Search

Titanium dioxide has been extensively studied in the rutile or anatase phase, while its high-pressure phases are less well-understood, despite that many are thought to have interesting optical, mechanical, and electrochemical properties. First-principles methods, such as density functional theory (DFT), are often used to compute the enthalpies of TiO2 phases at 0 K, but they are expensive and, thus, impractical for long time scale and large system-size simulations at finite temperatures. On the other hand, cheap empirical potentials fail to capture the relative stabilities of various polymorphs. To model the thermodynamic behaviors of ambient and high-pressure phases of TiO2, we design an empirical model as a baseline and then train a machine learning potential based on the difference between the DFT data and the empirical model. This so-called Δ-learning potential contains long-range electrostatic interactions and predicts the 0 K enthalpies of stable TiO2 phases that are in good agreement with DFT. We construct a pressure–temperature phase diagram of TiO2 in the range 0 < P < 70 GPa and 100 < T < 1500 K. We then simulate dynamic phase transition processes by compressing anatase at different temperatures. At 300 K, we predominantly observe an anatase-to-baddeleyite transformation at about 20 GPa via a martensitic two-step mechanism with a highly ordered and collective atomic motion. At 2000 K, anatase can transform into cotunnite around 45–55 GPa in a thermally activated and probabilistic manner, accompanied by diffusive movement of oxygen atoms. The pressures computed for these transitions show good agreement with experiments. Our results shed light on how to synthesize and stabilize high-pressure TiO2 phases, and our method is generally applicable to other functional materials with multiple polymorphs. [ABSTRACT FROM AUTHOR]

The solar system's outer planets, and many of their moons, are dominated by matter from the H-C-N-O chemical space, based on solar system abundances of hydrogen and the planetary ices H2O, CH4, and NH3. In the planetary interiors, these ices will experience extreme pressure conditions, around 5 Mbar at the Neptune mantle-core boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. While temperature will dictate the formation of compounds, groundstate density functional theory allows us to probe the chemical effects resulting from pressure alone. These structural developments in turn determine the planets' interior structures, thermal evolution, and magnetic field generation, among others. Despite its importance, the H-C-N-O system has not been surveyed systematically to explore which compounds emerge at high-pressure conditions, and what governs their stability. Here, we report on and analyze an unbiased crystal structure search among H-C-N-O compounds between 1 and 5 Mbar. We demonstrate that simple chemical rules drive stability in this composition space, which explains why the simplest possible quaternary mixture HCNO--isoelectronic to diamond--emerges as a stable compound and discuss dominant decomposition products of planetary ice mixtures. [ABSTRACT FROM AUTHOR]

The discovery of materials is increasingly guided by quantum‐mechanical crystal‐structure prediction, but the structural complexity in bulk and nanoscale materials remains a bottleneck. Here we demonstrate how data‐driven approaches can vastly accelerate the search for complex structures, combining a machine‐learning (ML) model for the potential‐energy surface with efficient, fragment‐based searching. We use the characteristic building units observed in Hittorf's and fibrous phosphorus to seed stochastic ("random") structure searches over hundreds of thousands of runs. Our study identifies a family of hierarchically structured allotropes based on a P8 cage as principal building unit, including one‐dimensional (1D) single and double helix structures, nanowires, and two‐dimensional (2D) phosphorene allotropes with square‐lattice and kagome topologies. These findings yield new insight into the intriguingly diverse structural chemistry of phosphorus, and they provide an example for how ML methods may, in the long run, be expected to accelerate the discovery of hierarchical nanostructures. [ABSTRACT FROM AUTHOR]

The discovery of materials is increasingly guided by quantum‐mechanical crystal‐structure prediction, but the structural complexity in bulk and nanoscale materials remains a bottleneck. Here we demonstrate how data‐driven approaches can vastly accelerate the search for complex structures, combining a machine‐learning (ML) model for the potential‐energy surface with efficient, fragment‐based searching. We use the characteristic building units observed in Hittorf's and fibrous phosphorus to seed stochastic ("random") structure searches over hundreds of thousands of runs. Our study identifies a family of hierarchically structured allotropes based on a P8 cage as principal building unit, including one‐dimensional (1D) single and double helix structures, nanowires, and two‐dimensional (2D) phosphorene allotropes with square‐lattice and kagome topologies. These findings yield new insight into the intriguingly diverse structural chemistry of phosphorus, and they provide an example for how ML methods may, in the long run, be expected to accelerate the discovery of hierarchical nanostructures. [ABSTRACT FROM AUTHOR]

NMR chemical shifts were calculated from first principles for well defined crystalline organic solids. These density functional theory calculations were carried out within the plane-wave pseudopotential framework, in which truly extended systems are implicitly considered. The influence of motional effects was assessed by averaging over vibrational modes or over snapshots taken from ab initio molecular dynamics simulations. It is observed that the zero-point correction to chemical shifts can be significant, and that thermal effects are particularly noticeable for shielding anisotropies and for a temperature-dependent chemical shift. This study provides insight into the development of highly accurate first principles calculations of chemical shifts in solids, highlighting the role of motional effects on well defined systems. [ABSTRACT FROM AUTHOR]

We have combined a computational search strategy with first-principles density-functional-theory calculations to identify metastable phases of H2O under pressure. The most stable structures consist of water molecules, while the most energetic metastable phases consist of oxygen and hydrogen molecules. In between lie many other metastable phases, consisting of various combinations of a few small molecules. It may be possible to synthesize some of these metastable phases, and we use our results to understand the nature of the crystalline metastable phase of H2O recently synthesized by Mao et al. [Science 314, 636 (2006)]. [ABSTRACT FROM AUTHOR]

The use of the spin-dependent pseudopotentials has been shown to markedly enhance the transferability of the commonly used spin-neutral pseudopotential method for the study of the structural and magnetic properties of transition-metal-containing materials. Unfortunately, because the method was based on the rather expensive norm-conserving pseudopotential formalism, the method was limited to the study of fairly small systems. Here we present an extension of the spin-dependent pseudopotential method for the far more computationally advantageous ultrasoft formalism and show that it is very easy to add such a feature to any preexisting computer code. We benchmark our new method by comparing to previously published results and then apply it to the study of several relevant test cases: bulk Ni, Fe, and Co, as well as a Pd atomic wire. [ABSTRACT FROM AUTHOR]

We present a method for the first principles density functional calculation of relativistic all-electron nuclear magnetic resonance chemical shifts using pseudopotentials. The method is based on the gauge including projector augmented wave approach of Pickard and Mauri [Phys. Rev. B 63, 245101 (2001)]. Relativistic effects are included at the level of the scalar-relativistic zeroth-order regular approximation. The method allows chemical shifts of large, low symmetry structures containing heavy elements to be calculated efficiently. We demonstrate its success for a range of Se and Te containing molecules. [ABSTRACT FROM AUTHOR]

Using first-principles calculations and crystal structure search methods, we found that many covalently bonded molecules such as H2, N2, CO2, NH3, H2O and CH4 may react with NaCl, a prototype ionic solid, and form stable compounds under pressure while retaining their molecular structure. These molecules, despite whether they are homonuclear or heteronuclear, polar or non-polar, small or large, do not show strong chemical interactions with surrounding Na and Cl ions. In contrast, the most stable molecule among all examples, N2, is found to transform into cyclo-N5− anions while reacting with NaCl under high pressures. It provides a new route to synthesize pentazolates, which are promising green energy materials with high energy density. Our work demonstrates a unique and universal hybridization propensity of covalently bonded molecules and solid compounds under pressure. This surprising miscibility suggests possible mixing regions between the molecular and rock layers in the interiors of large planets. [ABSTRACT FROM AUTHOR]

We predict by first-principles methods a phase transition in TiO2 at 6.5 Mbar from the Fe2P-type polymorph to a ten-coordinated structure with space group I4/mmm. This is the first report, to our knowledge, of the pressure-induced phase transition to the I4/mmm structure among all dioxide compounds. The I4/mmm structure was found to be up to 3.3% denser across all pressures investigated. Significant differences were found in the electronic properties of the two structures, and themetallization of TiO2 was calculated to occur concomitantly with the phase transition to I4/mmm. The implications of our findings were extended to SiO2, and an analogous Fe2P-type to I4/mmm transition was found to occur at 10 TPa. This is consistent with the lower-pressure phase transitions of TiO2, which are well-established models for the phase transitions in other AX2 compounds, including SiO2. As in TiO2, the transition to I4/mmm corresponds to the metallization of SiO2. This transformation is in the pressure range reached in the interiors of recently discovered extrasolar planets and calls for a reformulation of the equations of state used to model them. [ABSTRACT FROM AUTHOR]

Searches for low-energy tetrahedral polymorphs of carbon and silicon have been performed using density functional theory computations and the ab initio random structure searching approach. Several of the hypothetical phases obtained in our searches have enthalpies that are lower or comparable to those of other polymorphs of group 14 elements that have either been experimentally synthesized or recently proposed as the structure of unknown phases obtained in experiments, and should thus be considered as particularly interesting candidates. A structure of Pbam symmetry with 24 atoms in the unit cell was found to be a low-energy, low-density metastable polymorph in carbon, silicon, and germanium. In silicon, Pbam is found to have a direct band gap at the zone center with an estimated value of 1.4 eV, which suggests applications as a photovoltaic material. We have alsofound a low-energy chiral framework structure of P41212 symmetry with 20 atoms per cell containing fivefold spirals of atoms, whose projected topology is that of the so-called Cairo-type two-dimensional pentagonal tiling. We suggest that P41212 is a likely candidate for the structure of the unknown phase XIII of silicon. We discuss Pbam and P41212 in detail, contrasting their energetics and structures with those of other group 14 elements, particularly the recently proposed P42/ncm structure, for which we also provide a detailed interpretation as a network of tilted diamondlike tetrahedra. [ABSTRACT FROM AUTHOR]

Ab initio random structure searching (AIRSS) and density functional theory methods are used to predict structures of calcium and magnesium carbonate (CaCO3 and MgCO3) at high pressures. We find a previously unknown CaCO3 structure which is more stable than the aragonite and "post aragonite" phases in the range 32-48 GPa. At pressures from 76 GPa to well over 100 GPa the most stable phase is a previously unknown CaCO3 structure of the pyroxene type with fourfold coordinated carbon atoms. We also predict a stable structure of MgCO3 in the range 85-101 GPa. Our results lead to a revision of the phase diagram of CaCO3 over more than half the pressure range encountered within the Earth's mantle, and smaller changes to the phase diagram of MgCO3. We predict CaCO3 to be more stable than MgCO3 in the Earth's mantle above 100 GPa, and that CO2 is not a thermodynamically stable compound under deep mantle conditions. Our results have significant implications for understanding the Earth's deep carbon cycle. [ABSTRACT FROM AUTHOR]

We present here a fully first-principles method for predicting the atomic structure of interfaces. Our method is based on the ab initio random structure searching (AIRSS) approach, applied here to treat two-dimensional defects. The method relies on repeatedly generating random structures in the vicinity of the interface and relaxing them within the framework of density functional theory (DFT). The method is simple, requiring only a small set of parameters that can be easily connected to the chemistry of the system of interest, and efficient, ideally adapted to high-throughput first-principles calculations on modern parallel architectures. Being first-principles, our method is transferable, an important requirement for a generic computational method for the determination of the structure of interfaces. Results for two structurally and chemically very different interfaces are presented here, grain boundaries in graphene and grain boundaries in strontium titanate (SrTiO3). We successfully find a previously unknown low energy grain boundary structure for the graphene system, as well as recover the previously known higher energy structures. For the SrTiO3 system we study both stoichiometric and nonstoichiometric compositions near the grain boundary and find previously unknown low energy structures for all stoichiometries. We predict that these low energy structures have long-range distortions to the ground state crystal structure emanating into the bulk from the interface. [ABSTRACT FROM AUTHOR]

Carbon monoxide and nitrogen are among the potentially interesting high-energy density materials. However, in spite of the physical similarities of the molecules, they behave very differently at high pressures. Using density functional theory and structural prediction methods, we examine the ability of these systems to combine their respective properties and form novel mixed crystalline phases under pressures of up to 100 GPa. Interestingly, we find that CO catalyzes the molecular dissociation of N2, which means mixed structures are favored at a relatively low pressure (below 18 GPa), and that a three-dimensional framework with Pbam symmetry becomes the most stable phase above 52 GPa, i.e., at much milder conditions than in pure solid nitrogen. This structure is dynamically stable at ambient pressure and has an energy density of approximately 2.2 kJg-1, making it a candidate for a high-energy density material, and one that could be achieved at less prohibitive experimental conditions. [ABSTRACT FROM AUTHOR]

Computational searches for stable and metastable structures of water ice and other H:O compositions at TPa pressures have led us to predict that H2O decomposes into H2O2 and a hydrogen-rich phase at pressures of a little over 5 TPa. The hydrogen-rich phase is stable over a wide range of hydrogen contents, and it might play a role in the erosion of the icy component of the cores of gas giants as H2O comes into contact with hydrogen. Metallization of H2O is predicted at a higher pressure of just over 6 TPa, and therefore H2O does not have a thermodynamically stable low-temperature metallic form. We have also found a new and rich mineralogy of complicated water ice phases that are more stable in the pressure range 0.8-2 TPa than any predicted previously. [ABSTRACT FROM AUTHOR]

We have studied solid hydrogen up to pressures of 300 GPa and temperatures of 350 Κ using density functional theory methods and have found "mixed structures" that are more stable than those predicted earlier. Mixed structures consist of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. Quasiharmonic vibrational calculations show that mixed structures are the most stable at room temperature over the pressure range 250-295 GPa. These structures are stabilized with respect to strongly bonded molecular phases at room temperature by the presence of lower frequency vibrational modes arising from the graphene-like sheets. Our results for the mixed structures are consistent with the experimental Raman data [M. I. Eremets and 1. A. Troyan, Nat. Mater. 10, 927 (2011) and R. T. Howie et al., Phys. Rev. Lett. 108, 125501 (2012)]. We find that mixed phases are reasonable structural models for phase IV of hydrogen. [ABSTRACT FROM AUTHOR]

Molecular packing in two polymorphs of sibenadet hydrochloride (AR-C68397AA, ViozanTM) is investigated using a combined experimental ¹H double-quantum (DQ) solid-state magic-angle spinning nuclear magnetic resonance and computational (gauge including projected augmented wave calculation of chemical shifts) approach. For Form I, NH-NH and NH-OH ¹H DQ peaks are observed corresponding to nearest distances of 2.62 and 2.87Å, respectively, for the intermolecular hydrogen-bonding arrangement in the single-crystal X-ray diffraction structure. The same ¹H DQ peaks at the same ¹H chemical shifts are observed for Form II, for which there is no single-crystal diffraction structure, indicating the same intermolecular hydrogen-bonding arrangement of the benzothiazolone moieties as in Form I. ¹H DQ build-up (as a function of the DQ recoupling time) curves are presented for the resolved NH-NH and NH-OHDQpeaks for the two polymorphs. For Form I, the ratio of the maximum intensity for the NH-OH and NH-NH DQ peaks is in excellent agreement with the ratio of the summed squares of the H-H dipolar couplings, as determined using H-H distances from the crystal structure up to 4Å. Small differences in the ¹H DQ build-up behaviour for the two polymorphs are attributed to differences in the longer-range NH-OH distances associated with different interlayer arrangements. [ABSTRACT FROM AUTHOR]

The article discusses a study on the phases of carbon up to pressures of 1 petapascal (PPa) using first-principles density-functional-theory methods and a structure searching algorithm. In this study, the ab initio random structure searching (AIRSS) aapproach was used to perform a computational search for carbon structures and an ultrasoft pseudopotentials treating all six electrons explicitly was used to approach all-electron accuracy.

Studying materials at terapascal (TPa) pressures will provide insights into the deep interiors of large planets and chemistry under extreme conditions. The equation of state of aluminium is of interest because it is used as a standard material in shock-wave experiments and because it is a typical sp-bonded metal. Here we use density-functional-theory methods and a random-searching approach to predict stable structures of aluminium at multiterapascal pressures, finding that the low-pressure close-packed structures transform to more open structures above 3.2 TPa (nearly ten times the pressure at the centre of the Earth), with an incommensurate host–guest structure being stable over a wide range of pressures and temperatures. We show that the high-pressure phases may be described by a two-component model consisting of positive ions and interstitial electron ‘blobs’, and propose that such structures are common in sp-bonded materials up to multiterapascal pressures. [ABSTRACT FROM AUTHOR]

Ammonia is an important compound with many uses, such as in the manufacture of fertilizers, explosives and pharmaceuticals. As an archetypal hydrogen-bonded system, the properties of ammonia under pressure are of fundamental interest, and compressed ammonia has a significant role in planetary physics. We predict new high-pressure crystalline phases of ammonia (NH3) through a computational search based on first-principles density-functional-theory calculations. Ammonia is known to form hydrogen-bonded solids, but we predict that at higher pressures it will form ammonium amide ionic solids consisting of alternate layers of NH4+ and NH2− ions. These ionic phases are predicted to be stable over a wide range of pressures readily obtainable in laboratory experiments. The occurrence of ionic phases is rationalized in terms of the relative ease of forming ammonium and amide ions from ammonia molecules, and the volume reduction on doing so. We also predict that the ionic bonding cannot be sustained under extreme compression and that, at pressures beyond the reach of current static-loading experiments, ammonia will return to hydrogen-bonded structures consisting of neutral NH3 molecules. [ABSTRACT FROM AUTHOR]

Hydrogen, being the first element in the periodic table, has the simplest electronic structure of any atom, and the hydrogen molecule contains the simplest covalent chemical bond. Nevertheless, the phase diagram of hydrogen is poorly understood. Determining the stable structures of solid hydrogen is a tremendous experimental challenge, because hydrogen atoms scatter X-rays only weakly, leading to low-resolution diffraction patterns. Theoretical studies encounter major difficulties owing to the small energy differences between structures and the importance of the zero-point motion of the protons. We have systematically investigated the zero-temperature phase diagram of solid hydrogen using first-principles density functional theory (DFT) electronic-structure methods, including the proton zero-point motion at the harmonic level. Our study leads to a radical revision of the DFT phase diagram of hydrogen up to nearly 400 GPa. That the most stable phases remain insulating to very high pressures eliminates a major discrepancy between theory and experiment. One of our new phases is calculated to be stable over a wide range of pressures, and its vibrational properties agree with the available experimental data for phase III. [ABSTRACT FROM AUTHOR]

The 13C NMR chemical shifts for functionalized (7,0), (8,0), (9,0), and (10,0) single-walled carbon nanotubes (SWNTs) have been studied computationally using gauge-including projector-augmented plane-wave (GIPAW) density functional theory (DFT). The functional groups NH, NCH3, NCH2OH, and CH2NHCH2 have been considered, and different sites where covalent addition or substitution may occur have been examined. The shifts of the carbons directly attached to the group are sensitive to the bond which has been functionalized and may, therefore, be used to identify whether the group has reacted with a parallel or a diagonal CC bond. The addition of NH to a parallel bond renders the functionalized carbons formally sp³-hybridized, yielding shifts of around 44 ppm, independent of the SWNT radius. Reaction with a diagonal bond retains the formal sp² hybridization of the substituted carbons, and their shifts are slightly lower or higher than those of the unsubstituted carbon atoms. The calculated 1H NMR shifts of protons in the functional groups are also dependent upon the SWNT-group interaction. Upon decreasing the degree of functionalization for the systems where the group is added to a parallel bond, the average chemical shift of the unfunctionalized carbons approaches that of the pristine tube. At the same time, the shifts of the functionalized carbons remain independent upon the degree of functionalization. For the SWNTs where N-R attaches to a parallel bond, the average shift of the sp² carbons was found to be insensitive to the substituent R. Moreover, the shifts of the functionalized sp³ carbons, as well as of the carbons within the group itself, are independent of the SWNT radius. The results indicate that a wealth of knowledge may be obtained from the 13C NMR of functionalized SWNTs. [ABSTRACT FROM AUTHOR]

In the carbon science literature, there have been various reports over the past few decades of potentially novel crystalline forms of carbon emerging as nanometer scale fragments recovered from the explosive remnants of heated, shock compressed graphite and other precursors of C. Two nanometric and crystalline forms of C that are particularly prominent in these studies are known as n-diamond and i-carbon forms. In our previous work, we have shown that the commonly observed diffraction pattern of n-diamond nanocrystals, recorded by several research groups around the world, is consistent with the calculated diffraction pattern of a novel form of carbon that we propose to call glitter. Glitter is a tetragonal allotrope of carbon with a calculated density of ~3.08g/cm3, and the density functional theory (DFT) optimized lattice parameters given as a = 0.2560 nm and c = 0.5925 nm. In addition to the diffraction evidence for n-diamond having the glitter structure, the DFT calculated band structure of glitter shows it to be metallic, like the observed electrical characteristics of n-diamond. In this communication, we report on a comparison of the diffraction pattern observed for nanocrystalline i-carbon by the investigative team of Yamada et al. in 1994, with the calculated diffraction pattern of glitter based upon the optimized lattice parameters. The close fit of the latter dataset to that observed for i-carbon, as reported herein, suggests that indeed i-carbon may be of the same structure as n-diamond, and that they both may have the tetragonal glitter structure. [ABSTRACT FROM AUTHOR]

Machine-learned interatomic potentials are fast becoming an indispensable tool in computational materials science. One approach is the ephemeral data-derived potential (EDDP), which was designed to accelerate atomistic structure prediction. The EDDP is simple and cost-efficient. It relies on training data generated in small unit cells and is fit using a lightweight neural network, leading to smooth interactions which exhibit the robust transferability essential for structure prediction. Here, we present a variety of applications of EDDPs, enabled by recent developments of the open-source EDDP software. New features include interfaces to phonon and molecular dynamics codes, as well as deployment of the ensemble deviation for estimating the confidence in EDDP predictions. Through case studies ranging from elemental carbon and lead to the binary scandium hydride and the ternary zinc cyanide, we demonstrate that EDDPs can be trained to cover wide ranges of pressures and stoichiometries, and used to evaluate phonons, phase diagrams, superionicity, and thermal expansion. These developments complement continued success in accelerated structure prediction. [ABSTRACT FROM AUTHOR]

Most metals adopt simple structures such as body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) structures in specific groupings across the periodic table, and many undergo transitions to surprisingly complex structures on compression, not expected from conventional free-electron-based theories of metals. First-principles calculations have been able to reproduce many observed structures and transitions, but a unified, predictive theory that underlies this behavior is not yet in hand. Discovered by analyzing the electronic properties of metals in various lattices over a broad range of sizes and geometries, a remarkably simple theory shows that the stability of metal structures is governed by electrons occupying local interstitial orbitals and their strong chemical interactions. The theory provides a basis for understanding and predicting structures in solid compounds and alloys over a broad range of conditions. [ABSTRACT FROM AUTHOR]

Fluorite-perovskite heterointerfaces garner great interest for enhanced ionic conductivity for application in electronic and energy devices. However, the origin of observed enhanced ionic conductivity as well as the details of the atomic structure at these interfaces remain elusive. Here, systematic, multi-stoichiometry computational searches and experimental investigations are performed to obtain stable and exact atomic structures of interfaces between CeO2 and SrTiO3—two archetypes of the corresponding structural families. Local reconstructions take place at the interface because of mismatched lattices. TiO2 terminated SrTiO3 causes a buckled rock salt CeO interface layer to emerge. In contrast, SrO terminated SrTiO3 maintains the fluorite structure at the interface compensated by a partially occupied anion lattice. Moderate enhancement in oxygen diffusion is found along the interface by simulations, yet evidence to support further significant enhancement is lacking. Our findings demonstrate the control of interface termination as an effective pathway to achieve desired device performance. [ABSTRACT FROM AUTHOR]

We study the effects of atomic vibrations on the solid-state chemical shielding tensor using first principles density functional theory calculations. At the harmonic level, we use a Monte Carlo method and a perturbative expansion. The Monte Carlo method is accurate but computationally expensive, while the perturbative method is computationally more efficient, but approximate. We find excellent agreement between the two methods for both the isotropic shift and the shielding anisotropy. The effects of zero-point quantum mechanical nuclear motion are important up to relatively high temperatures: at 500 K they still represent about half of the overall vibrational contribution. We also investigate the effects of anharmonic vibrations, finding that their contribution to the zero-point correction to the chemical shielding tensor is small. We exemplify these ideas using magnesium oxide and the molecular crystals L-alanine and β-aspartyl-L-alanine. We therefore propose as the method of choice to incorporate the effects of temperature in solid state chemical shielding tensor calculations using the perturbative expansion within the harmonic approximation. This approach is accurate and requires a computational effort that is about an order of magnitude smaller than that of dynamical or Monte Carlo approaches, so these effects might be routinely accounted for. [ABSTRACT FROM AUTHOR]

The allotropes of boron continue to challenge structural elucidation and solid-state theory. Here we use machine learning combined with random structure searching (RSS) algorithms to systematically construct an interatomic potential for boron. Starting from ensembles of randomized atomic configurations, we use alternating single-point quantum-mechanical energy and force computations, Gaussian approximation potential (GAP) fitting, and GAP-driven RSS to iteratively generate a representation of the element's potential-energy surface. Beyond the total energies of the very different boron allotropes, our model readily provides atom-resolved, local energies and thus deepened insight into the frustrated β-rhombohedral boron structure. Our results open the door for the efficient and automated generation of GAPs, and other machine-learning-based interatomic potentials, and suggest their usefulness as a tool for materials discovery. [ABSTRACT FROM AUTHOR]

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 also been applied to the silicophosphate, Si5O(PO4)6, giving 31P–29Si-couplings which are in excellent agreement with experiment. [ABSTRACT FROM AUTHOR]

The article informs about the U.S National Ignition Facility machine, a laser-based inertial confinement fusion located in Livermore, California. Topics discussed include transformation of close-packed aluminum structure to a complicated non-close structure and application of Monte Carlo computational method to provide description of pressure-density regions and compression of diamond upto a record pressure of five tetrapascals to the density of lead.

We used ab initio random structure searching (AIRSS) to investigate polymorphism in C3N4 carbon nitride as a function of pressure. Our calculations reveal new framework structures, including a particularly stable chiral polymorph of space group P43212 containing mixed sp² and sp³ bonding, that we have produced experimentally and recovered to ambient conditions. As pressure is increased a sequence of structures with fully sp³-bonded C atoms and three-fold-coordinated N atoms is predicted, culminating in a dense Pnma phase above 250 GPa. Beyond 650 GPa we find that C3N4 becomes unstable to decomposition into diamond and pyrite-structured CN2. [ABSTRACT FROM AUTHOR]

The article describes the use of measured solid-state nuclear magnetic resonance (NMR) chemical shifts and first principles calculations to resolve the crystal structure of powdered microcrystalline β-L-aspartyl-L-alanine to within 0.13Å of the known structure. According to the authors, the plane wave density functional theory based optimization method allows the determination of crystal structures from powders using the molecular modeling with proton spin diffusion-NMR data.

Through a series of high-pressure x-ray diffraction experiments combined with in situ laser heating, we explore the pressure–temperature phase diagram of germanium (Ge) at pressures up to 110 GPa and temperatures exceeding 3000 K. In the pressure range of 64–90 GPa, we observe orthorhombic Ge-IV transforming above 1500 K to a previously unobserved high-temperature phase, which we denote as Ge-VIII. This high-temperature phase is characterized by a tetragonal crystal structure, space group I4/mmm. Density functional theory simulations confirm that Ge-IV becomes unstable at high temperatures and that Ge-VIII is highly competitive and dynamically stable at these conditions. The existence of Ge-VIII has profound implications for the pressure–temperature phase diagram, with melting conditions increasing to much higher temperatures than previous extrapolations would imply. [ABSTRACT FROM AUTHOR]

Cathodes are critical components of rechargeable batteries. Conventionally, the search for cathode materials relies on experimental trial-and-error and a traversing of existing computational/experimental databases. While these methods have led to the discovery of several commercially viable cathode materials, the chemical space explored so far is limited and many phases will have been overlooked, in particular, those that are metastable. We describe a computational framework for battery cathode exploration based on ab initio random structure searching (AIRSS), an approach that samples local minima on the potential energy surface to identify new crystal structures. We show that by delimiting the search space using a number of constraints, including chemically aware minimum interatomic separations, cell volumes, and space group symmetries, AIRSS can efficiently predict both thermodynamically stable and metastable cathode materials. Specifically, we investigate LiCoO2, LiFePO4, and LixCuyFz to demonstrate the efficiency of the method by rediscovering the known crystal structures of these cathode materials. The effect of parameters, such as minimum separations and symmetries, on the efficiency of the sampling is discussed in detail. The adaptation of the minimum interatomic distances on a species-pair basis, from low-energy optimized structures to efficiently capture the local coordination environment of atoms, is explored. A family of novel cathode materials based on the transition-metal oxalates is proposed. They demonstrate superb energy density, oxygen-redox stability, and lithium diffusion properties. This article serves both as an introduction to the computational framework and as a guide to battery cathode material discovery using AIRSS. [ABSTRACT FROM AUTHOR]

We propose here a two-dimensional material based on a single layer of violet or Hittorf's phosphorus. Using first-principles density functional theory, we find it to be energetically very stable, comparable to other previously proposed single-layered phosphorus structures. It requires only a small energetic cost of approximately 0.04 eV/atom to be created from its bulk structure, Hittorf's phosphorus, or a binding energy of 0.3-0.4 J/m2 per layer, suggesting the possibility of exfoliation in experiments. We find single-layered Hittorf's phosphorus to be a wide band gap semiconductor with a direct band gap of approximately 2.5 eV, and our calculations show it is expected to have a high and highly anisotropic hole mobility with an upper bound lying between 3000-7000 cm2 V-1 s-1. These combined properties make single-layered Hittorf's phosphorus a very good candidate for future applications in a wide variety of technologies, in particular for high frequency electronics, and optoelectronic devices operating in the low wavelength blue color range. [ABSTRACT FROM AUTHOR]

Phosphorus has received recent attention in the context of high-capacity and high-rate anodes for lithium- and sodium-ion batteries. Here, we present a first-principles structure prediction study combined with NMR calculations, which gives us insights into its lithiation/sodiation process. We report a variety of new phases found by the ab initio random structure searching (AIRSS) and the atomic species swapping methods. Of particular interest, a stable Na5P4-C2/m structure and locally stable structures found less than 10 meV/f.u. from the convex hull such as Li4P3-P212121, NaP5-Pnma, and Na4P3-Cmcm. The mechanical stability of Na5P4-C2/m and Li4P3-P212121 has been studied by first-principles phonon calculations. We have calculated average voltages, which suggest that black phosphorus (BP) can be considered as a safe anode in lithium-ion batteries due to its high lithium insertion voltage, 1.5 V; moreover, BP exhibits a relatively low theoretical volume expansion compared with other intercalation anodes, 216% (ΔV/V). We identify that specific ranges in the calculated shielding can be associated with specific ionic arrangements, results that play an important role in the interpretation of NMR spectroscopy experiments. Since the lithium-phosphides are found to be insulating even at high lithium concentrations, we show that Li-P-doped phases with aluminum have electronic states at the Fermi level suggesting that using aluminum as a dopant can improve the electrochemical performance of P anodes. [ABSTRACT FROM AUTHOR]

We present here a first-principles study of the ternary compounds formed by Ni, In, and As, a material of great importance for self-aligned metallic contacts in next-generation InAs-based MOS transistors. The approach we outline is general and can be applied to study the crystal structure and properties of a host of other new interface compounds. Using the ab initio random structure searching approach we find the previously unknown low-energy structures of NixInAs and assess their stability with respect to the known binary compounds of Ni, In, and As. Guided by experiments, we focus on Ni3InAs and find a rich energy landscape for this stoichiometry. We consider the five lowest-energy structures, with space groups Pmmn, Pbcm, P21/m, Cmcm, and R3. The five low-energy structures for Ni3InAs are all found to be metallic and nonmagnetic. By comparison to previously published TEM results we identify the crystal structure observed in experiments to be Cmcm Ni3InAs. We calculate the work function for Ni3InAs and, according to the Schottky-Mott model, expect the material to form an Ohmic contact with InAs. We further explicitly consider the interface between Cmcm Ni3InAs and InAs and find it to be Ohmic with an n-type Schottky barrier height of -0.55 eV. [ABSTRACT FROM AUTHOR]

We use first-principles calculations to study structural, vibrational, and superconducting properties of H2S at pressures P⩾200 GPa. The inclusion of zero-point energy leads to two different possible dissociations of H2S, namely 3H2S→2H3S+S and 5H2S → 3H3S+HS2, where both H3S and HS2 are metallic. For H3S, we perform nonperturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (λ≈2.64 at 200 GPa) and Tc. Anharmonicity hardens H-S bond-stretching modes and softens H-S bond-bending modes. As a result, the electron-phonon coupling is suppressed by 30% (λ≈1.84 at 200 GPa). Moreover, while at the harmonic level Tc decreases with increasing pressure, the inclusion of anharmonicity leads to a Tc that is almost independent of pressure. High-pressure hydrogen sulfide is a strongly anharmonic superconductor. [ABSTRACT FROM AUTHOR]

Alkali metals exhibit unexpected structures and electronic behavior at high pressures. Compression of metallic sodium (Na) to 200 GPa leads to the stability of a wide-band-gap insulator with the double hexagonal hP4 structure. Post-hP4 structures remain unexplored, but they are important for addressing the question of the pressure at which Na reverts to a metal. Here, we report the reentrant metallicity of Na at the very high pressure of 15.5 terapascal (TPa), predicted using first-principles structure searching simulations. Na is therefore insulating over the large pressure range of 0.2-15.5 TPa. Unusually, Na adopts an oP8 structure at pressures of 117-125 GPa and the same oP8 structure at 1.75-15.5 TPa. The metallization of Na occurs on the formation of a stable and striking body-centered cubic cI24 electride structure consisting of Na12 icosahedra, each housing at its center about one electron that is not associated with any Na ions. [ABSTRACT FROM AUTHOR]

High-throughput 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 role as Li-ion battery (LIB) anodes. The calculations comprise structural optimizations 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 groundstate stoichiometries, including the recently found Li17Si4 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 LfiSL and Li1 Ge1 P4/mmm layered phases which become the ground states above 2.5 and 5 GPa, respectively, and reveal silicon and germanium's propensity toform dumbbells in the LixSi, x = 2.33-3.25, stoichiometry range. DFT predicts the stability of the Li11Ge6 Cmmm, Li12Ge7 Pnma, and Li7Ge3 P3212 phases and several new Li-Ge compounds, with stoichiometries Li5Ge2, Li13Ge5, Li8Ge3 and Li13Ge4. [ABSTRACT FROM AUTHOR]

Studies of the Earth's atmosphere have shown that more than 90% of the expected amount of Xe is depleted, a finding often referred to as the 'missing Xe paradox'. Although several models for a Xe reservoir have been proposed, whether the missing Xe could be contained in the Earth's inner core has not yet been answered. The key to addressing this issue lies in the reactivity of Xe with Fe/Ni, the main constituents of the Earth's core. Here, we predict, through first-principles calculations and unbiased structure searching techniques, a chemical reaction of Xe with Fe/Ni at the temperatures and pressures found in the Earth's core. We find that, under these conditions, Xe and Fe/Ni can form intermetallic compounds, of which XeFe3 and XeNi3 are energetically the most stable. This shows that the Earth's inner core is a natural reservoir for Xe storage and provides a solution to the missing Xe paradox. [ABSTRACT FROM AUTHOR]

Abstract: Two hexanuclear niobium halide cluster compounds with a [Nb6X12]2+ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable B o field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of 95Mo nuclei in structurally related compounds containing [Mo6Br8]4+ cluster cores. Experimentally determined δ iso(93Nb) values are in the range from 2400 to 3000ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (V zz and δ 33) coinciding with the molecular four-fold axis of the [Nb6X12]2+ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (V zz ) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (V yy ) and the largest CS principal axis (δ 11) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by 79/81Br NQR and 35Cl solid-state NMR measurements. [Copyright &y& Elsevier]

Solid He is studied in the pressure and temperature ranges 1-40 TPa and 0-10 000 K using first-principles methods. Anharmonic vibrational properties are calculated within a self-consistent field framework, including the internal and free energies, density-pressure relation, stress tensor, thermal expansion, and the electron-phonon coupling renormalization of the electronic band gap. We find that an accurate description of electron-phonon coupling requires us to use a nonperturbative approach. The metallization pressure of 32.9 TPa at 0 K is larger than found previously. The vibrational effects are large; for example, at P=30 TPa the band gap is increased by 2.8 eV by electron-phonon coupling and a further 0.1 eV by thermal expansion compared to the static value. The implications of the calculated metallization pressure for the cooling of white dwarfs are discussed. [ABSTRACT FROM AUTHOR]

We have used the ab initio random structure searching method together with density functional theory calculations to find stable structures of strontium under pressures up to 50 GPa. We predict a sequence of structural phase transitions and the stability of an orthorhombic structure of Cmcm symmetry above 25 GPa. Our energy, lattice dynamics, and molecular dynamics calculations confirm the stability of the Cmcm structure. The electron-phonon coupling calculations show that superconductivity arises in the bcc structure of compressed Sr and that it continues to exist in the Cmcm structure. The calculated superconducting transition temperatures are in good agreement with experiment. Our study gives an excellent account of the experimental observations. [ABSTRACT FROM AUTHOR]

We have used density-functional-theory methods together with a structure searching algorithm to determine stable structures and stoichiometries of mixtures of iron and carbon at high pressures and zero temperature. The most favorable stoichiometries at Earth's inner-core pressures (~350 GPa) are those with between about 20% and 35% carbon atoms. The most stable stoichiometries were found to be (Fe and C), Fe3C, Fe7C3, and Fe2C. The latter has not to our knowledge been discussed previously in relation to the Earth's core. The stoichiometries Fe4C and Fe5C2 were found to be close to stability at Earth's inner-core pressures. We find that Fe7C3 is unstable to decomposition into Fe3C + 2Fe2C at pressures greater than ~330 GPa. At 150 GPa only Fe, C, Fe3C, and Fe7C3 are stable. Formation of Fe/C compounds is energetically more favorable at 350 GPa than at 150 GPa. We also report a new phase for Fe3C with Cmcm symmetry to be more stable than the well-known cementite phase at 350 GPa. A number of pressure-induced phase transitions are identified in Fe3C, Fe3C2, FeC, Fe8C, and FeC2. The lowest enthalpy Fe/C phases were found to be metallic at the pressures studied. [ABSTRACT FROM AUTHOR]

Noble metals adopt close-packed structures at ambient pressure and rarely undergo structural transformation at high pressures. Platinum (Pt) is normally considered to be unreactive and is therefore not expected to form hydrides under pressure. We predict that platinum hydride (PtH) has a lower enthalpy than its constituents solid Pt and molecular hydrogen at pressures above 21.5 GPa. PtH transforms to a hexagonal close-packed or face-centered cubic (fcc) structure between 70 and 80 GPa. Linear response calculations indicate that PtH is a superconductor at these pressures with a critical temperature of about 10-25 K. These findings help to shed light on recent observations of pressure-induced metallization and superconductivity in hydrogen-rich materials. We show that the formation of fcc noble metal hydrides under pressure is common and examine the possibility of superconductivity in these materials. [ABSTRACT FROM AUTHOR]

Abstract: The NMR parameters obtained from solid-state DFT calculations within the GIPAW approach for 17O- and 69/71Ga-sites in a range of predominantly oxide-based (group II monoxides, SrTiO3, BaZrO3, BaSnO3, BaTiO3, LaAlO3, LaGaO3, SrZrO3, MgSiO3 and Ba2In2O5), and gallate (α- and β-Ga2O3, LiGaO2, NaGaO2, GaPO4 and LaGaO3) materials are compared with experimental values, with a view to the future application of a similar approach to doped phases of interest as candidate intermediate temperature solid oxide fuel cell (ITSOFC) electrolytes. Isotropic and anisotropic chemical shift parameters, quadrupolar coupling constants, and associated asymmetries are presented and analyzed. The unusual GaO5 site occurring in LaGaGe2O7 is also fully characterised. In general, it is found that the theoretical results closely track the experimental trends, though some deviations are identified and discussed, particularly in regard to quadrupolar η Q-values. The high quality of the computed results suggests that this approach can be extended to study more complex and disordered phases. [Copyright &y& Elsevier]

Abstract: A recently developed method for calculating NMR J coupling in solid-state systems is applied to calculate hydrogen-bond-mediated 2hJNN couplings across intra- or intermolecular N-H∙∙∙N hydrogen bonds in two 6-aminofulvene-1-aldimine derivatives and the ribbon structure formed by a deoxyguanosine derivative. Excellent quantitative agreement is observed between the calculated solid-state J couplings and those previously determined experimentally in two recent spin-echo magic-angle-spinning NMR studies (Brown, S. P.; et al. Chem. Commun.2002, 1852-1853 and Pham, T. N.; et al. Phys. Chem. Chem. Phys. 2007, 9, 3416-3423). For the 6-aminofulvene-1 -aldimines, the differences in 2hJNN couplings in pyrrole and triazole derivatives are reproduced, while for the guanosine ribbons, an increase in 2hJNN is correlated with a decrease in the N-H∙∙∙N hydrogen-bond distance. Jcouplings are additionally calculated for isolated molecules of the 6-aminofulevene-1 -aldimines extracted from the crystal with and without further geometry optimization. Importantly, it is shown that experimentally observed differences between J couplings determined by solution- and solid-state NMR are not solely due to differences in geometry; long-range electrostatic effects of the crystal lattice are shown to be significant also. J couplings that are yet to be experimentally measured are calculated. Notably, 2hJNN couplings across N-H∙∙∙N hydrogen bonds are found to be of a similar magnitude to 2hJNN couplings, suggesting that their utilization and quantitative determination should be experimentally feasible. [ABSTRACT FROM AUTHOR]

Two-dimensional ¹H-13C MAS-J-HMQC solid-state NMR spectra of the two anomeric forms of maltose at natural abundance are presented. The experimental ¹H chemical shifts of the CH and CH₂ protons are assigned using first-principles chemical shift calculations that employ a plane-wave pseudo- potential approach. Further calculations show that the calculated change in the ¹H chemical shift when comparing the full crystal and an isolated molecule is a quantitative measure of intermolecular C-HߪO weak hydrogen bonding. Notably, a clear correlation between a large chemical shift change (up to 2 ppm) and both a short HߪO distance (<2.7 Å) and a CHO bond angle greater than 130° is observed, thus showing that directionality is important in C-HߪO hydrogen bonding. [ABSTRACT FROM AUTHOR]

We theoretically investigated the local structure of the aperiodic states of zircon (ZrSiO[sub4]) using ab initio quantum mechanical calculations. The low and high density liquid and solid glassy phases were obtained by constant volume Car-Parrinello molecular dynamics simulations, using the molar volume of metamict and crystalline zircon, respectively. As in naturally metamict zircons, the polymerization of Si units, the segregation of Zr atoms, and an overall decrease of the Zr coordination were observed. However, the local ordering of our theoretical glasses differs from that of the natural amorphous samples. In the theoretical glasses, the Zr-O distances in the first coordination polyhedra are significantly more distributed and five- and six-coordinated Si species were observed. The relaxation of the glass structure on a time scale exceeding the possibility of molecular modeling is a possible explanation for these discrepancies. The dielectric and [sup29]Si NMR responses of the glassy phases were computed, providing new constraints for the analysis of experimental data recorded from metamict zircons. The calculated NMR spectra are in good agreement with the experimental NMR spectra in the four-coordinated Si region. Our results show that the usual regular systematic decrease of the [sup29]Si chemical shift as a function of the polymerization of Si units (Q[supn] species) cannot be used to interpret the [sup29]Si NMR spectrum of amorphous zircon. In particular, whereas the empirical scale would indicate an average polymerization of Q³, the average polymerization of Q&sup1.5; observed in the low density zircon glass can account for the experimental data. In a non-uniform model of the structure of metamict zircon, this lower average polymerization is consistent with a clustering of cations limited to few coordination polyhedra. [ABSTRACT FROM AUTHOR]