Most of the oxidized carbon in the Earth's lower mantle is believed to be stored in the high-pressure forms of MgCO3 and/or CaCO3 or possibly even CO2. Recently, through ab initio evolutionary ...simulations and high-pressure experiments, a complete picture of phase transformations of CaCO3 at mantle pressures was obtained. Here, using the same approach, we investigate the high-pressure structures of MgCO3. Two new structure types were predicted to be stable in the relevant pressure range: one at 82–138 GPa and the other above 138 GPa. Both phases contain rings of corner-sharing CO4-tetrahedra. These predictions were largely confirmed by the experiments presented here. A number of structurally very different, but energetically competitive metastable polymorphs were found and reveal complex high-pressure chemistry of MgCO3, in contrast to CaCO3. For CO2, from 19 GPa to at least 150 GPa, we find β-cristobalite structure to be stable. Differences between high-pressure tetrahedral carbonates and low-pressure silicates are discussed in terms of rigidity of the T–O–T angles (flexible when T=Si and stiff when T=C). We show that through most of the P–T conditions of the mantle, MgCO3 is the major host of oxidized carbon in the Earth. We discuss the possibility of CO2 release at the very bottom of the mantle, which could enhance partial melting of rocks and explain the geodynamical differences between the Earth and Venus.
An effective fitting scheme within the thermo-viscoelastic model is proposed for estimation of dynamic eigenmodes of an ionic melt from ab initio molecular dynamics (AIMD) simulations. Dynamic ...eigenmodes are solutions of the generalized Langevin equation in matrix form, obtained on the basis set of eight dynamic variables. Fitting parameters are used only in the generalized hydrodynamic matrix T(k) for matrix elements involving heat density and heat current correlations only, because of huge computational efforts needed to calculate them directly in ab initio simulations. In the proposed scheme six AIMD-derived time correlation functions, three partial density-density and three partial current-current ones, are recovered by the proposed theoretical approach, which satisfies the exact sum rules up to the fourth frequency moments of partial dynamic structure factors. The suggested theoretical analysis was applied to estimation of propagating hydrodynamic and non-hydrodynamic eigenmodes in molten NaCl.
•a new ab initio methodology of analysis of collective dynamics in binary liquids and ionic liquids.•calculations by a combination of ab initio simulations and GCM theory of collective excitations in liquids.•dispersions of acoustic and optic eigenmodes in molten NaCl obtained by the new methodology.•molten NaCl is known for experimental observation of non-hydrodynamic optic modes.
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We study microstructural evolution of Silver (Ag) single-crystal nanocubes during high-velocity impacts, their dynamic recrystallization, and post-impact lattice structure using a ...combination of molecular dynamics and ab-initio simulations. Our study shows that, upon the impact, some preferential orientations can develop intricate, architected microstructures with grains of different sizes. These selected orientations correspond to the cases where at least eight or more slip systems are simultaneously activated, leading to an avalanche of dislocations. These dislocations interact and have the ability to produce severe plastic work, stimulating recrystallization in the nanocubes. On the other hand, dynamic recrystallization is not observed for the orientations with asynchronously activated slip systems besides large shock-wave pressures, plastic deformation, and large dislocation densities. Using thermalized ab-initio simulations, we find that the severe plastic deformation can trigger phase transformation of the initial face-centered cubic lattice structure to the 4H hexagonal closed-packed phase, which is thermodynamically more stable than the 2H hexagonal closed-packed phase. These results are in good agreement with experimental works. Our systematic numerical experiments shed light on the factors that promote dynamic recrystallization and provide a pathway to control the microstructure and atomic structure simultaneously by orienting nanocubes during the impact.
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Liquid metal surfaces have gained increased interest over the last decade due to new applications in synthesis of 2D materials, catalysis, or fusion reactors. Static properties such ...as the reflectivity and density profile have been determined, both experimentally and computationally, for numerous liquid metals and alloys. However, the characterization of the dynamic properties has remained a challenging task and only one experimental study by Reichert et al. has evaluated the depth-dependence of different dynamic properties in the liquid indium (l-In) surface. In this paper, we present an ab inito molecular dynamics study of the collective dynamic properties of this same system at different depths, obtaining very good agreement with the experimental data. In addition, we are able to compute the properties much closer to the surface than experimentally attainable, and have discovered that at these shallower depths, the properties drastically differ from those deeper in the slab. Therefore, this study sheds light into the behavior of dynamic properties at the atomic interface and highlights the ability of ab initio molecular dynamics to study such unknown dynamic behavior of liquid metals surfaces at depths not yet attainable experimentally but of crucial importance for liquid surface physics.
Magnetic energy conversion systems based on magnetocaloric effect promise to be an efficient and eco-friendly alternative to widespread gas cooling systems. Progress in this field initiates an active ...search for high-performance magnetic refrigerants with suitable characteristics. Among various caloric materials, rare-earth multi-principal element alloys are of particular interest to achieve these purposes. Since structure and properties in these materials strongly depend on synthesis conditions and fabrication prehistory, these issues need to be considered for each multi-element systems. This study is aimed to address structure formation, magnetic and magnetocaloric properties in ScGdTbDyHo high-entropy alloy fabricated under different conditions. We analyze the liquid and solid phases in this system using the ab initio molecular dynamics simulations and find that the alloy has a strong tendency to form a single-phase solid solution. Within the experimental study, as-cast, rapidly quenched, thermally annealed and severely cold-deformed alloy samples have been examined. We have found that all the fabricated specimens are single-phase hexagonal close-packed solid solutions characterized by different density of structural defects. The magnetic measurements reveal a complex magnetic structure in these materials. The Neel point in the studied alloys varies in the range of 139–144 K. A field-induced metamagnetic transition from antiferromagnetic to ferromagnetic state is observed in the alloy samples at all temperatures below the Neel point. In the magnetically ordered state, all the materials demonstrate large coercive force, reaching 4 kOe. Despite of the magnetic hysteresis, the alloys demonstrate the largest values of relative cooling power (920–984 J/kg at 5 T magnetic field) among similar rare-earth high-entropy systems reported so far. Analysis of the experimental results obtained for the alloy samples fabricated under different synthesis conditions allows us to conclude that there is a correlation between structural defectiveness and refrigerant capacity in this rare-earth system.
•The liquid and solid phases in the system are analyzed with the ab initio molecular dynamics simulations.•The fabricated materials are single-phase solid solutions with HCP crystalline structure.•The alloys exhibit a complex magnetic structure and a pronounced field-induced metamagnetic transition.•The relative cooling power in the alloys reaches values of 920–984 J/kg at 5 T magnetic field.•A correlation between structural defectiveness and the refrigerant capacity is revealed.
Detailed understanding of charge diffusion processes in a lithium-ion battery is crucial to enable its systematic improvement. Experimental investigation of diffusion at the interface between active ...particles and the electrolyte is challenging but warrants investigation as it can introduce resistances that, for example, limit the charge and discharge rates. Here, we show an approach to study diffusion at interfaces using muon spin spectroscopy. By performing measurements on LiFePO4 platelets with different sizes, we determine how diffusion through the LiFePO4 (010) interface differs from that in the center of the particle (i.e., bulk diffusion). We perform ab initio calculations to aid the understanding of the results and show the relevance of our interfacial diffusion measurement to electrochemical performance through cyclic voltammetry measurements. These results indicate that surface engineering can be used to improve the performance of lithium-ion batteries.
An approximate approach to quantum vibrational dynamics, “Brownian chain molecular dynamics (BCMD),” is proposed to alleviate the chain resonance and curvature problems in the imaginary time‐based ...path integral (PI) simulation. Here the non‐centroid velocity is randomized at each step when solving the equation of motion of path integral molecular dynamics. This leads to a combination of the Newton equation and the overdamped Langevin equation for the centroid and non‐centroid variables, respectively. BCMD shares the basic properties of other PI approaches such as centroid and ring polymer molecular dynamics: It gives the correct Kubo‐transformed correlation function at short times, conserves the time symmetry, has the correct high‐temperature/classical limits, gives exactly the position and velocity autocorrelations of harmonic oscillator systems, and does not have the zero‐point leakage problem. Numerical tests were done on simple molecular models and liquid water. On‐the‐fly ab initio BCMD simulations were performed for the protonated water cluster, H5O2+, and its isotopologue, D5O2+.
An approximate approach to quantum vibrational dynamics, “Brownian chain molecular dynamics,” is proposed to alleviate the chain resonance and curvature problems in the imaginary time‐based path integral simulation. Here the non‐centroid velocity is randomized at each step when solving the equation of motion of path integral molecular dynamics. This leads to a combination of the Newton equation and the overdamped Langevin equation for the centroid and non‐centroid variables, respectively.
GaP and related III-V semiconductors have attracted interest as high-efficiency photoelectrochemical electrodes for solar water splitting. However, their efficacy is linked to the presence, identity, ...and integrity of native surface oxides, which are structurally and chemically complex and evolve during operation. Using ambient pressure X-ray photoelectron spectroscopy (APXPS) coupled with ab initio simulations, we track key chemical motifs expressed during evolution of GaP(111) surface oxides, their associated reaction kinetics, and their correlations with electronic properties. We identify two distinct thermal regimes corresponding to kinetically and thermodynamically controlled oxidation. Below 600 K, exposure to O2 generates kinetically facile Ga-O-Ga configurations, whereas higher temperatures cause activated oxygen to insert into Ga–P bonds as part of a thermodynamically driven transformation into a complex, heterogeneous 3D network of surface POx (1≤x ≤ 4) groups and Ga2O3 species, the latter of which eventually dominates upon depletion of surface phosphorus. Our study highlights the critical competition between kinetic and thermodynamic factors during GaP oxidation, yielding insights for fabricating stable III-P-based photoelectrodes with precisely engineered surface properties.
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•Key atomic motifs associated with GaP(111) surface oxidation are identified.•Formation of oxide species correlates with evolution of electronic properties.•Kinetic analysis of GaP(111) oxidation agrees with DFT calculations.•The competition between kinetic and thermodynamic factors during GaP (111) oxidation is revealed.
GaP and related III-V semiconductors have attracted interest as high-efficiency photoelectrochemical electrodes for solar water splitting. However, their efficacy is linked to the presence, identity, ...and integrity of native surface oxides, which are structurally and chemically complex and evolve during operation. Using ambient pressure X-ray photoelectron spectroscopy (APXPS) coupled with ab initio simulations, we track key chemical motifs expressed during evolution of GaP(111) surface oxides, their associated reaction kinetics, and their correlations with electronic properties. We identify two distinct thermal regimes corresponding to kinetically and thermodynamically controlled oxidation. Below 600 K, exposure to O2 generates kinetically facile Ga-O-Ga configurations, whereas higher temperatures cause activated oxygen to insert into Ga–P bonds as part of a thermodynamically driven transformation into a complex, heterogeneous 3D network of surface POx (1≤x ≤ 4) groups and Ga2O3 species, the latter of which eventually dominates upon depletion of surface phosphorus. Here our study highlights the critical competition between kinetic and thermodynamic factors during GaP oxidation, yielding insights for fabricating stable III-P-based photoelectrodes with precisely engineered surface properties.
Blatter radical derivatives are very attractive due to their potential applications, ranging from batteries to quantum technologies. In this work, we focus on the latest insights regarding the ...fundamental mechanisms of radical thin film (long-term) degradation, by comparing two Blatter radical derivatives. We find that the interaction with different contaminants (such as atomic H, Ar, N, and O and molecular H2, N2, O2, H2O, and NH2) affects the chemical and magnetic properties of the thin films upon air exposure. Also, the radical-specific site, where the contaminant interaction takes place, plays a role. Atomic H and NH2 are detrimental to the magnetic properties of Blatter radicals, while the presence of molecular water influences more specifically the magnetic properties of the diradical thin films, and it is believed to be the major cause of the shorter diradical thin film lifetime in air.
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