Thermodynamically stable lithium silicides and germanides from density-functional theory calculations

Density-functional-theory (DFT) calculations have been performed on the Li-Si and Li-Ge systems. Lithiated Si and Ge, including their metastable phases, play an important technological r\^ole as Li-ion battery (LIB) anodes. The calculations comprise structural optimisations on crystal structures obtained by swapping atomic species to Li-Si and Li-Ge from the X-Y structures in the International Crystal Structure Database, where X={Li,Na,K,Rb,Cs} and Y={Si,Ge,Sn,Pb}. To complement this at various Li-Si and Li-Ge stoichiometries, ab initio random structure searching (AIRSS) was also performed. Between the ground-state stoichiometries, including the recently found Li$_{17}$Si$_{4}$ phase, the average voltages were calculated, indicating that germanium may be a safer alternative to silicon anodes in LIB, due to its higher lithium insertion voltage. Calculations predict high-density Li$_1$Si$_1$ and Li$_1$Ge$_1$ $P4/mmm$ layered phases which become the ground state above 2.5 and 5 GPa respectively and reveal silicon and germanium's propensity to form dumbbells in the Li$_x$Si, $x=2.33-3.25$ stoichiometry range. DFT predicts the stability of the Li$_{11}$Ge$_6$ $Cmmm$, Li$_{12}$Ge$_7$ $Pnma$ and Li$_7$Ge$_3$ $P32_12$ phases and several new Li-Ge compounds, with stoichiometries Li$_5$Ge$_2$, Li$_{13}$Ge$_5$, Li$_8$Ge$_3$ and Li$_{13}$Ge$_4$.Comment: 10 pages, 5 figure

Energetics of hydrogen/lithium complexes in silicon analyzed using the Maxwell construction

We have studied hydrogen/lithium complexes in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method for predicting structures. A method based on the Maxwell construction and convex hull diagrams is introduced which gives a graphical representation of the relative stabilities of point defects in a crystal and enables visualization of the changes in stability when the chemical potentials are altered. We have used this approach to study lithium and hydrogen impurities in silicon, which models aspects of the anode material in the recently-suggested lithium-ion batteries. We show that hydrogen may play a role in these anodes, finding that hydrogen atoms bind to three-atom lithium clusters in silicon, forming stable {H,3Li} and {2H,3Li} complexes, while the {H,2Li} complex is almost stable.Comment: (5 pages, 4 figures

A First Principles Theory of Nuclear Magnetic Resonance J-Coupling in solid-state systems

A method to calculate NMR J-coupling constants from first principles in extended systems is presented. It is based on density functional theory and is formulated within a planewave-pseudopotential framework. The all-electron properties are recovered using the projector augmented wave approach. The method is validated by comparison with existing quantum chemical calculations of solution-state systems and with experimental data. The approach has been applied to verify measured J-coupling in a silicophosphate structure, Si5O(PO4)6Comment: 9 page

Lithiation of silicon via lithium Zintl-defect complexes

An extensive search for low-energy lithium defects in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method shows that the four-lithium-atom substitutional point defect is exceptionally stable. This defect consists of four lithium atoms with strong ionic bonds to the four under-coordinated atoms of a silicon vacancy defect, similar to the bonding of metal ions in Zintl phases. This complex is stable over a range of silicon environments, indicating that it may aid amorphization of crystalline silicon and form upon delithiation of the silicon anode of a Li-ion rechargeable battery.Comment: 4 pages, 3 figure

Cognitive and psychological sequelae of hydrocephalus and spina bifida: turning interesting theoretical research into useful clinical intervention and guidelines

Background Ventricular shunts have significantly improved the prognosis of people with hydrocephalus and spina bifida; nevertheless, there are lifelong effects, which in many cases is associated with a cognitive profile of short term memory, attention and executive function difficulties (Iddon et al, 1996, 2001, 2003, 2004). There can also be psychological sequelae including depression and low self-esteem. The aim of this study is to develop clinical practice guidelines to inform intervention strategies to help patients and their carer to manage these difficulties. Materials and methods A three-stage project is being developed. Stage 1: A survey will be carried out to ascertain the needs of the client group, including gaps in local service provision and the impact of cognitive and psychological difficulties on everyday lives. Stage 2: Using data previously collected as well as new data, a detailed review will take place of specific cognitive difficulties and how these relate to everyday functions. Stage 3: A cognitive training programme will be developed as a guideline for practical clinical intervention. Results Data will be presented, showing the unmet need of people with hydrocephalus and spina bifida (N = 150, range of IQ's). Preliminary results of the new project will be presented and discussed. Conclusion It is not possible to provide a complete psychological support for individuals with hydrocephalus attending one London hospital clinic for their annual multi-disciplinary review. However, this project aims to bridge the theoretical/clinical gap we have previously identified in order to inform and guide their local services of the unmet needs of these clients, and to recommend appropriate interventions. This we anticipate will improve patients' quality of life

Quantum Monte Carlo study of a positron in an electron gas

Quantum Monte Carlo calculations of the relaxation energy, pair-correlation function, and annihilating-pair momentum density are presented for a positron immersed in a homogeneous electron gas. We find smaller relaxation energies and contact pair-correlation functions in the important low-density regime than predicted by earlier studies. Our annihilating-pair momentum densities have almost zero weight above the Fermi momentum due to the cancellation of electron-electron and electron-positron correlation effects

Cognitive and psychological sequelae of hydrocephalus and spina bifida: correlating subjective data and objective neuropsychological data to establish insight and inform clinical intervention and guidelines

Background Despite significant advances in treating and improving the prognosis of individuals with hydrocephalus and spina bifida, many of these individuals continue to experience specific cognitive difficulties in the areas of memory, language, attention and executive function and these can often have a significant negative impact on everyday functioning [1–3]. Materials and methods A comprehensive questionnaire was designed, based on known cognitive and emotional sequelae, to assess patient and caregiver perceptions of the specific difficulties experienced by people with hydrocephalus and spina bifida and the extent to which these are being addressed. In order to establish levels of insight, this questionnaire was correlated with detailed neuropsychological data to triangulate actual cognitive performance with subjective self-assessment obtained from patients and the objective view of caregivers. Results Questionnaire data will be presented that will highlight specific areas of discrepancy and concordance between patients and their caregivers (n = 60) and will be discussed in relation to actual performance on a range of cognitive tasks and the subsequent implications for strategic advice and intervention. Conclusion The data is being used to tailor specific cognitive strategies based on enhanced self-awareness, as part of small group and individual cognitive training interventions, within a multidisciplinary setting. It is hoped that the dissemination of the materials and methods designed for this study will inform best practice guidelines for these individuals and their caregivers and provide measurable outcomes for cognitive performance discrepancy, meta-awareness, strategy implementation and evaluation

Experimental evidence of new tetragonal polymorphs of silicon formed through ultrafast laser-induced confined microexplosion

Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties

An efficient k.p method for calculation of total energy and electronic density of states

An efficient method for calculating the electronic structure in large systems with a fully converged BZ sampling is presented. The method is based on a k.p-like approximation developed in the framework of the density functional perturbation theory. The reliability and efficiency of the method are demostrated in test calculations on Ar and Si supercells

Stochastic generation of complex crystal structures combining group and graph theory with application to carbon

A method is introduced to stochastically generate crystal structures with defined structural characteristics. Reasonable quotient graphs for symmetric crystals are constructed using a random strategy combined with space group and graph theory. Our algorithm enables the search for large-size and complex crystal structures with a specified connectivity, such as three-fold sp2 carbons, four-fold sp3 carbons, as well as mixed sp2-sp3 carbons. To demonstrate the method we randomly construct initial structures adhering to space groups from No.75 to No.230 and a range of lattice constants, and we identify 281 new sp3 carbon crystals. First-principles optimization of these structures show that most of them are dynamically and mechanically stable and are energetically comparable to those previously proposed. Some of the new structures can be considered as candidates to explain the experimental cold compression of graphite