Region Of Thermodynamic Stability Research Articles

DFT calculations of the stability field and properties of a predicted lanthanum–scandium–aluminum garnet La[formula omitted]Sc[formula omitted]Al[formula omitted]O12 and P–T phase diagram of Y[formula omitted]Sc[formula omitted]Al[formula omitted]O12

In this study we present the results of our density functional theory calculations within the quasi-harmonic approximation, which predict formation and properties of a new compound of the garnet family, La3Sc2Al3O12 (LSAG). LSAG was shown to be formed from 3LaAlO3 + Sc2O3 at ambient pressure and room temperature. The thermodynamic stability region of LSAG is limited by a temperature of 685 K, above which its decomposition into LaAlO3 (LAP) and Sc2O3 occurs. With increasing pressure, LSAG retains its stability up to 0.23 GPa, where the decomposition reaction LSAG↔3LAP＋ Sc2O3 becomes thermodynamically favorable. In this work, not only the thermodynamic stability of LSAG was assessed, but also its thermal conductivity, elastic constants and band gap were calculated. At 300 K the thermal conductivity of LSAG is 13.204 Wm−1K−1. LSAG has the smallest elastic constants compared with YSAG and YAG, which indicates weaker atomic bonding in LSAG than in YSAG and YAG. The width of band gap of LSAG calculated using an hybrid functional is equal to 5.24 eV. In addition, the P–T phase diagram of the known Y3Sc2Al3O12 (YSAG) was calculated for the first time. Unlike LSAG, YSAG is stable at fairly high temperatures, reaching values of at least 1500 K. With increasing temperature to 1500 K, the pressure of the phase transition increases slightly to 3.55 GPa, and the monovariant curve of the reaction YSAG↔3YAP+Sc2O3 is represented by an almost straight line.

Configurational entropy of ice XIX and its isotope effect

Ice XIX is a partly hydrogen-ordered polymorph related to disordered ice VI, similar to ice XV. We here investigate the order–order–disorder sequence ice XIX→ice XV→ice VI based on calorimetry at ambient pressure both for D2O and H2O-ice XIX. From these data we extract configurational entropy differences between ice XIX, ice XV and ice VI. This task is complex because, unlike for all other ices, the order–disorder transition from ice XIX to ice VI takes place in two steps via ice XV. Even more challenging, these two steps take place in an overlapping manner, so that careful separation of slow kinetics is necessary. This is evidenced best by changing the heating rate in calorimetry experiments: For fast heating experiments the second step, disordering of ice XV, is suppressed because the first step, formation of ice XV from ice XIX, is too slow. The transient state ice VI‡ that is initially produced upon ice XIX decay then does not have enough time to convert to ice XV, but remains disordered all along. In order to tackle the challenge to determine the entropy difference between ice XIX and VI as well as the entropy difference between ice XV and VI we employ two different approaches that allow assessing the impact of kinetics on the entropy change. “Single peak integration” defines a kinetically limited result, but “combined peak integration” allows estimation of the true thermodynamic values. Our best estimate for the true value shows ice XIX to be much more ordered than ice XV (25 ± 3% vs 9 ± 4% of the Pauling entropy). For D2Oice XIX samples we obtain 28% of order, but only when a small number of fast H-isotope defects are used. In the second part we use these results to estimate the location of the ice XIX phase boundary both for protiated and deuterated ice XIX. The initial Clapeyron slope at ambient pressure is determined from the combination of neutron powder diffraction volume differences and calorimetry entropy differences data to be 21 K GPa−1 with an order–disorder transition temperature To-d(0.0 GPa) = 103 ± 1 K. An in situ bracketing experiment at 1.8 GPa yields To-d(1.8 GPa) = 116 ± 3 K, i.e., the phase boundary slope flattens at higher pressures. These data allow us to determine the region of thermodynamic stability of ice XIX in the phase diagram and to explain the surprising isotope shift reversal at 1.6 GPa compared to 0.0 GPa, i.e., why D2O-ice XIX disorders at lower temperatures than H2O-ice XIX at 1.6 GPa, but at higher temperatures at ambient pressures.

On the Kinetic Theory of Formation and Growth of Synthetic Diamond in the Region of Thermodynamic Stability in Diffusion and Diffusion-Less Approximation

On the Kinetic Theory of Formation and Growth of Synthetic Diamond in the Region of Thermodynamic Stability in Diffusion and Diffusion-Less Approximation

Structural, electronic, mechanical and thermodynamic properties of lithium-rich layered oxides cathode materials for lithium-ion battery: Computational study

Because of their important energy density, low lithium reduction potential and compact size, lithium-ion batteries (LIBs) are the often-considered the greatest batteries ever used. Cathode materials, especially lithium-rich layered oxides are among the most appropriate candidates actualizing the remarkable energy densities of LIBs, due to their capacitance values that exceed 250 mAh/g. This study investigates the structural, electronic, mechanical and thermodynamic properties of two candidate materials: Li1.2Ni0.1Co0.2Mn0.5O2 (A) and Li1.03Ni0.77Co0.1Mn0.1O2 (B), by means of DFT-based computational simulations. The obtained results were further validated by comparison with existing experimental data. Computations were undertaken based on the Perdew-Burke-Ernzerhof (PBE) function on the Generalized Gradient Approximation (GGA) of the electron density, in the DMol3 module of Materials Studio software (BIOVIA Inc.), to determine the partial (PDOS) and total density of state (TDOS), electron density, band gap, etc. The results reveal the energy gap of material A to be 0.005 eV, as against 0.015 eV for material B, showing that A is more conductive than B. Structural and electronic defects in material B restrict its thermodynamic stability region to just 273 K, unlike A, which is thermodynamically more stable in the temperature range 273–278 K even up to 323 K, thus allowing material A to resist degradation. Moreover, material A is less resistant to changes in volume and shape at 288 and 308 K compared to material B, therefore confirming its resistance to temperature. Thus, A has a more desirable performance in terms of structural stability, reactivity, thermal stability, mechanical stability and conductivity compared to B. So, from experimental and computational results, A is a good candidate for cathode material of LIBs.

Effect of Sn oxides on the thermal conductivity of polycrystalline SnSe

SnSe is a promising thermoelectric material, with intrinsically low lattice thermal conductivity, κL. Surprisingly, in several reports, polycrystalline samples are found to have a higher thermal conductivity than single crystals. This disparity has been attributed to trace amounts of thermally conductive Sn oxides at the grain boundaries of polycrystalline samples. The same culprit was recently proposed to explain the reduction of κL in purified, oxide-free, SnSe polycrystals. Here, we test this hypothesis by: (i) tuning the type of oxide in SnSe by exploiting thermodynamic stability regions, since Sn-rich or Sn-poor compositions favour the formation of SnO or SnO2, respectively; and (ii) varying the quantity of SnO2 by intentionally oxidizing SnSe powder before consolidation, to obtain samples with quantifiable amounts - up to 15% - of SnO2. We find that the κL of SnSe is impervious to changes in the type or the amount of Sn oxide present in the samples. Our results show that a simple “rule of mixtures” cannot be used to estimate the effect of grain boundary oxides on the thermal conductivity of SnSe. These results call for an improved understanding of the intriguing thermal transport mechanisms in SnSe and numerous other systems where a two-phase transport is presumed.

Unusual solid-state transformations in LuFe2O4 films during their synthesis via MOCVD with further reduction

Thin films of lutetium ferrite LuFe2O4, potential components of multiferroic devices, are difficult objects for synthetic solid-state chemistry. In this paper, we report on the successful synthesis of epitaxial LuFe2O4 thin films on YSZ(111) single-crystalline substrates via a two-stage technique. The newly proposed synthesis route combines a novel variant of metal-organic chemical vapor deposition (MOCVD) with subsequent annealing at low oxygen partial pressure provided by the metal/oxide (Fe/FeO) getter. The phase composition and structural transformations during synthesis are studied. It is shown that the orienting effect of the substrate manifests itself in the fine structure of as-deposited films: the LuFe2O4.5 phase forms as a layer (∼5 ​nm thick) adjacent to the substrate. On top of it the layer of hexagonal LuFeO3 (h-LuFeO3) forms. The crystalline layer is then replaced by an amorphous mixture of lutetium and iron oxides. High-temperature annealing at 900 ​°C and partial oxygen pressure (pO2) laying in the region of thermodynamic stability of LuFe2O4 leads to the formation of films that are well-crystallized over the entire thickness of the sample. At the same time, it was discovered that a thin epitaxial layer (∼5 ​nm) of Lu2O3 forms at the interface with the substrate. The proposed scenario of the appearance of this layer implies the partial evaporation of the iron oxide (FeO) from the film surface during annealing, which is accompanied by diffusion of iron and oxygen ions from the interface to the upper surface of the film. It was also established that the solid-state reaction between h-LuFeO3 and FeO is an essential stage in the formation of LuFe2O4. Transmission electron microscopy (TEM) images of the model samples reveal that wustite penetrates the interlayer space between lutetium and iron-containing layers, which leads to the formation of well-ordered (FeO)2(LuO2)1 structure.

Total free energy analysis of fully hydrated proteins.

The total free energy of a hydrated biomolecule and its corresponding decomposition of energy and entropy provides detailed information about regions of thermodynamic stability or instability. The free energies of four hydrated globular proteins with different net charges are calculated from a molecular dynamics simulation, with the energy coming from the system Hamiltonian and entropy using multiscale cell correlation. Water is found to be most stable around anionic residues, intermediate around cationic and polar residues, and least stable near hydrophobic residues, especially when more buried, with stability displaying moderate entropy-enthalpy compensation. Conversely, anionic residues in the proteins are energetically destabilized relative to singly solvated amino acids, while trends for other residues are less clear-cut. Almost all residues lose intraresidue entropy when in the protein, enthalpy changes are negative on average but may be positive or negative, and the resulting overall stability is moderate for some proteins and negligible for others. The free energy of water around single amino acids is found to closely match existing hydrophobicity scales. Regarding the effect of secondary structure, water is slightly more stable around loops, of intermediate stability around strands and turns, and least stable around helices. An interesting asymmetry observed is that cationic residues stabilize a residue when bonded to its N-terminal side but destabilize it when on the C-terminal side, with a weaker reversed trend for anionic residues.

Atomistic structure and anomalous heat capacity of low-density liquid carbon: Molecular dynamics study with machine learning potential

Liquid carbon remains the source of several unsolved questions related to its structure and region of thermodynamic stability. Experiments demonstrate a drastic decrease in the density for liquid carbon along the graphite melting line in the pressure range P = 1–3 GPa and the nature of this phenomenon is unclear. A recent experimental study [A.M. Kondratyev and A.D. Rakhel, PRL (2019)] revealed another peculiar and still unexplained feature of the liquid carbon – its excessive heat capacity. Using classical molecular dynamics with machine learning potential GAP-20, we study the structural properties of liquid carbon and demonstrate that at P < 1–2 GPa it resembles a net of sp-hybridized chains, rather than a typical covalent liquid, with nanoscale porosity of this phase being responsible for the density decrease. We also show that excessive heat capacity could be a direct manifestation of a smooth transition from a high-density sp2-hybridized phase into a low-density sp-hybridized.

Estimation of thermodynamic stability of isoperiodic epitaxial structures whith GaInSbAs and GaInAsP solid solutions

The work studied the thermodynamic stability of GaInSbAs, GaInAsP heterosystems on different substrates. The isotherms of spinodal decomposition caused by chemical changes in the internal energy of the alloy and by elastic stresses at the layer-substrate interface are obtained with the model of quasiregular solutions. It has been found that elastic stresses lead to an expansion of the region of thermodynamic stability of isoperiodic solid solutions for GaSb substrates and a decrease in the critical temperature. The developed model can be using to selection of the technological modes and parameters of epitaxial growth.

What Is the Real State of Single-Atom Catalysts under Electrochemical Conditions—From Adsorption to Surface Pourbaix Plots?

The interest in single-atom catalysts (SACs) is increasing, as these materials have the ultimate level of catalyst utilization, while novel reactions where SACs are used are constantly being discovered. However, to properly understand SACs and to further improve these materials, it is necessary to consider the nature of active sites under operating conditions. This is particularly important when SACs are used as electrocatalysts due to harsh experimental conditions, including extreme pH values or high anodic and cathodic potential. In this contribution, density functional theory-based thermodynamic modelling is used to address the nature of metal centers in SACs formed by embedding single metal atoms (Ru, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au) into graphene monovacancy. Our results suggest that none of these SAC metal centers are clean at any potential or pH in the water thermodynamic stability region. Instead, metal centers are covered with Hads, OHads, or Oads, and in some cases, we observed the restructuring of the metal sites due to oxygen incorporation. Based on these findings, it is suggested that setting up theoretical models for SAC modelling and the interpretation of ex situ characterization results using ultra-high vacuum (UHV) techniques requires special care, as the nature of SAC active sites under operating conditions can significantly diverge from the basic models or the pictures set by the UHV measurements.

Alloy decomposition under pressure: Bi–Sb incommensurate phase as a case study

The high-pressure phase behavior of the Bi–Sb system was investigated experimentally and modeled thermodynamically to study the effect of the transition to the incommensurate Bi-III type structure, which is adopted by the high-pressure phases Bi-III and Sb-II of the pure elements. The thermodynamic model of the Bi–Sb phase diagram including the effect of pressure in the isomorphous regime was extended to include solid–solid phase transitions of the pure elements. Consequently, the alloy phase diagram was found to transform to the eutectic form with pressure and is predicted here in the pressure range of 3–6 GPa. The alloy was found to transform from the ambient, rhombohedral A7 structure to the incommensurate phase. The transformation occurred at intermediate pressures between those observed in elemental Bi and Sb. The region of thermodynamic stability was determined by calculating the spinodal curves. To verify these predictions, the structure of a Sb-rich alloy was determined as a function of pressure by X-ray diffraction measurements in a diamond anvil cell, demonstrating the transition from A7 to the incommensurate Sb-II-like structure. It was found that the decomposition of the alloy under pressure is determined by the thermodynamic instability located at the spinodal curve. In contrast, alloy compositions located in the metastable region between the binodal phase line and the spinodal curve did not decompose under pressure, thus indicating a new region of phase space to be explored in high-pressure studies of alloys.

Fluorides of Silver Under Large Compression*.

The silver-fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag+ and Ag2+ cations (Ag3 F4 and Ag2 F3 ) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors under ambient pressure conditions. For Ag2 F5 , containing both Ag2+ and Ag3+ , we find that strong 1D antiferromagnetic coupling is retained throughout the pressure-induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed-valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF4 as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed-valence compounds of silver at high pressure.

Complete Photonic Band Gaps with Nonfrustrated ABC Bottlebrush Block Polymers.

Bottlebrush block polymers are a promising platform for self-assembled photonic materials, yet most work has been limited to one-dimensional photonic crystals based on the lamellar phase. Here we demonstrate with simulation that nonfrustrated ABC bottlebrush block polymers can be used to self-assemble three-dimensional photonic crystals with complete photonic band gaps. To show this, we have developed a computational approach that couples self-consistent field theory (SCFT) simulations to Maxwell's equations, thereby permitting a direct link between molecular design, self-assembly, and photonic band structures. Using this approach, we calculate the phase diagram of nonfrustrated ABC bottlebrush block polymers and identify regions where the alternating gyroid and alternating diamond phases are stable. By computing the photonic band structures of these phases, we demonstrate that complete band gaps can be found in regions of thermodynamic stability, thereby suggesting a route to realize these photonic materials experimentally. Furthermore, we demonstrate that gap size depends on volume fraction, segregation strength, and polymer architecture, and we identify a design strategy based on symmetry breaking that can achieve band gaps for lower values of refractive index contrast. Taken together, the approach presented here provides a powerful and flexible tool for predicting both the self-assembly and photonic band structures of polymeric materials.

AlGaInSbAs Solid Solutions Grown on InAs Substrates by Zone Recrystallization with a Temperature Gradient

The growth of AlGaInSbAs solid solutions on InAs substrates from the liquid phase in a temperature-gradient field is discussed. The calculation of parameters is performed, and the luminescence properties and spectral characteristics of AlGaInSbAs solid solutions isoperiodic to InAs substrates are investigated. The heterophase equilibria in the Al–Ga–In–Sb–As system are analyzed in the scope of the model of regular solutions. Regions of thermodynamic stability to the spinodal decomposition of AlGaInSbAs solid solutions and isoperiodicity ranges to the InAs substrate are revealed.

Accelerating Development of Materials for Industrial and High-Tech Applications with Data-Driven Analysis and Simulations

ABSTRACTWe describe how the development of advanced materials via high-throughput experimentation at Intermolecular® is accelerated using guidance from modelling, machine learning (ML) and other data-driven approaches. Focusing on rapid development of materials for the semiconductor industry at a reasonable cost, we review the strengths and the limitations of data-driven methods. ML applied to the experimental data accelerates the development of record-breaking materials, but needs a supply of physically meaningful descriptors to succeed in a practical setting. Theoretical materials design greatly benefits from the external modelling ecosystems that have arisen over the last decade, enabling a rapid theoretical screening of materials, including additional material layers introduced to improve the performance of the material stack as a whole, “dopants” to stabilize a given phase of a polymorphic material, etc. We discuss the relative importance of different approaches, and note that the success rates for seemingly similar problems can be drastically different. We then discuss the methods that assist experimentation by providing better phase identification. Finally, we compare the strengths of different approaches, using as an example the problem of identifying regions of thermodynamic stability in multicomponent systems.

Thermodynamic Modeling of Phase Equilibria in the FeO-MgO-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; System

Thermodynamic modeling of coordinates of phase diagrams’ liquidus lines of the FeO–MgO, FeO–Al2O3, MgO–Al2O3 systems and coordinates of phase diagram’s liquidus surface of the FeO–MgO–Al2O3 system has been carried out. In the course of work, a thermodynamic model which describes activity of oxide melt had been selected for each of the systems; energy parameters of the model have been determined. Regions of thermodynamic stability of solid phases which are at equilibrium with the oxide melt have been determined. Results of the modeling have been compared with experimental data existing in the literature. Modeling technique has also allowed evaluating enthalpies and entropies of FeAl2O4 and MgAl2O4 compounds’ formation out of components of the oxide melt. The obtained results are of interest for steelmaking industry processes when determining the melt temperature of a slag containing oxides of iron, magnesium and aluminum.

Thermodynamic mechanism of stability and polymorphic transformation behaviors of enantiotropic polymorphs of glycolide

In this paper, the heat capacity data of two crystal forms (Form1 and Form 2) and liquid state of glycolide were obtained by differential scanning calorimetry (DSC). Their Gibbs energies were calculated by thermodynamic data and relevant thermodynamic equations. The melting point and melting enthalpy of Form 1 which can’t be experimentally determined were successfully calculated. According to the calculated Gibbs energy, thermodynamic stability regions of Form 1 and Form 2 of glycolide were obtained. Through analyzing data of Gibbs energy and activation energy of polymorphic transformation process, the thermodynamic mechanism of the stability and polymorphic transformation behaviors of the two forms of glycolide was proposed and discussed.

Structural state and thermodynamic stability of Al–Cu alloys

It is known that processes occurring in binary system melts affect the crystallization process and the phase composition of alloys. To predict these processes, we should determine the region of thermodynamic stability of the melt. In this paper, the structural properties of hypoeutectic and hypereutectic alloys in Al–Cu system are studied depending on the heating temperature above the liquidus line and aftercooling rate. It is shown that overheating of Al–Cu melts to 150 K above the liquidus line and further cooling leads to complete suppression of the process of formation of primary aluminum crystals in hypoeutectic alloys and [Formula: see text] phase in hypereutectic alloys. For the first time, by accounting in Gibbs energy of binary Al–Cu alloy for the first degree approximation of high-temperature expansion of thermodynamic potential, the dependence of temperature of line of the melt thermodynamic stability on copper content in alloy is obtained.

Thermodynamic Analysis of the Existing Forms of Iron Oxides in Neutral Oxygen-Containing Aqueous Media of the NPP Power-Generating Units

Foreign and domestic software products designed for quantitative description of the flow accelerated corrosion (FAC) rate and modeling erosion–corrosion use physicochemical computational models based on the assumption that the oxide film that protects the surface of steel consists of magnetite. Investigations of the phase composition and structural characteristics of oxide films on the inner surfaces of pipelines and equipment of coolant-circulation circuits of the NPP power-generating units with RBMK high-power channel-type reactors or the third circuits of the BN-800 fast reactors in which correction-free or oxygenated water chemistry conditions are implemented have revealed the presence on the steels' surface of not only magnetite but also other iron oxide forms. At present, the presence of the forms of iron oxides and hydroxides other than magnetite is considered in the computational modeling of the FAC by the introduction of empirical dependences. In the work, thermodynamic analysis of the existence forms of solid iron oxide and hydroxide phases in contact with the aqueous coolant in neutral oxygen-containing solutions has been made for temperatures of 25–200°C. To solve the problem set, the method of minimization of free energies of the system was used that allows one to determine which of the solid phases—iron oxides or hydroxides—will be thermodynamically stable in aqueous process media. On the assumption that the free energy of a system is minimal in the equilibrium state, the method allows determining the composition of the solid phase that is thermodynamically stable under the given conditions. The dependence of the phase composition of the oxide film on the temperature and chemical composition of the aqueous medium in contact with the oxide in question is shown. It is suggested to use the calculated results for refining the physicochemical models of the dependence of the FAC on the temperature for estimation of changes in the solubility of the iron oxides on the temperature and the thermodynamic stability regions of the basic phases in which iron oxides exist in the presence of oxygen and hydrogen.

Can clathrates heterogeneously nucleate ice?

Methane hydrates can be preserved at ambient pressure, beyond their region of thermodynamic stability, by storing them at temperatures from 240 to 270 K. The origin of this anomalous self-preservation is the formation of an ice coating that covers the clathrate particles and prevents further loss of gas. While there have been several studies on self-preservation, the question of what is the mechanism by which ice nucleates on the decomposing clathrate hydrates has not yet been fully explained. Here, we use molecular simulations, thermodynamic analysis, and nucleation theory to investigate possible scenarios for the nucleation of ice: heterogeneous nucleation at the clathrate/vapor or clathrate/liquid interfaces and homogeneous nucleation from supercooled water. Our results indicate that clathrates cannot heterogeneously nucleate ice and that ice nucleation is due to the cooling of water at the decomposing clathrate/liquid interface, which suffices to trigger homogeneous ice nucleation. We find that the (111) face of the sII structure clathrate can bind to the (111) plane of cubic ice or the basal plane of hexagonal ice through domain matching, resulting in a weak binding that-while insufficient to promote heterogeneous ice nucleation-suffices to produce epitaxy and alignment between these crystals. We use thermodynamic relations, theory, and the contact angles of ice at the (111) sII clathrate/liquid interface to determine-for the first time-the interfacial free energy of this most favorable ice-clathrate interface, 59 ± 5 mJ/m2. We discuss the implications of our results for the feasibility of heterogeneous nucleation of gas clathrates at ice/vapor interfaces.