaction between atoms, and provide a rapid way to explore the energy landscape. These potentials are based on the idea that atoms in similar environments will contribute similarly to the total energy. The building of empirical (or data driven) potentials can be very difficult to perfect. There is great interest in using data fitting techniques that form the bedrock of modern machine learning (ML) - such as deep neural networks or gaussian processes, to automate and improve this process.

In this paper researchers from the University of Cambridge Engineering and Materials Science and Metallurgy Departments have combined Random Structure Searches  (RSS) with Gaussian Approximation Potentials (GAPs) to study boron. They obtain the first general purpose empirical boron potential, and open up the possibility of resolving the many long standing puzzles surrounding elemental boron - such as the microscopic structure of beta-boron.

Data-Driven Learning of Total and Local Energies in Elemental Boron

Volker L. Deringer, Chris J. Pickard and Gábor Csányi

Physical Review Letters, 120, 156001 (2018) [Editors Suggestion]

 

Figure Caption: In contrast to quantum mechanical approaches, interatomic potentials allow the total energy to be broken down into atom by atom contributions. An analysis of the RSS-GAP model for beta-boron shows that partially occupying the B13 site greatly reduces the local energy of the remaining ones.

A real-space way to do your electrostatic sums

last modified Jan 22, 2018 03:50 PM

Computing the energy of infinite arrays of point charges, interacting with each other through Coulomb’s potential, is a central task in materials modelling. Typically, modern implementations of Ewald's method, which splits the problem into two separately convergent sums; one in real space and the other in reciprocal space, are used to do this. 

Density functional theorybased electronic structure methods (such as CASTEP) require the evaluation of the ion-ion repulsive energy, neutralised by a uniform background charge. In this article a purely real-space approach is described which is straightforward to implement, and computationally efficient. It offers linear scaling and when applied to the evaluation of the electrostatic energy of neutral ionic crystals, it is shown to be closely related to the well known method due to Wolf and co-workers.

 

Real-space pairwise electrostatic summation in a uniform neutralizing background

Chris J. Pickard

Phys. Rev. Materials 2, 013806 (2018)

DOI:https://doi.org/10.1103/PhysRevMaterials.2.013806

 

 Real Space Summation Figure

Figure Caption: a) With no neutralising background the electrostatic sum of an array of positive charges diverges with increasing cutoff sphere radius (Rc). Adding a neutralising background sphere with the same radius stops the sum increasing, but the result oscillates strongly. Choosing a radius (Ra), which adapted so as to enforce charge neutrality, ensures convergence. b) Smearing (or damping) the charge density due to the array of charges a little leads to very accurate sums.

 

Complex atomic networks

last modified Jan 22, 2018 03:52 PM
Complex atomic networks

Figure Caption: Two alternative modular decompositions of the metal-organic framework MOF-5 obtained from community detection applied to a complex atomic network. The point groups and number of each module in the original structures are provided.

Complex network analysis is a computational tool that be been used to study problems as diverse as uncovering hidden social groupings and divining new tasty recipes. A collaboration between researchers in the University of Cambridge Materials and Physics Departments have introduced a way to use complex network analysis to break materials down into their constituent fragments (or modules), and then put them back together in different ways, assisting the computational discovery of new materials.
 
A community detection algorithm is applied to a complex atomic network (built up from the distances between pairs of atoms). The resulting communities, or modules, are examined and the most simple solutions (in terms of the amount of information required to describe them) are selected. This approach is applied to a variety of crystal structures, and used to uncover potential polytypism in a dense phase of boron. It is suggested that the method could be applied to biomolecular systems.
 
Revealing and exploiting hierarchical material structure through complex atomic networks
Sebastian E. Ahnert, William P. Grant and Chris J. Pickard
npj Computational Materials, (2017) 3:35; doi:10.1038/s41524-017-0035-x

Total Energy and Force Methods 2018

last modified Jan 22, 2018 03:51 PM

Dear Colleagues,

We would like to invite you to Total Energy and Force Methods 2018, the next workshop in the “mini” series of Computational Physics and Materials Science events. The workshop will take place in Cambridge, UK, from 9thJanuary to 11th January 2018.

The workshop will focus on the most recent developments in the field of electronic structure methods from the first-principles perspective, their diverse applications and mathematical foundations.

The confirmed speakers are:

Prof Bogdan Bernevig, Princeton University, USA
Dr George Booth, King’s College London, UK
Prof Kieron Burke, UC Irvine, USA
Prof Eric Cancès, Ecole des Ponts ParisTech, France
Dr Giuseppe Carleo, ETH Zürich, Switzerland
Prof Michele Ceriotti, EPFL, Switzerland
Prof Garnet Chan, Caltech, USA
Prof Daan Frenkel, University of Cambridge, UK
Dr Luca Ghiringhelli, Fritz Haber Institute, Berlin, Germany
Dr Andreas Grüneis, MPI Stuttgart, Germany
Prof Kristjan Haule, Rutgers University, USA
Prof Motoko Kotani, Tohoku University, Japan
Mr Michael Medvedev, Russian Academy of Sciences, Moscow, Russia
Prof Katarzyna Pernal, TU Łódź, Poland
Prof Fabio Pietrucci, Université Pierre et Marie Curie, Paris, France
Dr Mariana Rossi, Fritz Haber Institute, Berlin, Germany
Prof Atsuto Seko, Kyoto University, Japan

Please visit http://onlinesales.admin.cam.ac.uk/conferences-and-events/materials-science-metallurgy/total-energy-and-force-methods for further information and to register.

If you would like to display a poster at the workshop, please email the abstract to totalenergy2018@msm.cam.ac.uk by the 25th November for consideration.

We hope to see you in January,

Chris and Gabor

(On behalf of the Workshop Organisers, Chris Pickard, Gabor Csanyi, Mike Payne, Richard Needs, Michiel Sprik, and Mike Finnis)

A possible route to room-temperature superconductivity under pressure

last modified Jan 22, 2018 03:51 PM
A possible route to room-temperature superconductivity under pressure

The predicted YH10 structure is calculated to have a Tc of up to 303K at 400GPa.

The hunt for high temperature superconductivity has been reinvigorated by the experimental discovery that compressed H2S exhibits a Tc of up to 203K at megabar pressures (1Mbar=100GPa). A collaboration between the University of Cambridge and Jilin University has published the results of a computational search for materials that might superconduct at even higher temperatures.

 

An extensive search for the stable structures and compositions of rare earth hydrides was performed using first principles density functional theory based methods. The superconducting transition temperatures for the stable metallic compounds were calculated using the same theoretical techniques that were used to anticipate the superconductivity in dense hydrogen sulphide. The highest temperatures were predicted for pressures that are around those found in the centre of the Earth. It is a challenge for the future to find materials that superconduct at high temperatures and everyday low pressures.

 

Hydrogen Clathrate Structures in Rare Earth Hydrides at High Pressures: Possible Route to Room-Temperature Superconductivity

Feng Peng, Ying Sun, Chris J. Pickard, Richard J. Needs, Qiang Wu, and Yanming Ma

Physical Review Letters, 119, 107001 – 8 September 2017 (Editors Suggestion)

DOI:https://doi.org/10.1103/PhysRevLett.119.107001

Public release of the AIRSS package

last modified Aug 15, 2017 03:18 PM

Ab initio Random Structure Searching (AIRSS) is a very simple, yet powerful and highly parallel, approach to structure prediction. The concept was introduced in 2006 and its philosophy more extensively discussed in 2011.

AIRSS has been used in a number of landmark studies in structure prediction, from the structure of SiH4 under pressure to providing the theoretical structures which are used to understand dense hydrogen (and anticipating the mixed Phase IV), incommensurate phases in aluminium under terapascal pressures, and ionic phases of ammonia.

The AIRSS package is tightly integrated with the CASTEP first principles total energy code. However, it is relatively straightforward to modify the scripts to use alternative codes to obtain the core functionality, and examples are provided.

The AIRSS package is today released under the GPL2 licence. 

Click here to find our more, and download version 0.9.

The end of water

last modified Oct 18, 2016 08:57 PM
The end of water

A thorough computational search of possible structures, based on quantum mechanics, has revealed that water ice, at pressures approaching 1 TPa, adopts a more complex structure than previously anticipated. According to these calculations, at even higher p

Computational searches for stable structures of ice and other compositions of water and hydrogen have predicted a new range of complicated water-ice phases of previously unknown structure types.

This finding has led to a revision of the predicted phase diagram of H2O at extremely high pressures.

One of the results of the study is the prediction that H2O decomposes into hydrogen peroxide (H2O2) and a hydrogen-rich phase at pressures of a little over 5 terapascals (TPa).

Water ice under high pressures is an important component of gas giant planets, and it has been speculated that it is present in the core of Jupiter at pressures as high as 6.4 TPa. The pressures at the centres of massive exoplanets can reach 10 TPa or more, however establishing the properties of materials at these high pressures is very difficult due to the experimental challenges of replicating these conditions on Earth.  We can, however, make progress using theoretical approaches.

Hydrogen and oxygen are, respectively, the most abundant and the third most abundant elements in the solar system. The spatial distributions of elements within planets are understood to some extent, but it is not in general known what chemical compounds are stable at TPa pressures.

The theoretical study, published in Physical Review Letters, also concluded that if H2O occurs under conditions of excess hydrogen, for example, at the core-mantle boundary in a gas giant planet, the decomposed hydrogen-rich phase might act as a “hydrogen sponge”.  The researchers suggest that these sponges might soak up the hydrogen from the mantle and could play a role in the erosion of the ice component in the core of gas giants.

It is also predicted that H2O will become metal at a higher pressure of just over 6 TPa, and therefore H2O does not have a thermodynamically stable low-temperature metallic form.

Lead author of this paper, Professor Chris Pickard, said  of the discovery, "This result reminds us how little we know about chemistry and mineralogy of these remote planetary bodies. The breakdown of water to produce hydrogen peroxide is the precise opposite of what we would expect. I am sure that there are many more surprises to come."

Overall, this study suggests that H2O is not a stable compound at the highest pressures at which it has been suggested to occur within Jupiter.

Journal link:  Decomposition and Terapascal Phases of Water Ice, Phys. Rev. Let. DOI: 10.1103/PhysRevLett.110.245701

Carbon and oxygen at extremes

last modified Oct 18, 2016 08:56 PM

Research has revealed the fate of the vital elements carbon and oxygen when subjected to the extremes of compression encountered in giant planets and stars.

The centres of planets and stars are well hidden from us. Squeezing samples between two diamonds (diamond anvil cell experiments) have allowed some materials to be examined at pressures approaching those found at the centre of the Earth. But Jupiter, Saturn, Uranus, Neptune and a host of recently discovered exoplanets are much larger, and gravitational forces generate considerably greater pressures at their cores.

The laws of physics that govern the behaviour of matter, even to these extreme pressures, were established in the early 20th century - those of quantum mechanics and special relativity. But we are far from understanding all the consequences of these rules, and scientists have been consistently surprised by what they observe. Nature is notoriously more inventive than the most imaginative theoretical physicist.

So how might we hope to predict what is going on in these hidden away parts of the universe, in the centre of massive planets and stars? The approach taken by Chris Pickard  and Richard Needs has been to perform a thorough computational search through the possible configurations that the atoms may take, using a reliable quantum mechanical description of the electrons that bind them together.

Dubbed Ab Initio Random Structure Searching, following its introduction in 2006 it has been responsible for numerous predictions, some of which already confirmed experimentally, and others being actively investigated by experimenters.

In two papers, published in the same issue of Physical Review Letters, the nature of carbon and oxygen under extreme compression has been explored. Four new phases of carbon were found, with one of them (pictured right, top) being found to be an “electride” - or a binary ionic compound, the electrons taking on the role of the anions. Oxygen molecules were found to survive to much higher pressures than expected, followed by a complicated series of electronic transitions including a metal-insulator transition as the oxygen polymerises (pictured right, bottom).

Thermodynamically Stable Phases of Carbon at Multiterapascal Pressures.  Miguel Martinez-Canales, Chris J. Pickard and Richard J. Needs.  Physical Review Letters108, 045704 (2012)

Persistence and Eventual Demise of Oxygen Molecules at Terapascal Pressures. Jian Sun, Miguel Martinez-Canales, Dennis D. Klug, Chris J. Pickard and Richard J. Needs. Physical Review Letters, 108, 045503 (2012)

A potential way to make graphene superconducting

last modified Oct 18, 2016 08:55 PM
A potential way to make graphene superconducting

Calcium atoms (orange spheres) arranged between graphene planes (blue honeycomb) creates a superconductor CaC6. (Credit: Greg Stewart / SLAC)

A group of scientists including Professor Chris Pickard, have discovered a potential way to make graphene – a single layer of carbon atoms with great promise for future electronics – superconducting. The study, performed in collaboration with UCL, Stanford University and the SLAC National Accelerator Laboratory is published in Nature Communications.

Graphene, a single sheet of carbon atoms arranged in a honeycomb pattern, is the thinnest and strongest known material and a great conductor of electricity, among other remarkable properties. Scientists hope to eventually use it to make ultra fast transistors, sensors and transparent electrodes. Despite its array of exciting properties, superconducting graphene, in which electricity is conducted without resistance, is yet to be achieved.

The classic way to make graphene is by peeling atomically thin sheets from a block of graphite. But it is possible to isolate these carbon sheets by chemically interweaving graphite with arrays of calcium atoms to form calcium intercalated graphite or CaC6. While it's been known for nearly a decade that this material is superconducting the new study uncovers the mechanism of superconductivity in unprecedented detail. The work therefore points to a pathway to make graphene itself superconducting – something the scientific community has dreamed about for a long time, but failed to achieve.

The combination of high purity samples and state-of-the-art light scattering experiments permitted the scientists to distinguish what the electrons in each layer were doing, revealing details of their behaviour that had not been seen before. Before now researchers were unable to confirm whether CaC6’s superconductivity came from the calcium layer, the graphene layer or both. The new discovered that electronic states associated both the calcium layer and graphene together with the resulting interaction between the two are essential for superconductivity. Thus to realise superconductivity in single graphene sheets, arrays of metal atoms must be first arranged on its surface.

Although applications of superconducting graphene are speculative and far in the future they could include ultra-high frequency analogue transistors, nanoscale sensors and electromechanical devices and quantum computing devices. 

Journal link: Superconducting graphene sheets in CaC6 enabled by phonon-mediated interband interactions. S.-L. Yang, J. A. Sobota, C. A. Howard, Chris J. Pickard, M. Hashimoto, D. H. Lu, S.-K. Mo, P. S. Kirchmann & Z.-X. Shen

Aluminium at terapascal pressures

last modified Oct 18, 2016 08:57 PM
Aluminium at terapascal pressures

Host-guest phase of aluminium

Under normal conditions matter is mostly empty space. Atoms are built from a dense nucleus of protons and neutrons, with the void between filled by a comparably tenuous cloud of electrons shuttling about, following quantum mechanical rules. High pressure physicists squeeze hard on this empty space (using mechanical presses, diamond anvils, or shock waves from sudden impacts or laser light), forcing the electrons to occupy smaller volumes.

The incredibly small distances between particles in the collisions generated by the Large Hadron Collider (LHC) to simulate the compressed universe shortly after the big bang can be thought of as the extreme limit of high pressure physics. But there is a vast gap in our knowledge of the behaviour of dense matter laying between high energy particle physics and the rather modest pressures generated routinely in the laboratory. A new generation of laser shock experiments, to be performed using machines such as the National Ignition Facility (NIF) in the United States, is set to change this - and while no one knows what they will find, it had been generally assumed that the atoms would be rather boringly well (or close) packed.

In 2006 Chris Pickard and Richard Needs (Cambridge) introduced a simple and effective method for the prediction of material structure at high pressure. Their method, dubbed "ab initio random structure searching" (AIRSS) combines quantum mechanics with trying many random configurations for the starting positions of the atoms. Confidence has grown in the method, following impressive "blind" (not depending on any experimental observations) predictions of unexpected materials, which have subsequently been found to exist in diamond anvil cell experiments.

Encouraged, Pickard and Needs have started to explore the behaviour of matter at extremely high pressures - those to be explored by the laser shock experiments at the NIF, and encountered in the centres of the recently discovered large exoplanets, and gas giants of our own solar system. They have found (published in Nature Materials) that far from being boring, even such a standard and elemental material such as aluminium adopts intriguing complex structures (pictured) at pressures ten times those found in the centre of the earth. They track down the origin of these mysterious atomic arrangements to the squeezing of electrons from the atoms to the gaps between them, forming what is known as an 'electride' - or an ionic structure in which the anions are electrons alone.

The full article features in Nature Materials, accompanied by a News and Views by Malcolm McMahon and Graeme Ackland, and in this month's PsiK newsletter.

Predicting interface structures

last modified Oct 06, 2016 05:51 PM
Predicting interface structures

Graphene grain boundary structure between armchair and zigzag regions. The red transparent region marks the interface between the two grains, forming a continuous chain of pentagons and heptagons in the otherwise pristine graphene.

We have developed a general first-principles approach to predict the crystal structure of interfaces in materials, a technique that represents a major step towards computationally developing materials with specially designed interfaces. It is based on the ab initio random structure searching approach (AIRSS): It relies on generating random structures in the vicinity of the interface and followed by relaxation of the structures within the framework of density functional theory.

We have studied grain boundaries in two structurally and chemically very different materials, graphene and strontium titanate, and found new low energy structures for both systems. Both materials are important to the microelectronics industry, but more importantly each also represents a larger class of materials, and a better understanding of the crystal structure and properties of their grain boundaries has the potential to make significant long-term impact in this and other industry sectors.

G Schusteritsch, C J Pickard, "Predicting interface structures: From SrTiO3 to graphene", Physical Review B90(3), Article No. 035424 (2014) DOI: 10.1103/PhysRevB.90.035424

Two Dimensional Ice from First Principles: Structures and Phase Transitions

last modified Oct 06, 2016 03:45 PM
Two Dimensional Ice from First Principles: Structures and Phase Transitions

The structure of two-dimensional ice built purely from water pentagons. This space filling structure is made possible by arranging water molecules in a so-called Cairo tiling (CT) pattern.

Scientists at UCL and Cambridge predict new two-dimensional ice structures on the basis of state-of-the-art computer simulations.

A systematic computer simulation study has led to predictions about how water molecules freeze into a single layer of ice. These simulations, published in Physical Review Letters, reveal several models for 2D ice, including a hexagonal, a Cairo tiling pentagonal, a square and a rhombic structure. The new 2D ice structures, obtained on the basis of first principles simulations and unbiased structure search methods, extend the knowledge of ice in nature and are potentially important in understanding phenomena such as cloud microphysics and tribology.

The authors also predict a sequence of phase transitions that happens as a function of pressure and confinement, leading to the determination of a phase diagram of 2D ice. Overall this work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures formed to the confining pressure and confinement width. The observation of the flat square structure supports recent experimental observations of square ice confined within graphene sheets. The authors also discuss how other structures such as the Cairo tiling pentagonal structure may be observed by slightly altering the conditions used so far in experiments.

J. Chen, G. Schusteritsch, C.J. Pickard, C.G. Salzmann, and A. Michaelides, "Two Dimensional Ice from First Principles: Structures and Phase Transitions", Phys. Rev. Lett. 116, 025501 – Published 13 January 2016 - DOI: 10.1103/PhysRevLett.116.025501 

 

Reproducibility in density functional theory calculations of solids

last modified Oct 07, 2016 04:37 PM
Reproducibility in density functional theory calculations of solids

A major international study has shown that modern codes for the first principles prediction of materials properties agree with each other.

A large scale community research collaboration, including Chris Pickard [1], has shown that widely used computer codes for first principles materials property prediction agree with each other. 

These computational tools all start from the same fundamental equations of quantum mechanics, however choices are made in the design of efficient computer algorithms. A typical package comprises hundreds of thousands of lines of source code, and are written by large teams of researchers over decades who will have made distinct design decisions. 

In the light of an increased scrutiny of computer codes following reports of a "reproducibility crisis" in science, the current study is reassuring.

[1] Chris Pickard is a lead developer of the CASTEP code which is licensed by Cambridge Enterprise to Dassault Systemes BIOVIA Ltd.

K. Lejaeghere et al., "Reproducibility in density functional theory calculations of solids", Science 351, 1415 (2016) DOI: 10.1126/science.aad3000

Single-Layered Hittorf’s Phosphorus: A Wide-Bandgap High Mobility 2D Material

last modified Oct 10, 2016 05:59 PM
Single-Layered Hittorf’s Phosphorus: A Wide-Bandgap High Mobility 2D Material

The structure of single-layered Hittorf’s phosphorus and the band gap for different thicknesses of Hittorf’s phosphorus, going from bulk to its single-layered form

The field of two-dimensional materials has seen enormous growth since the discovery of graphene, largely driven by the promise of exotic electronic properties that can be exploited in novel applications. Unfortunately many of the two-dimensional materials studied thus far exhibit either large band gaps or high mobilities but not both.

Scientists at Cambridge and EPFL have studied the properties of a single layer of violet, or Hittorf’s, phosphorus using first-principles density functional theory (DFT).

The single-layered form of Hittorf’s phosphorus is found to exhibit quite exceptional electronic properties. This two-dimensional material is predicted to have a large, direct band gap of around 2.5 eV and to have a very high hole mobility with an upper bound lying between 3000-7000 cm2 V-1 s-1. Being a direct semiconductor makes it very attractive for optical applications. In addition, the binding energy per layer is found to be very small (similar to graphite), suggesting that exfoliation may be experimentally possible.

The rare combination of properties in single-layered Hittorf’s phosphorus render it an exceptional candidate for use in future applications spanning a wide variety of technologies, in particular for high-frequency electronics and optoelectronic devices operating in the low-wavelength blue colour range.

Georg Schusteritsch, Martin Uhrin and Chris J. Pickard, "Single-Layered Hittorf’s Phosphorus: A Wide-Bandgap High Mobility 2D Material", Nano Lett. 16 (5), (2016) pp 2975–2980 DOI: 10.1021/acs.nanolett.5b05068

Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system

last modified Oct 10, 2016 06:00 PM
Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system

The crystal structure of H3S - the highest temperature superconductor currently known.

An international collaboration, including the University of Cambridge, has shown that quantum mechanical effects are central to the high temperature (up to 203K, -70 degrees Celsius) superconductivity that has been measured in hydrogen sulphide under extreme compression.

Earlier results of the collaboration addressed the superconducting mechanism[1] and the structure and decomposition[2] of hydrogen sulphide under pressure. In the current article it is shown that the crystal structure that exhibits the highest temperature superconductivity is adopted a lower pressures than would be expected in a purely classical picture.

[1] "High-pressure hydrogen sulfide from first principles: a strongly anharmonic phonon-mediated superconductor", I Errea, M Calandra, CJ Pickard, J Nelson, RJ Needs, Y Li, H Liu, Y Zhang, , Yanming Ma, Francesco Mauri Physical review letters 114 (15), 157004, 2015

[2] "Dissociation products and structures of solid H2S at strong compression", Y Li, L Wang, H Liu, Y Zhang, J Hao, CJ Pickard, JR Nelson, RJ Needs, Wentao Li, Yanwei Huang, Ion Errea, Matteo Calandra, Francesco Mauri, Yanming Ma Physical Review B 93 (2), 020103, 2016

University of Cambridge news article: 

http://www.cam.ac.uk/research/news/quantum-effects-at-work-in-the-worlds-smelliest-superconductor

Ion Errea, Matteo Calandra, Chris J. Pickard, Joseph R. Nelson, Richard J. Needs, Yinwei Li, Hanyu Liu, Yunwei Zhang, Yanming Ma and Francesco Mauri, "Quantum hydrogen-bond symmetrization in the superconducting hydrogen sulfide system", Nature 532, 81-84 (07 April 2016) DOI: 10.1038/nature17175

Xenon oxides Xe2O5 and Xe3O2

last modified Oct 18, 2016 08:53 PM
Xenon oxides Xe2O5 and Xe3O2

The crystal structure of the newly discovered xenon oxides, Xe2O5. The oxidation states of xenon are labelled.

Under normal conditions the noble gases, helium, neon and the heavier argon, krypton, xenon and radon, are unreactive. One of the enduring geochemical mysteries is the apparent lack of xenon in the Earth's crust and atmosphere. It has long been speculated that xenon might be locked up in compounds under extreme compression within the Earth - but little is known about xenon compounds, or even whether they can exist.

Combining theoretical structure prediction techniques, with diamond anvil cell high pressure experiments, two xenon oxides have been synthesised and characterised below 1 Mbar (100 GPa). [1] The xenon adopts mixed oxidation states and forms extended networks that incorporate oxygen-sharing XeO4 squares. Xe2O5 additionally incorporates oxygen-sharing XeO5 pyramids. In combination with previous theoretical work [2], xenon's rich chemistry under extreme conditions is being revealed. 

 

[1] Agnes Dewaele, Nicholas Worth, Chris J. Pickard, Richard J. Needs, Sakura Pascarelli, Olivier Mathon, Mohamed Mezouar & Tetsuo Irifune, "Synthesis and stability of xenon oxides Xe2O5 and Xe3O2 under pressure", Nature Chemistry (2016) DOI: 10.1038/nchem.2528


[2] Li Zhu, Hanyu Liu, Chris J. Pickard, Guangtian Zou & Yanming Ma, "Reactions of xenon with iron and nickel are predicted in the Earth's inner core ", Nature Chemistry6, 644-648 (2014) DOI: 10.1038/nchem.1925