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]

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]

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]

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]

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]

We investigate the high-pressure behavior of beryllium, magnesium, and calcium difluorides using ab initio random structure searching and density functional theory (DFT) calculations, over the pressure range 0-70 GPa. Beryllium fluoride exhibits extensive polymorphism at low pressures, and we find two new phases for this compound--the silica moganite and CaCl2 structures--which are stable over the wide pressure range 12-57 GPa. For magnesium fluoride, our searching results show that the orthorhombic "O-I" TiO2 structure (Pbca,Z=8) is stable for this compound between 40 and 44 GPa. Our searches find no new phases at the static-lattice level for calcium difluoride between 0 and 70 GPa; however, a phase with P 62m symmetry is close to stability over this pressure range, and our calculations predict that this phase is stabilized at high temperature. The P 62m structure exhibits an unstable phonon mode at large volumes which may signal a transition to a superionic state at high temperatures. The group-II difluorides are isoelectronic to a number of other AB2-type compounds such as SiO2 and TiO2, and we discuss our results in light of these similarities. [ABSTRACT FROM AUTHOR]

The alloying mechanism of high-capacity tin anodes for sodium-ion batteries is investigated using a combined theoretical and experimental approach. Ab initio random structure searching (AIRSS) and high-throughput screening using a species-swap method provide insights into a range of possible sodium-tin structures. These structures are linked to experiments using both average and local structure probes in the form of operando pair distribution function analysis, X-ray diffraction, and 23Na solid-state nuclear magnetic resonance (ssNMR), along with ex situ 119Sn ssNMR. Through this approach, we propose structures for the previously unidentified crystalline and amorphous intermediates. The first electrochemical process of sodium insertion into tin results in the conversion of crystalline tin into a layered structure consisting of mixed Na/Sn occupancy sites intercalated between planar hexagonal layers of Sn atoms (approximate stoichiometry NaSn3). Following this, NaSn2, which is predicted to be thermodynamically stable by AIRSS, forms; this contains hexagonal layers closely related to NaSn3, but has no tin atoms between the layers. NaSn2 is broken down into an amorphous phase of approximate composition Na1.2Sn. Reverse Monte Carlo refinements of an ab initio molecular dynamics model of this phase show that the predominant tin connectivity is chains. Further reaction with sodium results in the formation of structures containing Sn-Sn dumbbells, which interconvert through a solid-solution mechanism. These structures are based upon Na5-x Sn2, with increasing occupancy of one of its sodium sites commensurate with the amount of sodium added. ssNMR results indicate that the final product, Na15Sn4, can store additional sodium atoms as an off-stoichiometry compound (Na15+x Sn4) in a manner similar to Li15Si4. [ABSTRACT FROM AUTHOR]

[Mo6X14]2- octahedral molybdenum clusters are the main building blocks of a large range of materials. Although 95Mo nuclear magnetic resonance was proposed to be a powerful tool to characterize their structural and dynamical properties in solution, these measurements have never been complemented by theoretical studies which can limit their interpretation for complex systems. In this Article, we use quantum chemical calculations to evaluate the 95Mo chemical shift of three clusters: [Mo6Cl14]2-, [Mo6Br14]2-, and [Mo6I14]2-. In particular, we test various computational parameters influencing the quality of the results: size of the basis set, treatment of relativistic and solvent effects. Furthermore, to provide quantum chemical calculations that are directly comparable with experimental data, we evaluate for the first time the 95Mo nuclear magnetic shielding of the experimental reference, namely, Mo042- in aqueous solution. This is achieved by combining ab initio molecular dynamics simulations with a periodic approach to evaluate the 95Mo nuclear shielding?. The results demonstrate that, despite the difficulty to obtain accurate 95Mo chemical shifts, relative values for a cluster series can be fairly well-reproduced by DFT calculations. We also show that performing an explicit solvent treatment for the reference compound improves by ~50 ppm the agreement between theory and experiment. Finally, the standard deviation of ~70 ppm that we calculate for the 95Mo nuclear shielding of the reference provides an estimation of the accuracy we can achieve for the calculation of the 95Mo chemical shifts using a static approach. These results demonstrate the growing ability of quantum chemical calculations to complement and interpret complex experimental measurements. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nanodiamonds (ND) with 1-5 nm dimensions found in meteorites or produced by chemical vapour deposition (CVD) and detonation synthesis are typically described in terms of an sp3-bonded carbon network. However, ultra-high-resolution transmission electron microscopy (uHRTEM) combined with density functional theory (DFT) modelling leads to a different structural interpretation. uHRTEM imaging and nanodiffraction studies of many NDs show six-fold symmetry features whose identity has long been controversial. We also observe diffraction patterns with twelve equally-spaced and symmetrically but unequally arranged reflections, indicating structures with crystallographically-forbidden ideal and distorted twelve-fold symmetry. Structural models based on our DFT calculations lead to an interpretation of these unusual features found throughout the meteoritic and CVD samples in terms of sp3 domains arranged around and coherently bonded to graphitic domains embedded within the diamond matrix. The bonding at the sp2-sp3 interface can explain the unusual features observed in electron energy-loss spectra (EELS) below the onset of the main diamond C1s core-loss edge leading to predictions of low-dimensional conductivity behaviour. The presence of sp2- as well as sp3-bonded regions allows us to interpret previously unexplained features of the Raman spectra and EELS data of ND materials. [Display omitted] • Nanosized grains exhibiting six- and twelve-fold diffraction symmetry occur in natural and synthetic NDs. • Structurally distinct features arise from diaphite domains with few-layered graphene layers contained within and coherently bonded to the host diamond matrix. • Diaphite-structured nanocarbons are widespread in ND material and their presence is responsible for unusual and previously unexplained features of their Raman and electron-energy loss spectra. • The new class of nanocarbons leads to new interpretations of the formation and properties of natural and synthetic diamond-related materials. [ABSTRACT FROM AUTHOR]

We introduce two-dimensional borane, a single-layered material of BH stoichiometry, with promising electronic properties. We show that, according to density functional theory calculations, two-dimensional borane is semimetallic, with two symmetry-related Dirac cones meeting right at the Fermi energy Eƒ. The curvature of the cones is lower than in graphene, thus closer to the ideal linear dispersion. Its structure, formed by a puckered trigonal boron network with hydrogen atoms connected to each boron atom, can be understood as distorted, hydrogenated borophene [Mannix et al., Science 350, 1513 (2015)]. Chemical bonding analysis reveals the boron layer in the network being bound by delocalized four-center two-electron s bonds. Finally, we suggest high pressure could be a feasible route to synthesize two-dimensional borane. [ABSTRACT FROM AUTHOR]

Review: 81 refs. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) has been used in many fields of the physical sciences, but none so successfully as in the solid state. From its origins in condensed matter physics, it has expanded into materials science, high-pressure physics and mineralogy, solid-state chemistry and more, powering entire computational subdisciplines. Modern DFT simulation codes can calculate a vast range of structural, chemical, optical, spectroscopic, elastic, vibrational and thermodynamic phenomena. The ability to predict structure-property relationships has revolutionized experimental fields, such as vibrational and solid-state NMR spectroscopy, where it is the primary method to analyse and interpret experimental spectra. In semiconductor physics, great progress has been made in the electronic structure of bulk and defect states despite the severe challenges presented by the description of excited states. Studies are no longer restricted to known crystallographic structures. DFT is increasingly used as an exploratory tool for materials discovery and computational experiments, culminating in ex nihilo crystal structure prediction, which addresses the long-standing difficult problem of how to predict crystal structure polymorphs from nothing but a specified chemical composition. We present an overview of the capabilities of solid-state DFT simulations in all of these topics, illustrated with recent examples using the CASTEP computer program. [ABSTRACT FROM AUTHOR]