Search

Iron is considered to be the main component of the Earth's core. Substantial efforts have been made to understand its phase diagram and physical properties at extreme conditions. However, it remains debated about how the atoms in solid iron are arranged at Earth's core conditions, where possible candidates include hexagonal close-packed (hcp), body-centred cubic (bcc), and face-centred cubic (fcc) structures. As crystal structure and physical properties are closely related, there is also a significant uncertainty in the properties of Earth's core, such as elasticity, heat conductivity, and density, making the accurate interpretation of seismic observations difficult. Here we aim to study the phase stability of solid iron at Earth's core conditions. For this, a deep-learning interatomic potential was developed with ab initio accuracy but is more cost-effective. To further check the performance of such potential, we examine the elastic and plastic behaviour of hcp iron and the effects of structural defects at inner core conditions [1]. We then compute the Gibbs free energy of the bcc, fcc, hcp and liquid phases by performing large-scale molecular dynamics simulations. The calculated free energy allows for determining the phase stability of solid iron in Earth's core.[1] Li, Z., & Scandolo, S. (2022). Elasticity and viscosity of hcp iron at Earth's inner core conditions from machine learning-based large-scale atomistic simulations. Geophysical Research Letters, 49, e2022GL101161.

At high pressures, simple metals such as potassium have a rich phase diagram including an insulating electride phase in which electrons have a localized, anionic character. Measurements in the liquid phase have shown a transition between two states, but experimental challenges have prevented detailed thermodynamic measurements. Using potassium as an example, we present numerical evidence that the liquid–liquid transition is a continuous transformation from free electron to electride behaviour. We show that the transformation manifests in anomalous diffusivity, thermal expansion, speed of sound, coordination number, reflectivity and heat capacity across a wide range of pressures. The abnormalities stem from a significant change in the local electronic and ionic structure. Although primarily a pressure-induced phenomenon, there is also a thermal expansion anomaly. By establishing the electride nature of the high-pressure liquid phase, we resolve the long-standing mystery of how a liquid can be denser than a close-packed solid. Our work is relevant for high-pressure thermodynamic properties of all alkali metal liquids. Alkali metals at high pressures have a liquid–liquid transition that is difficult to study in detail. Numerical calculations now suggest that the higher-pressure state is an electride liquid, in which electrons behave like localized anions.

We present a thermodynamic model which explains the presence of a negative slope in the melt curve, as observed in systems as diverse as the alkali metals and molecular hydrogen at high pressure. We assume that components of the system can be in one of two well defined states---one associated with low energy, the other with low volume. The model exhibits a number of measurable features which are also observed in these systems and are therefore expected to be associated with all negative Clapeyron-slope systems: first order phase transitions and thermodynamic anomalies along Widom lines. The melt curve maximum is a feature of the model, but appears well below the pressures where the change in state occurs in the solid: the solid-solid transition is related to the melt line minimum. An example of the model fitted to the electride transition in potassium is discussed.

The Raman frequencies of diamond subjected to non-hydrostatic uniaxial stress along the [001] and [111] crystallographic axes have been calculated with density-functional-theory methods and the results fitted to a simple analytical form. The data are analyzed in the context of the recently proposed use of the shift of the high-frequency edge of the Raman band of diamond as a pressure scale in diamond-anvil cell experiments. Combining theoretical and experimental data, we are able to determine the stress state of the diamond anvil in ultra-high-pressure experiments. We find that shear stresses close to the tip of the anvil can reach values exceeding 1 Mbar.

The experimental study of the ${\mathrm{CO}}_{2}$ phase diagram is hampered by strong kinetic effects leading to wide regions of metastability and to large uncertainties in the location of some phase boundaries. Here, we determine ${\mathrm{CO}}_{2}$'s thermodynamic phase boundaries by means of ab initio calculations of the Gibbs free energy of several solid phases of ${\mathrm{CO}}_{2}$ up to 50 Gigapascals. Temperature effects are included in the quasiharmonic approximation. Contrary to previous suggestions, we find that the boundary between molecular forms and the nonmolecular phase V has, indeed, a positive slope and starts at 21.5 GPa at $T=0\text{ }\text{ }\mathrm{K}$. A triple point between phase IV, V, and the liquid phase is found at 35 GPa and 1600 K, indicating a broader region of stability for the nonmolecular form than previously thought. The experimentally determined boundary line between ${\mathrm{CO}}_{2}\text{\ensuremath{-}}\mathrm{II}$ and ${\mathrm{CO}}_{2}\text{\ensuremath{-}}\mathrm{IV}$ phases is reproduced by our calculations, indicating that kinetic effects do not play a major role in that particular transition. Our results also show that ${\mathrm{CO}}_{2}\text{\ensuremath{-}}\mathrm{III}$ is stabilized at high temperature and its stability region coincides with the $P\text{\ensuremath{-}}T$ conditions where phase VII has been reported experimentally; instead, phase II is the most stable molecular phase at low temperatures, extending its region of stability to every $P\text{\ensuremath{-}}T$ condition where phase III is reported experimentally.

Copper-modified titania photocatalysts are known to display a higher activity toward water splitting than pure titania, but the mechanisms of this enhancement are unclear. We investigated oxidation of water on nanostructured Cu2O/TiO2 and CuO/TiO2 systems by density functional theory. Under photoelectrochemical conditions, the most stable structure has a CuO stoichiometry and yields an overpotential of 0.66 V for water oxidation, with the active site being at the interface between copper oxide and titanium dioxide. A lower overpotential of 0.48 V is found on a metastable Cu2O nanostructure. These values are lower than the values for pure titania and for copper oxide. Moreover, in the case of the most stable structure, the active site is directly at the interface between CuO and TiO2. We therefore argue that a cooperative catalytic effect is at play in this system, beside the known increased photoabsorption coming from the lower band gap. The nanostructures display a stronger adsorption of OH with respect ...

Materials science is arguably the most important discipline in the physical sciences that should be developed in Africa given its rich resources of minerals, energy and biological diversity. Developing materials science should therefore be an important goal for Africa with important opportunities for economic benefits and quality people development. There are already many successful materials science activities that are underway in Africa. This needs to gain wider international attention and should become contact points for international collaborations. The African School for Electronic Structure Methods and Applications (ASESMA)[1,2,3,4] is one such successful initiative that has been in existence for the past decade. ASESMA has shown that it is possible to build a network across sub-Saharan Africa with world-class research with a relatively low budget. The greatest asset is the commitment of the lecturers and mentors, the team-work of the local organisers and the idealism of the participants who rank amongst the brightest of young minds from Africa, many of whom come from impoverished backgrounds but still dare to reach for the stars.

High-pressure single-crystal X-ray diffraction method with precise control of hydrostatic conditions, typically with helium or neon as the pressure-transmitting medium, has significantly changed our view on what happens with low-density silica phases under pressure. Coesite is a prototype material for pressure-induced amorphization. However, it was found to transform into a high-pressure octahedral (HPO) phase, or coesite-II and coesite-III. Given that the pressure is believed to be hydrostatic in two recent experiments, the different transformation pathways are striking. Based on molecular dynamic simulations with an ab initio parameterized potential, we reproduced all of the above experiments in three transformation pathways, including the one leading to an HPO phase. This octahedral phase has an oxygen hcp sublattice featuring 2 × 2 zigzag octahedral edge-sharing chains, however with some broken points (i.e., point defects). It transforms into α-PbO2 phase when it is relaxed under further compression. We show that the HPO phase forms through a continuous rearrangement of the oxygen sublattice toward hcp arrangement. The high-pressure amorphous phases can be described by an fcc and hcp sublattice mixture.

Copper oxides deposited at titania surfaces have a beneficial effect on the photocatalytic activity of TiO2, but their role is not fully understood. In this work, possible nanostructures of copper oxide on TiO2 (101) have been investigated by simulations based on density functional theory. Various stoichiometries, from Cu2O to CuO, and morphologies, from clusters to nanowires, have been considered. Nanowire structures were consistently more stable than isolated clusters. In these structures, a Cu2O stoichiometry was found to be thermodynamically more stable than CuO at room conditions, in contrast to what happens in bulk copper. Occupied Cu 3d and O 2p states extend well into the band gap of titania, whereas the nature of the lowest-lying empty states depend on the stoichiometry: for Cu2O they consist mostly of Ti 3d orbitals, while in CuO unoccupied Cu 3d orbital ∼0.8 eV above the Fermi level are present. Thus, both oxides reduce the band gap of the system with respect to pure titania, but only Cu2O shou...

The experimental study of the CO_{2} phase diagram is hampered by strong kinetic effects leading to wide regions of metastability and to large uncertainties in the location of some phase boundaries. Here, we determine CO_{2}'s thermodynamic phase boundaries by means of ab initio calculations of the Gibbs free energy of several solid phases of CO_{2} up to 50 Gigapascals. Temperature effects are included in the quasiharmonic approximation. Contrary to previous suggestions, we find that the boundary between molecular forms and the nonmolecular phase V has, indeed, a positive slope and starts at 21.5 GPa at T=0 K. A triple point between phase IV, V, and the liquid phase is found at 35 GPa and 1600 K, indicating a broader region of stability for the nonmolecular form than previously thought. The experimentally determined boundary line between CO_{2}-II and CO_{2}-IV phases is reproduced by our calculations, indicating that kinetic effects do not play a major role in that particular transition. Our results also show that CO_{2}-III is stabilized at high temperature and its stability region coincides with the P-T conditions where phase VII has been reported experimentally; instead, phase II is the most stable molecular phase at low temperatures, extending its region of stability to every P-T condition where phase III is reported experimentally.

Nature has provided us with many more types of phases than the elementary ones—solid, liquid, and gas—that we learn in school textbooks. Some of them can be rather exotic, such as the one considered in the PNAS article by Robinson et al. (1), which deals with a remarkable but not so uncommon state of matter found in several elemental solids when they are compressed to gigapascal pressures (1 GPa = 10,000 bar). The solid in this specific case, elemental rubidium, crystallizes in a tetragonal structure composed of two interpenetrating lattices with incommensurate periodicity (Fig. 1). When heated, the “host” chain sublattice loses its long-range periodic order while the “guest” sublattice remains crystalline. Fig. 1. Crystal structure of the incommensurate host–guest structure observed in sodium, potassium, and rubidium at high pressure. The guest chains (dark green atoms) fill the channels of the host sublattice (light green atoms). At low temperature, the periodicity of the two sublattices is incommensurate along the chain direction. At high temperature, the chain sublattice loses long-range order. Adapted with permission from ref. 7, which is licensed under CC BY 4.0. Microscopically complex state changes such as the ones observed in host–guest structures raise fundamental questions regarding how the system disorders as temperature is raised. Is chain disordering a phase transition? Why do the two interpenetrating lattices melt at different temperatures? Do different types of order, or rather disorder, appear? Determining the nature of phase transitions, and especially temperature-induced phase transitions, … [↵][1]1Email: scandolo{at}ictp.it. [1]: #xref-corresp-1-1

A classical force field for MgSiO 3 polymorphs is presented and tested for the perovskite, post-perovskite and enstatite phases. In this force field each ion has a fixed partial charge and a fixed polarizability and the potential energy is minimised with respect to the ions’ dipole moments at each step of molecular dynamics by iterating them to self-consistency. The potential parameters are obtained by fitting to forces, stresses and energies calculated by density functional theory. The phase transition from perovskite to post-perovskite is predicted to take place at a pressure of 60 GPa at zero temperature. The potential reproduces well structural and thermodynamic properties as well as defects formation in a wide range of pressures, and therefore will be useful for the investigation of MgSiO 3 based materials in a geophysical context.

The phase diagram of solid oxygen at terapascal pressures and several thousand Kelvin has been studied with ab initio density functional theory within the quasiharmonic approximation for the vibrational free energy. Our work extends previous theoretical studies done at zero temperature and shows that temperature has a dramatic effect on the sequence of phases. At low temperature, the transition from onefold (molecular) to fourfold coordination takes place through two intermediate phases with twofold coordination and space groups $I{4}_{1}/acd$ and $Cmcm$. Above 8000 K we find that these two intermediate phases are no longer stable, and oxygen transforms directly from a molecular phase to a fourfold coordinated phase of space group $Fmmm$. We also find that the transition between $Cmcm$ and $Fmmm$ can be ascribed as a second-order transition driven by an electronic instability.

The high pressure properties of methane hydrates have been the subject of intense research in the last decade due to their relevance in geophysics and planetary sciences. Earlier studies have indicated that the compound is remarkably stable under pressure, however a recent report has shown that the compound decomposes into its constituents, ice and methane, above 3 GPa. Here, we report the results of density-functional theory calculations and show that methane hydrate is energetically unstable against decomposition above a few GPa. The enthalpy difference between the compound and the constituents reaches a value of 0.45 eV per CH4 molecule at 80 GPa, in favor of the constituents.

X-ray absorption near-edge spectroscopy (XANES) spectra of solid and liquid iron at pressure and temperature obtainable at the Earth's mantle conditions have been calculated. The spectra have been obtained using the supercell with an electron core-hole approach, in an ab initio scheme based on the continued-fraction approach and norm-conserving pseudo-potentials. The atomic structures for the spectra calculations were obtained from classical molecular dynamics simulations performed using an optimized modified embedded-atom potential. Comparisons to experimental XANES data are made for both solid and liquid phases of iron. For the liquid configuration, we find excellent agreement. Calculated spectra obtained for the solid configurations are also in good agreement with the experiment, and those obtained via other theoretical methods. We discuss the electronic origin of the XANES features.

We have performed ab-initio calculations of the formation and migration energies of intrinsic defects (interstitials, vacancies and Frenkel defects) in barium fluoride. The calculations were performed within density-functional theory and the generalized-gradient approximation, employing pseudopotentials and a plane-wave basis set. The results agree reasonably well with available experimental data. They are also compatible with calculations and experimental data on calcium fluoride. We found that Frenkel pairs are composed of pairs of charged defects and that their formation energies are 3.44 eV and 1.88 eV for cation and anion, respectively. The lowest barrier for defect migration was found to correspond to the migration of the anion vacancy along the (100) direction (energy barrier of 0.53 eV), which compares well with the experimental value of 0.59 eV. Cation vacancy migration was instead found to require an energy of at least 2.22 eV along the easiest migration path, < 100 >. (C) 2013 Elsevier Ltd. All rights reserved.

Starting from ab initio calculations and using a force matching procedure, we have developed a new force field for molecular dynamics simulations of self-assembled monolayers of methylthiolate (MT) on Au(111) surfaces. This new force field is able to reproduce several observed features of SAMs of MT on Au(111) surface, such as the formation of gold vacancy islands and the (root 3 x root 3)R30 lattice. We have studied the dynamics of Au adatoms and monatomic vacancies on the Au(111) surface for the SAM of MT at room temperature. It is observed that monatomic vacancies coarsen to form large vacancy islands while the adatoms group to form clusters. Both results are in agreement with experiments. At elevated temperatures, Au adatoms that are lifted from the surface leave an atomic vacancy on it. The liquid-like diffusion of gold adatoms on the SAM surface occurs by hopping between pairs of methylthiolate to which the adatom is temporarily bound. Our findings indicate that structural models of the c(4 x 2) unit cell including adatoms and vacancies at room temperature need to be revisited.

The photocatalytic activity of titanium dioxide is sometimes observed to increase upon addition of silica, but the origin of this enhancement is not clear. To shed light on this effect, we investigate the titania/silica system in case of perfect segregation using density functional theory. Two situations have been considered: a single silica monolayer covering a titania surface and a bulk titania/silica interface. In both cases, we find that the presence of silica strongly modifies the electronic structure of the catalyst. In the case of the bulk interface, the analysis of the projected density of states reveals that interface electronic states give a large contribution to the edges of valence and conduction bands. For a silica monolayer on a (101) surface of anatase, the spatial localization of the states near the edge of the valence band depends on the hydroxylation state of the monolayer. Similar to the bulk case, the hydrogen-free monolayer hosts interface states, whereas in the fully hydroxylated system, these disappear and all top valence band states are located inside the titania slab. These results can be rationalized in terms of the number of Ti atoms available for bonding with bridging oxygens at the interface, suggesting that the ratio of Ti-O-Si and 2Ti-O-Si linkages allowed by the interface bonding topology may v play an important role in photoabsorption processes.

Ab initio electronic structure calculations have been performed to study the formation and migration of Kr impurities, and the stability of small Kr–vacancy clusters for clusters with up to four vacancies and four Kr atoms, in hcp Ti. Both the substitutional and the interstitial configurations of Kr are found to be stable. The octahedral configuration is however found to be more stable than the tetrahedral. Interstitial Kr atoms are shown to have attractive interactions and a low migration barrier, suggesting that, at low temperature, Kr bubble formation is possible, even in the absence of vacancies. We also find vacancy clusters to be stable. The binding energies of an interstitial Kr atom and a vacancy to a Kr–vacancy cluster are obtained from the calculated formation energies of the clusters. The stability of small-vacancy clusters is found to be dependent on Kr–vacancy ratio. The trends of the calculated binding energies are discussed in terms of providing further insights on the behaviour of Kr in implanted Ti.

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration....

Understanding the structural transformations of solid CO 2 from a molecular solid characterized by weak intermolecular bonding to a 3-dimensional network solid at high pressure has challenged researchers for the past decade. We employ the recently developed metadynamics method combined with ab initio calculations to provide fundamental insight into recent experimental reports on carbon dioxide in the 60–80 GPa pressure region. Pressure-induced polymeric phases and their transformation mechanisms are found. Metadynamics simulations starting from the CO 2 -II (P4 2 /mnm) at 60 GPa and 600 K proceed via an intermediate, partially polymerized phase, and finally yield a fully tetrahedral, layered structure (P-4m2). Based on the agreement between calculated and experimental Raman and X-ray patterns, the recently identified phase VI [Iota V, et al. (2007) Sixfold coordinated carbon dioxide VI. Nature Mat 6:34–38], assumed to be disordered stishovite-like, is instead interpreted as the result of an incomplete transformation of the molecular phase into a final layered structure. In addition, an α-cristobalite-like structure (P4 1 2 1 2), is predicted to be formed from CO 2 -III (Cmca) via an intermediate Pbca structure at 80 GPa and low temperatures (2 -III transforms to an amorphous form, consistent with experiment [Santoro M, et al. (2006) Amorphous silica-like carbon dioxide. Nature 441:857–860], but the simulation yields additional structural details for this disordered solid.

Annealing to several hundred Kelvin has been recently shown to induce densification in compressed SiO2 glass. By means of an ab-initio parameterised interatomic potential for SiO2 and molecular dynamic simulations, we studied the structural properties of compressed glass by cold compression at room temperature and by quenching the liquid at selected pressures. The noticeable differences found below 10 GPa between the two results are interpreted in the context of the experimentally reported temperature-induced densification and of the speculated occurrence of a first-order transition between two distinct polyamorphs. Calculated X-ray and neutron structure factors agree well with available experiments, and indicate that structural differences between annealed and cold-compressed forms take place at distances of 3.5–4 A. In contrast, short-range order (atomic coordination, bond length and angular distribution functions) is identical in the two forms of silica glass.

Clarifying the structure/dynamics relation of water hydrogen-bond network has been the aim of extensive research over many decades. By joining anvil cell high-pressure technology, femtosecond 2D infrared spectroscopy, and molecular dynamics simulations, we studied, for the first time, the spectral diffusion of the stretching frequency of an HOD impurity in liquid water as a function of pressure. Our experimental and simulation results concordantly demonstrate that the rate of spectral diffusion is almost insensitive to the applied pressure. This behavior is in contrast with the previously reported pressure-induced speed up of the orientational dynamics, which can be rationalized in terms of large angular jumps involving sudden switching between two hydrogen-bonded configurations. The different trend of the spectral diffusion can be, instead, inferred considering that the first solvation shell preserves the tetrahedral structure with pressure and the OD stretching frequency is only slight perturbed.

Theoretical studies of the interaction of HCl with small water clusters have so far neglected the effect of temperature, which ranges from a few tens of kelvin in cluster experiments, up to about 250 K in typical atmospheric conditions. We study the dynamical behavior of a selected set of HCl(H2O)6 clusters, representative of undissociated and dissociated configurations, by means of DFT-based first principles molecular dynamics. We find that the thermodynamcal stability of different configurations can be affected by temperature. We also present the infrared spectra of dissociated and undissociated configurations at 200 K and discuss the origin of the spectral features.

Novel extended tetrahedral forms of CO2 have been synthesized recently under high-pressure conditions. We perform ab initio density functional theory calculations to investigate whether doping with Si can extend the stability range of such tetrahedral forms of CO2 to ambient pressure. Calculations are performed with a simple cubic cell containing eight formula units in a β-cristobalite-like structure. Though we find that all the SixC1−xO2 structures considered by us are thermodynamically unstable with respect to decomposition into the end members at ambient pressures, the energy differences are small, suggesting that it might be possible for such phases to exist in metastable forms. At higher pressures, the heat of formation is found to be negative. The bonding between C and O atoms is more covalent than that between Si and O atoms. We also find indications that some C atoms may prefer three-fold coordination at low pressure.

Pressure is extremely efficient to tune intermolecular interactions, allowing the study of the mechanisms regulating, at the molecular level, the structure and dynamics of condensed phases. Among the simplest molecules, water represents in many respects a mystery despite its primary role in ruling most of the biological, physical, and chemical processes occurring in nature. Here we report a careful characterization of the dynamic regime change associated with low-density and high-density forms of liquid water by measuring the line shape of the OD stretching mode of HOD in liquid water along different isotherms as a function of pressure. Remarkably, the high-pressure studies have been here extended down to 240 K, well inside the supercooled regime. Supported by molecular dynamics simulations, a correlation among amorphous and crystalline solids and the two different liquid water forms is attempted to provide a unified picture of the metastable and thermodynamic regimes of water.

Liquid water has a primary role in ruling life on Earth in a wide temperature and pressure range as well as a plethora of chemical, physical, geological, and environmental processes. Nevertheless, a full understanding of its dynamical and structural properties is still lacking. Water molecules are associated through hydrogen bonds, with the resulting extended network characterized by a local tetrahedral arrangement. Two different local structures of the liquid, called low-density (LDW) and high-density (HDW) water, have been identified to potentially affect many different chemical, biological, and physical processes. By combining diamond anvil cell technology, ultrafast pump-probe infrared spectroscopy, and classical molecular dynamics simulations, we show that the liquid structure and orientational dynamics are intimately connected, identifying the P-T range of the LDW and HDW regimes. The latter are defined in terms of the speeding up of the orientational dynamics, caused by the increasing probability of breaking and reforming the hydrogen bonds.

We present a computational study of elastic properties of disordered systems with voids. The influence of hydrogen and voids on the elastic properties has been investigated by means of atomistic simulations using empirical potentials for hydrogenated amorphous silicon. The elastic constants have been obtained from the fluctuations of the simulation cell in Monte Carlo simulations at constant pressure and temperature. Our results indicate that the softening of the elastic constants of a-Si:H observed experimentally upon increasing hydrogen content cannot be explained by a weakening of the network induced by the reduced coordination but should be attributed instead to the formation of voids. A simple relation between Young’s modulus and the density is presented.

We have isolated at T < 150 K a weakly adsorbed dimethyl disulfide (DMDS) layer on Au(111) and studied how the vibrational states, S core hole level shifts, valence band photoemission, and work function measurements evolve upon transforming this system into chemisorbed methylthiolate (MT) self-assembled monolayers (SAM) by heating above 200 K. By combining these observations with detailed theoretical electronic structure simulations, at the density functional level, we have been able to obtain a detailed picture of the electronic interactions at the interface between Au and adsorbed thiolates and disulfides. All of our measurements may be interpreted with a simple model where MT is bound to the Au surface with negligible charge transfer. Interfacial dipoles arising from Pauli repulsion between molecule and metal surface electrons are present for the weakly adsorbed DMDS layer but not for the chemisorbed species. Instead, for the chemisorbed species, interfacial dipoles are exclusively controlled by the molecular dipole, its interaction with the dipoles on neighboring molecules, and its orientation to the surface. The ramifications of these results for alignment of molecular levels and interfacial properties of this class of materials are discussed.

Understanding the behavior of simple molecular systems under the combined effect of high pressure (P) and temperature (T) has important implications in the modeling of planetary interiors, in the chemistry of detonation, and in the synthesis of new materials. Extreme conditions of P and T can be partially accessed in the laboratory either in heated diamond-anvil cells or in shock wave experiments, where dramatic changes in the chemical and physical properties of simple molecules have been reported. Depending on the detailed situation, compression may result in the formation of intermolecular bonds, or in disproportionation, or even in full dissociation into the constituent elements. In such a context, the theoretical prediction of stable or metastable structures and of their properties is highly desirable. However, this requires an accurate quantum description, capable of discriminating the often subtle changes in the nature of chemical bonds. First-principles molecular dynamics methods with variable-cell, constant-pressure dynamics—a natural derivation of the two revolutionary methods introduced more than 20 years ago by Michele Parrinello together with Roberto Car and Anees Rahman—offer a unique tool for this kind of exploration. Molecular CO2 was independently observed [5] and predicted to transform into extended quartz-like covalent solids at moderately high pressures (about 50 GPa). The actual structure of this high-pressure phase, as well as its P–T range of thermodynamic stability, are still widely debated. High temperatures (thousands of Kelvin) were crucially required in the synthesis of an extended covalent network, both in experiments and in theoretical simulations, which indicated that large reaction barriers are overcome in the synthesis. One possible explanation is that these barriers are associated with the rehybridization of carbon atomic orbitals, from linear (sp) in the molecule, to tetrahedral (sp) in all candidates for the extended solid proposed so far. Carbon is well-known to change its hybridization state with pressure. For example, carbon’s hybridi

We have developed a theoretical formalism to calculate the orientational phase diagram of ortho–para (or odd-J/even-J) mixtures of homonuclear diatomic molecules in the low-pressure solid phases. In particular, our formalism allows for the explicit disorder present in such mixtrues. While the formalism is general, here we apply it to the quantum anisotropic planar rotor model, a two-dimensional model of coupled rotors. Our calculated phase diagram, separating phases of disorder and short-range order is reentrant, when an equilibrium mixture of odd-J/even-J species is considered. A reentrant phase diagram separating states of disorder and long-range order is known to exist in all-J species in both two and three dimensions. The phase diagram we find for the thermal mixture of odd-J/even-J species exhibits reentrance over a wider range of coupling constants than the corresponding all-J species. We also investigate systems where the odd-J fraction is fixed as a function of temperature. We find that even 1% odd-J mixture exhibits a phase diagram different from the pure even-J case, indicating that the even-J molecules play important role in orientational ordering.

Ab initio simulations play an increasingly important role in the studies of deep planetary interiors. Here we review the current state of this field, concentrating on studies of the materials of the Earth’s deep interior (MgO—SiO2—FeO—Al2O3, Fe—Si—S—O) and of the interiors of giant planets (H—He system, H2O—CH4—NH3 system). In particular, novel phases and phase diagrams, insights into structural and electronic phase transitions, melting curves, thermoelasticity and the effects of impurities on physical properties of planet-forming materials are discussed.

We present a classical interatomic force field for liquid SiO2 which has been parametrized using the forces, stresses and energies extracted from ab initio calculations. We show how inclusion of more electronic effects in a phenomenological way and parametrization at the relevant conditions of pressure and temperature allow the creation of more accurate force fields. We compare the results of simulations with this force field both to experiment and to the results of ab initio molecular dynamics simulations and show how our procedure leads to comparisons which are greatly improved with respect to the most widely used force fields for silica.

We show, by electronic structure based molecular dynamics simulations, that an extra electron injected in crystalline polyethylene should fall spontaneously into a self-trapped state, a shallow donor with a large novel distortion pattern involving a pair of trans-gauche defects. Parallel calculations show instead that a hole will remain free and delocalized. We trace the difference of behavior to the intrachain nature of the hole, as opposed to the interchain one of the electron, and argue that applicability of this concept could be more general. Thus electrons (but not holes) should tend to self-trap in saturated organic insulators, but not for example in aromatic insulators, where both carriers are intrachain. © 2002 Elsevier Science B.V. All rights reserved.

Oxygen, one of the most common and important elements in nature, has an exceedingly well explored phase diagram under pressure, up and beyond 100 GPa. At low temperatures, the low pressures antiferromagnetic phases below 8 GPa where O$_2$ molecules have spin S=1 are followed by the broad apparently nonmagnetic $\epsilon$ phase from about 8 to 96 GPa. In this phase which is our focus molecules group structurally together to form quartets while switching, as believed by most, to spin S=0. Here we present theoretical results strongly connecting with existing vibrational and optical evidence, showing that this is true only above 20 GPa, whereas the S=1 molecular state survives up to at about 20 GPa. The $\epsilon$ phase thus breaks up into two: a spinless $\epsilon_0$ (20-96 GPa), and another $\epsilon_1$ (8-20 GPa) where the molecules have S=1 but possess only short range antiferromagnetic correlations. A local spin liquid-like singlet ground state akin to some earlier proposals and whose optical signature we identify in existing data, is proposed for this phase. Our proposed phase diagram thus has a first order phase transition just above 20 GPa, extending at finite temperature and most likely terminating into a crossover with a critical point near 30 GPa and 200 K.

We describe a procedure to take into account the spatial dispersion of the optical excitations in the susceptibility sum rules. We show that this implies that relativistic corrections of the same order must be considered. The final result is a decrease of the total oscillator strength equal to the ratio of the average electron kinetic energy with mc2. We propose experiments with synchrotron radiation sources on crystals of heavy elements to observe the described effect.

We discuss our recent studies on the low-temperature instabilities of the 3 × 3 phase of tetravalent adatoms on (1 1 1) semiconductor surfaces. The cases of interest include the surface charge density wave (CDW) systems Pb/Ge(1 1 1) and Sn/Ge(1 1 1), as well as the Mott insulators Si/SiC(0 0 0 1) and K/Si(1 1 1):B. We have approached the problem in two ways: first, by employing a one-band model Hamiltonian of the Hubbard–Holstein type, in order to understand general features of the phase diagram as a function of the strength of electron–electron (e–e) and electron–phonon (e–ph) interactions; second, by performing realistic ab initio calculations within the local spin density approximation (LSDA) for the case of Sn/Si(1 1 1), and of a hypothetical 3 × 3 Si/Si(1 1 1) mimicking K/Si(1 1 1):B. The collinear LSDA calculation for both Sn/Ge(1 1 1) and Si/Si(1 1 1) predicts a spin density wave (SDW) state with a uniform magnetization mz=1/3 and a small secondary CDW. We discuss and stress the likely important role played by e–e interactions in explaining the phenomenology of all these systems, as opposed to the secondary role played by the e–ph coupling, which would at most drive the lattice, for Pb–Sn/Ge(1 1 1), after the electrons have caused the transition.

A school on computational materials science that drew expert teachers and talented participants marks a new approach to the development of research in Africa.

The structural, dynamical, and thermal properties of crystalline polyethylene are addressed with first-principles methods based on the density functional theory. The relatively low accuracy of the local density approximation for molecular crystals is corrected with the generalized gradient approximation, supplemented with empirical van der Waals' corrections, so as to optimize the description of the static (structural and elastic) properties of crystalline polyethylene. Based on this description, we perform first-principles finite temperature molecular dynamics simulations of warm, solid polyethylene. We analyze in particular thermal disorder effects, revealing the spontaneous appearance of trans-gauche defects close to the melting temperature (430 K).

WHEN the physicist Marvin Ross announced in 1981 that a giant mine of diamonds might lie buried deep in the interiors of Neptune and Uranus, his paper on the subject – 'Diamonds in the sky' – came as a complete shock to the planetary-science community. But it was a real shock – a shock wave, in fact – that led him to such a bizarre conclusion. At the time, shock-wave studies were the only way that experimentalists could 'see' how matter behaves at the extremes of pressure and temperature that exist in the totally inaccessible interiors of every planet of our solar system – including the Earth.

Total-energy pseudopotential calculations are performed on several competing reconstructions for the Sn-covered and clean Ge(111) surfaces. Our results confirm that the Sn-Ge(111) $\ensuremath{\alpha}$ phase $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{})$ with 1/3 ML Sn coverage, prevails over the adatom--rest-atom $2\ifmmode\times\else\texttimes\fi{}2$ phase (with 1/4 ML Sn coverage) for all allowed values of the Sn chemical potential. This is to be contrasted with the $2\ifmmode\times\else\texttimes\fi{}2$-based reconstructions which are known to prevail on the clean Ge(111) and Si(111) surfaces. Our results suggest that the stabilization of the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ phase for Sn-Ge(111) is caused by the larger size of the adatom, which lowers the energy of the half-filled surface state band, and also increases hybridization with the underlying second-third layer bonding state. The same argument may explain why $\ensuremath{\alpha}$ phases are observed also in Pb-Ge(111), Pb-Si(111), and Sn-Si(111).

We show that two distinct $3 \times 3$ ground states, one nonmagnetic, metallic, and distorted, the other magnetic, semimetallic (or insulating) and undistorted, compete in $\alpha$-phase adsorbates on semiconductor (111) surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with experiment, that the distorted metallic ground state prevails. The reason for stability of this state is analysed, and is traced to a sort of bond density wave, specifically a modulation of the antibonding state filling between the adatom and a Ge-Ge bond directly underneath.