Ab Initio Thermodynamics Research Articles

Insights on the stability of cuprous chloride under high pressure: An equilibrium ab initio atomistic thermodynamics study

Density functional theory, in conjunction with ab initio atomistic thermodynamics, were used to investigate the stability of the low index cuprous chloride surfaces, and predict the morphology of CuCl nanocrystals under high pressures of chlorine. The rocksalt orientation of the lattice was adopted to resemble a CuCl crystal under high pressure. Under Cl-lean conditions, the defective CuCl(100)+Cl structure was found to be the most stable configuration among all surfaces investigated. Under Cl-rich conditions, a flawless, CuCl(111) surface with Cl-termination becomes the most stable configuration. It was shown that in a diluted chlorine environment, the CuCl in zinc blende structures are more stable than the CuCl in rocksalt configurations, while under a chlorine rich environment, the CuCl in rocksalt structures are more stable than the CuCl in zinc blende configurations. At low pressure of chlorine, it was found that the (100) facets are the most dominant to the Wulff construction, giving the crystal a coffin shape. As the pressure of chlorine increases, however, the involvement of the (111) facet increases giving the crystal a semi dodeca-hedron shape. At high pressure of chlorine, the major contributors to the Wulff construction were found to be the (111) facets, resulting in the formation of dodeca-hedron nanocrystals.

Ab Initio Thermodynamics of Hydrocarbons Relevant to Graphene Growth at Solid and Liquid Cu Surfaces

Using ab initio thermodynamics, the stability of a wide range of hydrocarbon adsorbates under various chemical vapor deposition (CVD) conditions (temperature, methane and hydrogen pressures) used in experimental graphene growth protocols at solid and liquid Cu surfaces has been explored. At the employed high growth temperatures around the melting point of Cu, we find that commonly used thermodynamic models such as the harmonic oscillator model may no longer be accurate. Instead, we account for the translational and rotational mobility of adsorbates using a recently developed hindered translator and rotator model or a two-dimensional ideal gas model. The thermodynamic considerations turn out to be crucial for explaining experimental results and allow us to improve and extend the findings of earlier theoretical studies regarding the role of hydrogen and hydrocarbon species in CVD. In particular, we find that smaller hydrocarbons will completely dehydrogenate under most CVD conditions. For larger clusters our results show that metal-terminated and hydrogen-terminated edges have very similar stabilities. While both cluster types might thus form during the experiment, we show that the low binding strength of clusters with hydrogen-terminated edges could result in instability towards desorption.

Evolution of the free energy of the GaN(0001) surface based on first-principles phonon calculations

In this paper, we present systematic improvements to a method that allows us to determine the surface phase diagrams based on ab initio calculations and atomistic thermodynamics. In particular, we include a contribution derived from thermal vibrations of the surface, which is usually omitted in such analysis. On the basis of first-principles phonon calculations, we determine a zero-point vibrational energy and thermal dependencies of vibrational entropy and free energy for atoms and molecules adsorbed on the surface. A series of polar GaN(0001) surfaces are examined. Significant changes are observed in the surface phase diagrams compared to the diagrams obtained by standard density-functional theory calculations without phononic contributions.

Stability of non-metal dopants to tune the photo-absorption of TiO2 at realistic temperatures and oxygen partial pressures: A hybrid DFT study

TiO2 anatase is considered to play a significant importance in energy and environmental research. However, for developing artificial photosynthesis with TiO2, the major drawback is its large bandgap of 3.2 eV. Several non-metals have been used experimentally for extending the TiO2 photo-absorption to the visible region of the spectrum. It’s therefore of paramount importance to provide theoretical guidance to experiment about the kind of defects that are thermodynamically stable at a realistic condition (e.g. Temperature (T), oxygen partial pressure ({{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}}), doping). However, disentangling the relative stability of different types of defects (viz. substitution, interstitial, etc.) as a function of charge state and realistic T, {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} is quite challenging. We report here using state-of-the-art first-principles based methodologies, the stability and meta-stability of different non-metal dopants X (X = N, C, S, Se) at various charge states and realistic conditions. The ground state electronic structure is very accurately calculated via density functional theory with hybrid functionals, whereas the finite T and {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} effects are captured by ab initio atomistic thermodynamics under harmonic approximations. On comparing the defect formation energies at a given T and {{boldsymbol{p}}}_{{{bf{O}}}_{{bf{2}}}} (relevant to the experiment), we have found that Se interstitial defect (with two hole trapped) is energetically most favored in the p-type region, whereas N substitution (with one electron trapped) is the most abundant defect in the n-type region to provide visible region photo-absorption in TiO2. Our finding validates that the most stable defects in X doped TiO2 are not the neutral defects but the charged defects. The extra stability of {({bf{S}}{bf{e}}{bf{O}})}_{{bf{O}}}^{+{bf{2}}} is carefully analyzed by comparing the individual effect of bond-making/breaking and the charge carrier trapping energies.

The stability and oxidation of supported atomic-size Cu catalysts in reactive environments

Atomic-scale species (atoms and clusters) have attracted much attention as potential highly active catalysts. Synthesizing such catalysts that may be stable under synthesis or reaction conditions is a challenge. In this work, we used density functional theory to model the growth of Cu clusters on the TiO2 support, including sintering and oxidation. Oxidation of supported Cu was preferred over sintering due to metal-support interactions. Ab initio thermodynamics calculations showed that O2 readily oxidized most clusters, while H2O was a mild oxidant. CO2 did not oxidize any clusters at relevant temperatures. Thermodynamics would suggest that oxidation of Cu atoms/clusters would readily occur, but kinetic analysis suggested otherwise. O2 adsorption was weak over the TiO2 surface, as well as most oxidized clusters. O2 dissociation barriers were low over nonoxidized Cu clusters, but quite high (1.88 eV) over single Cu atoms. Our results suggest that lone Cu atoms are stabilized on the surface, due to a high diffusion barrier (necessary for sintering) and a high O2 dissociation barrier (necessary for oxidation). We performed experiments that indeed support the premise that lone Cu atoms occur on the surface. Cu species were deposited on TiO2, and any Cu2+ species (indicative of oxidized Cu clusters) were removed after thermal treatment in various environments. Only Cu0 and Cu1+ species existed after thermal treatment. Lone adsorbed Cu atoms had a +1 oxidation state. Combined, our calculations and experiments indicate that Cu1+ species (lone adsorbed Cu atoms) are dominant. The kinetics of oxidation/diffusion rather than thermodynamics limits the growth/oxidation of Cu. In summary, we show that metal-support interactions are key for synthesizing stable atomic-scale catalysts, since they can strongly influence key processes such as diffusion/oxidation.

Thermodynamics of Fatigue: Degradation-Entropy Generation Methodology for System and Process Characterization and Failure Analysis.

Formulated is a new instantaneous fatigue model and predictor based on ab initio irreversible thermodynamics. The method combines the first and second laws of thermodynamics with the Helmholtz free energy, then applies the result to the degradation-entropy generation theorem to relate a desired fatigue measure—stress, strain, cycles or time to failure—to the loads, materials and environmental conditions (including temperature and heat) via the irreversible entropies generated by the dissipative processes that degrade the fatigued material. The formulations are then verified with fatigue data from the literature, for a steel shaft under bending and torsion. A near 100% agreement between the fatigue model and measurements is achieved. The model also introduces new material and design parameters to characterize fatigue.

Thermodynamic stability and electronic structure of bimetallic clusters (TMxMgyOz)

We study the (T,pO2) dependence of the composition, structure, and stability of the various isomers for different sizes of clusters NixMgyOz (x + y ≤ 3) in an oxygen atmosphere. This is achieved by evaluating the free energy of the clusters (via ab initio atomistic thermodynamics) with Density Functional Theory input. Structures of all the clusters are identified via cascade genetic algorithm. Our results show advanced hybrid functional is indispensable to address both the structural and thermodynamic stability of the system. We identify that the presence of Mg-coordinated-O is playing a pivotal role to reduce the fundamental gap in the O-rich clusters.

Ab initio thermodynamics of carbon segregation on dislocation cores in bcc iron

The equilibrium segregation of carbon atoms in the core of screw dislocations in body-centred cubic Fe is modelled using a generalized Ising model parametrized on density functional theory calculations and solved using both mean-field calculations and Monte Carlo simulations. Recently, a strong carbon-dislocation attraction was evidenced, resulting in a spontaneous reconstruction of the dislocation core towards a hard core configuration, where the carbon atom is located at the centre of a regular trigonal prism called a prismatic site. Here we show that the fourth neighbour octahedral sites of the reconstructed core are also attractive for carbon with a binding energy similar to that of the prismatic core site reported previously. This suggests that the dislocation may be decorated by lines of carbon atoms on both types of sites. Moreover, all carbon–carbon interactions including intra-line and inter-line interactions, are found repulsive. Segregation therefore results from a competition between the dislocation-carbon attraction and the carbon–carbon repulsion, leading to complex ordering phenomena that are analysed here in detail. Notably, we evidence a high temperature regime, in the regime of dynamical strain ageing of steels, where the prismatic line is half-occupied on average with every other prismatic site occupied by a carbon atom while the octahedral lines are empty. The iso-concentrations for the prismatic and octahedral lines obtained with the mean-field approach and with Monte Carlo simulations are qualitatively similar.

SAMPLE: Surface structure search enabled by coarse graining and statistical learning

In this publication we introduce SAMPLE, a structure search approach for commensurate organic monolayers on inorganic substrates. Such monolayers often show rich polymorphism with diverse molecular arrangements in differently shaped unit cells. Determining the different commensurate polymorphs from first principles poses a major challenge due to the large number of possible molecular arrangements. To meet this challenge, SAMPLE employs coarse-grained modeling in combination with Bayesian linear regression to efficiently map the minima of the potential energy surface. In addition, it uses ab initio thermodynamics to generate phase diagrams. Using the example of naphthalene on Cu(111), we comprehensively explain the SAMPLE approach and demonstrate its capabilities by comparing the predicted with the experimentally observed polymorphs.

Activity‐Stability Volcano Plots for Material Optimization in Electrocatalysis

AbstractIn the last two decades, research in electrocatalysis has been spurred by theoretical calculations and predictions based on the concept of ab initio thermodynamics, which has become a valuable tool for computational researchers in material screening. These investigations most commonly result into the construction of activity‐based volcano plots in order to predict potential electrocatalysts for the application in practice. However, the prototypical activity‐based volcano concept neither captures the influence of the applied overpotential on the activity nor the stability of electrode surfaces. Herein, the well‐established volcano approach is expanded by constructing activity‐stability volcano plots, which, beside the activity, also enclose the stability and the applied overpotential into the analysis.

Unraveling Thermodynamic Stability, Catalytic Activity, and Electronic Structure of [TMxMgyOz]+/0/– Clusters at Realistic Conditions: A Hybrid DFT and ab Initio Thermodynamics Study

Aiming toward catalytic applications, a large data set is generated on [TMxMgyOz]+/0/– clusters (TM = Cr, Fe, Co, Ni, x + y ≤ 5) using a massively parallel cascade genetic algorithm (cGA) approach at the hybrid density functional level of theory. The low-energy isomers are further analyzed via ab initio atomistic thermodynamics to estimate their free energy of formation at a realistic temperature T and partial pressure of oxygen pO2. A thermodynamic phase diagram is drawn by minimizing Gibbs free energy of formation to identify the stable phases of neutral and charged [TMxMgyOz]+/0/– clusters. From this analysis, we notice that neutral and negatively charged clusters are stable in the wide range of (T, pO2). The negatively charged clusters are more effective as a catalyst to lower the C–H bond activation barrier for oxidation of methane. We find that the nature of TM atoms toward controlling the activation barrier is less important. However, the TM gives rise to different structural motifs in the cluster, which may act as active centers for catalysis.

Growth of ultrathin Ru oxide films on perovskite and corundum substrates

The ability to control the dispersion of RuO2 monolayers on a range of perovskite- and corundum-structured oxide substrates was investigated using complementary experiment and theory. Controlled deposition under UHV conditions and subsequent in situ characterization by x-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED) was used to follow the chemical state, dispersion and structure of the RuO2 layers as a function of growth and post-growth processing conditions. The experimental results reveal that monolayer RuO2 on ferroelectric PbxZr1-xTiO3(001) and non-polar SrTiO3(001) α-Al2O3(11¯02) tends to dewet the surface to form three-dimensional clusters, though some kinetic stability of dispersed RuO2 monolayers was achieved on the SrTiO3 surface. Although an RuO2 termination is not favored for SrRuO3(001), deposition onto SrRuO3(001) yields a stable RuO2 surface which ISS suggests is capped by chemisorbed oxygen. The complementary density functional theory modeling and ab initio thermodynamics indicate that Ru oxide monolayers on the perovskite substrates tend to be unstable under the low oxygen pressure experimental conditions; however, either lower temperatures or higher oxygen pressures can lead to a kinetic and even thermodynamic stability. The theoretical findings also provide guidance on how the instability of dispersed RuO2 monolayers can be overcome by inserting buffer layers and increasing the partial pressure of molecules that selectively, strongly adsorb at coordinatively unsaturated Ru sites on the oxide surface.

Recent Advancements Towards Closing the Gap between Electrocatalysis and Battery Science Communities: The Computational Lithium Electrode and Activity-Stability Volcano Plots.

Despite of the fact that the underlying processes are of electrochemical nature, electrocatalysis and battery research are commonly perceived as two disjointed research fields. Herein, recent advancements towards closing this apparent community gap by discussing the concepts of the constrained ab initio thermodynamics approach and the volcano relationship, which were originally introduced for studying heterogeneously catalyzed reactions by first-principles methods at the beginning of the 21st century, are summarized. The translation of the computational hydrogen electrode (CHE) approach or activity-based volcano plots to a computational lithium electrode (CLiE) or activity-stability volcano plots, respectively, for the investigation of electrode surfaces in batteries may refine theoretical modeling with the aim that enhancements of the underlying concepts are transferred between the research communities. The presented strategy of developing novel approaches by interdisciplinary research activities may trigger further progress of improved theoretical concepts in the near future.

Thermodynamic Approach to Develop Lead-Free Halide Perovskites: Theoretical Study

Metal halide perovskites (MHPs) are one of the promising candidates for next-generation photovoltaic materials. Since the Miyasa group first adopt methylammonium lead tri-iodides as a sensitizer of dye-sensitized solar cells, the efficiency of MHP solar cells has improved at an unprecedented speed. According to the recent report of national renewable energy laboratory, the certified power conversion efficiency (PCE) of MHP solar cells exceeded 23%. However, stability and toxicity are primary concerns for MHP solar cell commercialization. Among various efforts to develop Pb-free perovskite solar cells, Sn-based materials have shown the PCE. However, the stability of Sn-based perovskites is worse than Pb-based solar cells, and the multivalent features of Sn are considered to be the origin of the unstable performance. Compositional engineering and additive control can be the effective ways to develop highly stable and efficient Pb-free perovskite solar cells. This study presents the effects of compositional engineering and various additives on the stabilization of Pb-free MHP materials via ab initio thermodynamics calculations. The concept of the ‘anti-stabilizer’ for the proper selection of the preferred phase is suggested by our previous report[1]. We present that the anti-stabilizing concept can be successfully applied to stabilize the proper band gap polymorph of black CsSnI3[2]and Bi-based layered perovskites. Additionally, the roles of additives including small alkali metal interstitials are discussed concerning thermodynamics. [1] J. Jeon, T. Eom, E. Lee, S. Kim, S. Kim, K.-H. Hong, H. Kim, J. Phys. Chem. C 121 (2017) 9508–9515. [2] J.H. Heo, J. Kim, H. Kim, S.H. Moon, S.H. Im, K.-H. Hong, J. Phys. Chem. Lett. 9 (2018) 6024–6031.

Ab Initio Thermodynamics Insight into the Structural Evolution of Working IrO2 Catalysts in Proton-Exchange Membrane Electrolyzers

At the cell voltages required to reach technologically viable current densities in proton-exchange membrane (PEM) electrolyzers, IrO2 catalysts are suspected to undergo a transformation to an amorphous hydrous form. Here, we present a systematic ab initio thermodynamics study analyzing the shape and stability of IrO2 nanoparticles in this potential range. Our results confirm a thermodynamic instability of the rutile crystal structure induced by the stabilization of highly oxidized O species at the surface already at onset potentials for the oxygen evolution reaction (OER). Intriguingly, this is preceded by a transformation of the equilibrium shape at even lower potentials. Instead of the well-studied IrO2(110) facets, this shape is dominated by IrO2(111) facets that have hitherto barely received attention. Our findings highlight the need to extend detailed characterization studies to this high-potential range, not the least to establish more suitable active-site models for the OER that may then serve as t...

Does Twinning Impact Structure/Property Relationships in Diamond Nanoparticles?

Although the majority of computational predictions of the properties of diamond nanoparticles (nanodiamonds) are based on sets of exclusively single-crystal structures, most experimental observations contain twins. The influence of twins is difficult to characterize computationally since they are metastable and their relative stability will depend on the thermochemical conditions. The properties of entire samples of nanodiamonds will also depend on how many twins there actually are. In this study, we have used a combination of electronic structure simulations, ab initio thermodynamics, and a simple statistical method called ensemble filtering to investigate the probability of twinning in nanodiamond and their impact on charge transfer properties such as the ionization potential, the electron affinity, and the electronic band gap. We find that, provided some degree of control can be exercised over the surfaces, increasing the number of twinned particles in samples could shift the selectivity of the electron affinity and the band gap and could greatly improve the quality of samples by decreasing the bandwidth, improving the sensitivity and specificity.

Pyiron: An integrated development environment for computational materials science

To support and accelerate the development of simulation protocols in atomistic modelling, we introduce an integrated development environment (IDE) for computational materials science called pyiron (http://pyiron.org). The pyiron IDE combines a web based source code editor, a job management system for build automation, and a hierarchical data management solution. The core components of the pyiron IDE are pyiron objects based on an abstract class, which links application structures such as atomistic structures, projects, jobs, simulation protocols and computing resources with persistent storage and an interactive user environment. The simulation protocols within the pyiron IDE are constructed using the Python programming language. To highlight key concepts of this tool as well as to demonstrate its ability to simplify the implementation and testing of simulation protocols we discuss two applications. In these examples we show how pyiron supports the whole life cycle of a typical simulation, seamlessly combines ab initio with empirical potential calculations, and how complex feedback loops can be implemented. While originally developed with focus on ab initio thermodynamics simulations, the concepts and implementation of pyiron are general thus allowing to employ it for a wide range of simulation topics.

Ab initio thermodynamics of complex alloys: The case of Al- and Mn-doped ferritic steels

In the context of Al and Mn-doped ferritic steels, we progressively elaborate an atomic-scale energy model to reproduce the thermodynamic behaviour of quaternary FeAlMnC on a bcc lattice. This model is built on physical concepts: DFT calculations, pair Hamiltonians, non-configurational thermal effects, these elements being combined in a reasoned way to lead to a mastered and predictive formulation. In particular, this approach allows to explore the correlation between ordering in substitutional ternary FeAlMn and interstitial carbon, which brings new elements to the metallurgy of carbon in ferritic steels.

Quantifying the Impact of Magnesium on the Stability and Water Binding Energy of Hydrated Calcium Carbonates by Ab Initio Thermodynamics.

Determining conditions that drive carbonate formation is important for many phenomena such as paleoindicators for mineral deposition, the carbon cycle, biomineralization, and industrial applications such as scale inhibition and manufacturing of cement and concrete. Magnesium and incorporated water have been observed to play critical roles in nonclassical crystallization pathways of calcium carbonate, the dominant carbonate found in nature, through promoting formation of low energy metastable intermediates such as monohydrocalcite (CaCO3·H2O), ikaite (CaCO3·6H2O), and amorphous calcium carbonate (CaCO3·H2O). The impact of Mg on the thermodynamics and water binding ability of these hydrated intermediates is challenging to measure and is not understood at the molecular level. In this work, density-functional theory and ab initio thermodynamics are used to quantify the impact of Mg on structure, thermodynamics, and water binding energies of the crystalline hydrated Ca carbonates as a function of temperature in aqueous and ultrahigh vacuum conditions, as well as CO2-rich environments relevant to carbon sequestration. For monohydrocalcite, Mg incorporation is found to destabilize the structure despite a dramatic increase in the water binding energy, thus confirming that Mg promotes monohydrocalcite formation kinetically rather than thermodynamically. For ikaite, however, Mg promotes its formation both kinetically and thermodynamically, expanding the stability region for ikaite in cold water.

Ab initiothermodynamics study of ambient gases reacting with amorphous carbon

Amorphous carbon (a-C) occurs as a tribologically induced phase in diamond or diamond-like carbon coatings. The interaction of ambient gases (${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, ${\mathrm{H}}_{2}\mathrm{O}$, ${\mathrm{CO}}_{2}$) with a-C of varying mass density is studied by means of ab initio thermodynamics simulations. Different scenarios such as moist air or pure gases are investigated under different pressure and temperature conditions. Equilibrium concentrations of chemisorbed and fragmented final states are found to only exhibit a minor dependence on the specific conditions of the gas phase reservoir. The differences in local structure of the a-C samples with varying mass density such as pore size and coordination and dangling bonds, as well as the competition among different gas molecules for a-C atoms as reactants, affect the equilibrium concentrations to a greater degree. The availability and reactivity of a-C atoms are thus found to mainly control the chemical composition of a-C interacting with ambient gases in thermodynamic equilibrium. Trends found in the analysis of chemical groups occurring in equilibrium can possibly be transferred to a more realistic nonequilibrium tribological scenario.