Chemical Potential Phase Diagram Research Articles

Efficient DFT prediction of chemical and structural stability using van der Waals correction: application for A3B2Ga3O12 garnets (A = Lu, Y and B = Al, Sc)

Several seamless van der Waals (vdW) correction methods available for a wide range of systems could be expected to enhance stability predictions by accounting for the vdW effect. The stability of material can be evaluated using chemical potential phase diagram (CPD) which reveals the elemental chemical potential conditions for a successful synthesis. In this work, viability of various vdW correction approaches in improving the accuracy of stability prediction for A3B2Ga3O12 garnets (A = Lu, Y and B = Al, Sc) has been studied. From the results, we have found that vdW-df-cx, Grimme-D3, vdW-df-c09, and vdW-df2-c09 significantly improve ΔH prediction with MAPE of >5.0% lower than PBE, which exhibit their potential for stability prediction based on the CPD analysis. For CPD construction whose reliability is based on ΔH prediction, vdW-df-cx which can minimize the MAPE in ΔH, relative to experimental data, is selected as the best method among all studied vdW approaches. A more accurate description of total energy of O2 molecule and the competing compounds with layered structure can be also acquired by incorporating vdW interaction. However, the MAPE in lattice constant reveals that there is no significant improvement of lattice constant prediction for the studied garnets and their competing compounds. The vdW method which gives the MAPE in lattice constant slightly lower than that of PBE is vdW-df2-b86r. Although we found that the vdW corrections can improve material stability prediction, there is still room for the development of a novel DFT-based vdW method capable of accurately predicting both the lattice constant and ΔH of solids, including complex materials like garnets.

Ferromagnetic instability in itinerant fcc lattice electron systems with higher-order van Hove singularities: Functional renormalization group study

We investigate the possibility of ferromagnetic ordering in the non-degenerate Hubbard model on the face-centered cubic lattice within the functional renormalization group technique using temperature as a scale parameter. We assume the relations between nearest, next-nearest, and next-next-nearest hopping parameters providing higher order (giant) van Hove singularity of the density of states. The ferromagnetic instability formation with lowering temperature is described consistently in the one-loop approximation for a one-particle irreducible vertex of two-particle electron interaction. The chemical potential versus temperature phase diagrams are calculated. We find ferromagnetic order only for sufficiently strong divergence of the density of states and fillings in the vicinity of van Hove singularity. The obtained Curie temperature is more than an order of magnitude smaller than the results of the random-phase approximation. The main origin of the suppression of ferromagnetism is the screening of interaction in the particle-particle channel. We also do not find the pronounced tendency towards incommensurate order when the Fermi level is moved away from a van Hove singularity, such that the first order quantum phase transitions from the ferro- to paramagnetic phase are obtained.

Electronic structures and stability investigation of large band gap topological insulators MTl4Te3 ( M = Cd, Hg)

By means of ternary chemical potential phase diagram and phonon spectrum calculations, we propose that $M$Tl$_4$Te$_3$ ($M$ = Cd, Hg), the derivatives of Tl$_5$Te$_3$, are thermodynamically and dynamically stable in the body centered tetragonal crystal structure with $I$4/$mcm$ symmetry. Our electronic structures calculations confirm that a robust $s$-$p$ band inversion occurs near the Fermi level in $M$Tl$_4$Te$_3$, and a topological band gap about 0.13 eV in CdTl$_4$Te$_3$ is induced by the spin-orbit coupling. These results suggest that $M$Tl$_4$Te$_3$ are a new class of large band gap 3D strong topological insulators that are stable and synthesizable in experiments, which could be used to design efficient spin torque equipment and spin device.

Stability and fundamental properties of Cu x O1-x as optoelectronic functional materials.

Binary CuxO1−x compounds have some advantages as optoelectronic functional materials, but their further development has encountered some bottlenecks, such as inaccurate bandgap values and slow improvement of photoelectric conversion efficiency. In this work, all possible stoichiometric ratios and crystal structures of binary CuxO1−x compounds were comprehensively analyzed based on a high-throughput computing database. Stable and metastable phases with different stoichiometric ratios were obtained. Their stability in different chemical environments was further analyzed according to the component phase diagram and chemical potential phase diagram. The calculation results show that Cu, Cu2O and CuO have obvious advantages in thermodynamics. The comparison and analysis of crystal microstructure show that the stable phase of CuxO1−x compounds contains the following two motifs: planar square with Cu atoms as the center and four O atoms as the vertices; regular tetrahedron with O atoms as the center and four Cu atoms as the vertices. In different stoichiometric ratio regions, the electron transfer and interaction modes between Cu and O atoms are different. This effect causes energy differences between bonding and antibonding states, resulting in the different conductivity of binary CuxO1−x compounds: semi-metallic ferromagnetic, semiconducting, and metallicity. This is the root of the inconsistent and inaccurate bandgap values of CuxO1−x compounds. These compositional, structural, and property variations provide greater freedom and scope for the development of binary CuxO1−x compounds as optoelectronic functional materials.

Phase Diagrams for sII Clathrate Hydrates of CO2 from First-Principles Thermodynamics.

Clathrate hydrates are crystalline solid compounds consisting of a water caged framework and guest molecules such as CH4, C2H6, and CO2. Understanding the phase equilibrium conditions of hydrates is significantly important for the industrial exploitation and experimental synthesis of hydrates. Based on the correct description of the intermolecular noncovalent interactions of clathrate hydrates with vdW-DF2, we studied the crystal structures and the chemical potential phase diagrams of sII hydrates encapsulated with CO2 molecules to provide a deep understanding of the stability mechanism of hydrates. Under the given p-T conditions, the partially occupied hydrates (136H2O·1CO2 and 136H2O·16CO2) and fully occupied hydrates (136H2O·24CO2) are thermodynamically stable, and the equilibrium temperature decreases as the relative CO2 chemical potential increases at the same pressure. We expect that the present study may provide vital information on the stability conditions of CO2 hydrates and trigger new experiments to establish an effective replacement strategy for CO2/CH4.

Interface Behavior of Brazing between Zr-Cu Filler Metal and SiC Ceramic

The interface behavior of brazing between Zr-Cu filler metal and SiC ceramic was investigated. Based on the brazing experiment, the formation of brazing interface products was analyzed using OM, SEM, XRD and other methods. The stable chemical potential phase diagram was established to analyze the possible diffusion path of interface elements, and then the growth behavior of the interface reaction layer was studied by establishing relevant models. The results show that the interface reaction between the active element Zr and SiC ceramic is the main reason in the brazing process the interface products are mainly ZrC and Zr2Si and the possible diffusion path of elements in the product formation process is explained. The kinetic equation of interfacial reaction layer growth is established, and the diffusion constant (2.1479 μm·s1/2) and activation energy (42.65 kJ·mol−1) are obtained. The growth kinetics equation of interfacial reaction layer thickness with holding time at different brazing temperatures is obtained.

Phase diagram for strongly interacting matter in the presence of a magnetic field using the Polyakov–Nambu–Jona-Lasinio model with magnetic field dependent coupling strengths

We study the phase diagram for strongly interacting matter using the 't Hooft determinant extended Nambu--Jona-Lasinio model with a Polyakov loop in the light and strange quark sectors (\emph{up}, \emph{down} and \emph{strange}) focusing on the effect of a magnetic field dependence of the coupling strengths of these interactions. This dependence was obtained so as to reproduce recent lattice QCD results for the magnetic field dependence of the quarks dynamical masses. A finite magnetic field is known to induce several additional first-order phase transition lines with the respective Critical End Points (CEP) in the temperature-quark chemical potential phase diagram when compared to the zero magnetic field case. A study of the magnetic field dependence in the range $eB=0-0.6~\mathrm{GeV}^2$ of the location of these CEPs reveals that the initial one as well as several of the new ones only survive up to a critical magnetic field. Only two remain in the upper limit of the studied magnetic field strength. A comparison of the results obtained with versions of the model with and without Polyakov loop is also done. We also found that the inclusion of the magnetic field dependence on the coupling strengths, while not changing the qualitative features of the phase diagram, affects the location of these CEPs. The comparison of results with and without a regularization cutoff in the medium part of the integrals does not show a significant change.

Computationally Predicted High-Throughput Free-Energy Phase Diagrams for the Discovery of Solid-State Hydrogen Storage Reactions.

The design of multinary solid-state material systems that undergo reversible phase changes via changes in temperature and pressure provides a potential means of safely storing hydrogen. However, fully mapping the stabilities of known or newly targeted compounds relative to competing phases at reaction conditions has previously required many stringent experiments or computationally demanding calculations of each compound's change in Gibbs energy with respect to temperature, G(T). In this work, we have extended the approach of constructing chemical potential phase diagrams based on ΔGf(T) to enable the analysis of phase stability at non-zero temperatures. We first performed density functional theory calculations to compute the formation enthalpies of binary, ternary, and quaternary compounds within several compositional spaces of current interest for solid-state hydrogen storage. Temperature effects on solid compound stability were then accounted for using our recently introduced machine learned descriptor for the temperature-dependent contribution Gδ(T) to the Gibbs energy G(T). From these Gibbs energies, we evaluated each compound's stability relative to competing compounds over a wide range of conditions and show using chemical potential and composition phase diagrams that the predicted stable phases and H2 release reactions are consistent with experimental observations. This demonstrates that our approach rapidly computes the thermochemistry of hydrogen release reactions for compounds at sufficiently high accuracy relative to experiment to provide a powerful framework for analyzing hydrogen storage materials. This framework based on G(T) enables the accelerated discovery of active materials for a variety of technologies that rely on solid-state reactions involving these materials.

Low μ and imaginary μ signals of a critical point in the phase diagram of an exactly soluble chiral symmetry breaking theory

Holography has allowed the exact solution of a small number of large Nc gauge theories. Amongst these is an N=2 SYM theory of quarks interacting with N=4 gauge fields. The temperature chemical potential phase diagram for this theory in the presence of a magnetic field is exactly known and shows first and second order chiral symmetry restoration transitions and a critical point. Here we extend this phase diagram to imaginary chemical potential to seek structure at small real mu and imaginary mu that help to reconstruct the large real mu phase structure. We also explore a phenomenologically deformed version of the theory where the critical point can be moved into the imaginary chemical potential plane. In particular we observe that when the transition is second order in these theories there are naturally two distinct transitions - one for the onset of density and one for chiral symmetry restoration. In addition, the phase diagram has boundaries of regions where metastable vacua exist and these boundaries, as well as the phase boundaries, converge at the critical point. These observations may point to techniques for the study of the QCD critical point either on the lattice or using heavy ion collision data.

Theoretical study of stability and optical absorption properties of ferroelectric materials ZnXO3 (X=Ge, Sn and Pb)

The chemical potential equilibrium phase diagram and optical absorption properties of ZnXO3 (X = Ge, Sn, and Pb) of LiNbO3 type (LN- ZnXO3) have been calculated by density functional theory. We find that LN-ZnXO3 (X = Ge, Sn, and Pb) do not form a stable chemical potential area with respect to the binary compounds, but could be stabilized under external pressures. The non-equilibrium chemical potential areas in LN-ZnXO3 (X = Ge, Sn, and Pb) are in a sequence of Ge > Sn > Pb. The calculated tendency of transition pressures and binary impurities compound during the synthesis of LN-ZnXO3 (X = Ge, Sn and Pb) are consistent with the experimental results, which are explained by the structure and electronic structure analysis. The calculated optical absorption property indicates that LN-ZnPbO3 could be a good ferroelectric photovoltaic material, while LN-ZnXO3 (X = Ge, Sn) are a good visible light transparent material. The diversity of the feasible transition between VBM and CBM charge with small band gap play important roles in good performance of optical absorption property in LN-ZnPbO3. These inclusions could expand the applications of LN-ZnXO3 (X = Ge, Sn, Pb).

Theoretical study of M–H (M=Ti, V, Zr or Nb) structure phase diagram at high pressures

Abstract We have performed structure searches for titanium (Ti) hydrides, vanadium (V) hydrides, zirconium (Zr) hydrides and niobium (Nb) hydrides under ambient and high pressure up to high hydrogen content using CALYPSO method combined with first-principles calculations. Ten novel stable phases, R 3 ¯ -TiH3, P42/mnm-TiH3, Ibam-TiH2.5, Cccm-VH, Pna21-ZrH4, R 3 ¯ -ZrH3, Pm 3 ¯ n_1-ZrH3, R 3 ¯ m-ZrH2.5, P2/c-NbH2.5 and P42/mcm-NbH2.5 were discovered under various pressures. In particular, the high hydrogen content Pna21-ZrH4 can be stabilized at 4–12 GPa. The detailed crystal structure, dynamical stability and electronic structure are also investigated for those novel stable phases. Combined with earlier predicted structures, comprehensive pressure–chemical potential (P- Δ μ H ) phase diagrams are constructed for these M−H systems.

Physics of intrinsic point defects in bismuth oxychalcogenides: A first-principles investigation

As quasi two-dimensional semiconductors, bismuth oxychalcogenides (BOXs) have been demonstrated as potential candidates for high-speed and low-power electronics because of their exceptional environmental stability and high carrier mobility. Here, thermodynamics of growth and a series of intrinsic defects in BOXs are studied using first-principles calculations. Comparing the chemical potential phase diagrams of BOXs, we find that it is easier to grow Bi2O2Se than to grow Bi2O2S or Bi2O2Te. It is most difficult to grow stable Bi2O2Te because of the existence of various binary phases. Under Se-poor conditions, the intrinsic point defects of Bi replacing Se (BiSe) and Se vacancy (VSe) can form easily and behave as donors because of low formation energy, which is the reason for the n-type character of as-grown Bi2O2Se in experiments. For Bi2O2S, the donor point defect of Bi substituting S (BiS) is also dominant, leading to an n-type carrier. This study of thermodynamics and the physics of intrinsic point defects provides a valuable understanding of BOXs.

Temperature and coverage effects on the stability of epitaxial silicene on Ag(111) surfaces

Silicene, the single layer of silicon atoms arranged in a honeycomb lattice, has been synthesized in recent experiments and attracted significant attentions. Silicene is promising in future nanoelectronic devices due to its outstanding electronic properties. In experiments, however, different silicene superstructures coexist on Ag(111) substrate. For the device applications, homogenous silicene sheet with large scale and high quality is highly desired. Here, for the first time, we investigate both the temperature and the coverage effects on the thermal stability of epitaxial silicene on Ag(111) surface by ab initio molecular dynamics simulations. The relationship between the stability of various silicene superstructures and the growth conditions, including temperature and coverage of silicon atoms, is revealed by plotting the chemical potential phase diagram of silicene on Ag(111) surfaces at different temperatures. Our results are helpful for understanding the observed diversity of silicene phases on Ag(111) surfaces and provide some useful guidance for the synthesis of homogenous silicene phase in experiments.

Possible Formation of Graphyne on Transition Metal Surfaces: A Competition with Graphene from the Chemical Potential Point of View

Graphyne (GY), a two-dimensional (2D) allotrope of carbon with mixed sp/sp2 hybridization, is predicted to exist in many stable phases and has recently received great attention. However, it is energetically less stable than graphene and remains difficult to be synthesized in experiment to date. In this report, the possible environments for synthesis of graphyne on Ru(0001), Rh(111), and Pd(111) substrates are investigated by considering three typical phases of GY (α, β, and γ). Their structures, interactions with metal substrates, as well as thermodynamic stability are calculated using first-principles calculations. The chemical potential phase diagram of GYs and graphene on metal substrates is constructed. For all these substrates, the α phase of GY can form in the carbon-poor environment, while formation of graphene dominates in the carbon-rich condition.

Phase diagrams for clathrate hydrates of methane, ethane, and propane from first-principles thermodynamics.

Natural gas hydrates are inclusion compounds composed of major light hydrocarbon gaseous molecules (CH4, C2H6, and C3H8) and a water clathrate framework. Understanding the phase stability and formation conditions of natural gas hydrates is crucial for their future exploitation and applications and requires an accurate description of intermolecular interactions. Previous ab initio calculations on gas hydrates were mainly limited by the cluster models, whereas the phase diagram and equilibrium conditions of hydrate formation were usually investigated using the thermodynamic models or empirical molecular simulations. For the first time, we construct the chemical potential phase diagrams of type II clathrate hydrates encapsulated with methane/ethane/propane guest molecules using first-principles thermodynamics. We find that the partially occupied structures (136H2O·1CH4, 136H2O·16CH4, 136H2O·20CH4, 136H2O·1C2H6, and 136H2O·1C3H8) and fully occupied structures (136H2O·24CH4, 136H2O·8C2H6, and 136H2O·8C3H8) are thermodynamically favorable under given pressure-temperature (p-T) conditions. The theoretically predicted equilibrium pressures for pure CH4, C2H6 and C3H8 hydrates at the phase transition point are consistent with the experimental data. These results provide valuable guidance for establishing the relationship between the accurate description of intermolecular noncovalent interactions and the p-T equilibrium conditions of clathrate hydrates and other molecular crystals.

Effect of lattice coarsening and exclusion on phase-transition properties of the Bell–Lavis model

The temperature and order of the phase transition are studied for a less-coarsened Bell–Lavis model. The interactions exist between triangular molecules being on third-neighbour (3NN) sites, while the nearest-neighbour (1NN) interactions are subject to a hard core exclusion. Using Monte Carlo calculations, we obtain the temperature, and chemical potential phase diagram for a whole range of values, where the honeycomb phase exists. We find that the entropy gain prevails at low molecular densities (high ). Therefore, the first-order disordered-to-honeycomb phase transition found in the 3NN model has lower temperature than that of the 1NN model. At higher molecular densities (low ), below the tricritical point, the exclusions start to play some role and the entropy decreases. This leads to the occurrence of the intermediate short-range ordered phase in between disordered and honeycomb phases. The upper phase transition is found at higher temperature, while the lower one at the same temperature as that of the 1NN model. The critical exponents obtained for the lower phase transition demonstrate the values typical for the three-state Potts model universality class.

Phase diagram and thermodynamic and dynamic anomalies in a pure repulsive model

Using Monte Carlo simulations a lattice gas model with only repulsive interactions was checked for the presence of anomalies. We show that this system exhibits the density (temperature of maximum density—TMD) and diffusion anomalies as present in liquid water. These anomalous behaviors exist in the region of the chemical potential vs temperature phase diagram where two structured phases are present. A fragile-to-strong dynamic transition is also observed in the vicinity of the TMD line.

Density function theory investigation on the thermodynamic properties of the Li–N–H system

The electronic structures and formation enthalpies of compounds in the Li–N–H system have been studied by using the density functional theory. In order to evaluate the competition between each reaction in the system, the chemical potential phase diagrams of compounds in the Li–N–H system have been computed and discussed. Our calculations show that for LiNH2, Li+ combines with [NH2]- by an ionic bond. For Li2NH, the N–H bond displays covalent characteristics. The calculated formation enthalpy of compounds in the Li–N–H system is in agreement with previous results, the LiNH2 is −212.27 kJ mol−1, LiH is −91.66 kJ mol−1, Li2NH is −243.14 kJ mol−1, Li4NH is −309.72 kJ mol−1, Li3N is −189.11 kJ mol−1, and NH3 is −102.27 kJ mol−1, respectively. Using the chemical potential phase diagrams, six reversible reactions are discussed. It is found that Li4NH takes part in the three reversible reactions and some NH3 formed in the system react with other compounds in the Li–N–H system. These reversible reactions are confirmed by the proposed mechanism from experiments.

Atomistic simulation of CO2 solubility in poly(ethylene oxide) oligomers

We have performed atomistic molecular dynamics simulations coupled with thermodynamic integration to obtain the excess chemical potential and pressure-composition phase diagrams for CO2 in poly(ethylene oxide) oligomers. Poly(ethylene oxide) dimethyl ether, CH3O(CH2CH2O)nCH3 (PEO for short) is a widely applied physical solvent that forms the major organic constituent of a class of novel nanoparticle-based absorbents. Good predictions were obtained for pressure–composition–density relations for CO2 + PEO oligomers (2 ≤ n ≤ 12), using the Potoff force field for PEO [J. Chem. Phys. 136, 044514 (2012)] together with the TraPPE model for CO2 [AIChE J. 47, 1676 (2001)]. Water effects on Henry’s constant of CO2 in PEO have also been investigated. Addition of modest amounts of water in PEO produces a relatively small increase in Henry’s constant. Dependence of the calculated Henry’s constant on the weight percentage of water falls on a temperature-dependent master curve, irrespective of PEO chain length.

Role of chemical processes on shear zone formation: an example from the Grimsel metagranodiorite (Aar massif, Central Alps)

AbstractAlpine deformation in the Grimsel granodiorite (Aar massif, Central Alps) at greenschist facies conditions (6.5 ± 1 kbar for 450°C ± 25°C) is characterized by the development of a network of centimetre to decametre localized shear zones that surround lenses of undeformed granodiorite. Localization of deformation is assumed to be the result of a first stage of extreme localization on brittle precursors (nucleation stage) followed by a transition to ductile deformation and lateral propagation into the weakly deformed granodiorite (widening stage). A paradox of this model is that the development of the ductile shear zone is accompanied by the crystallization of large amounts of phyllosilicates (white mica and chlorite) that maintains a weak rheology in the localized shear zone relative to the host rock so that deformation is localized and prevents shear zone widening. We suggest that chemical processes, and more particularly, the metamorphic reactions and metasomatism occurring during re‐equilibration of the metastable magmatic assemblage induced shear zone widening at these P–T–X conditions. These processes (reactions and mass transfer) were driven by the chemical potential gradients that developed between the thermodynamically metastable magmatic assemblage at the edge of the shear zone and the stable white mica and chlorite rich ultramylonite formed during the first stage of shear zone due to localized fluid infiltration metasomatism. P–T and chemical potential projections and sections show that the process of equilibration of the wall rocks (μ–μ path) occurs via the reactions: kf + cz + ab + bio + MgO + H2O = mu + q + CaO + Na2O and cz + ab + bio + MgO + H2O = chl + mu + q + CaO + Na2O. Computed phase diagram and mass balance calculations predict that these reactions induce relative losses of CaO and Na2O of ∼100% and ∼40% respectively, coupled with hydration and a gain of ∼140% for MgO. Intermediate rocks within the strain gradient (ultramylonite, mylonite and orthogneiss) reflect various degrees of re‐equilibration and metasomatism. The softening reaction involved may have reduced the strength at the edge of the shear zone and therefore promoted shear zone widening. Chemical potential phase diagram sections also indicate that the re‐equilibration process has a strong influence on equilibrium mineral compositions. For instance, the decrease in Si‐content of phengite from 3.29 to 3.14 p.f.u, when white mica is in equilibrium with the chlorite‐bearing assemblage, may be misinterpreted as the result of decompression during shear zone development while it is due only to syn‐deformation metasomatism at the peak metamorphic condition. The results of this study suggest that it is critical to consider chemical processes in the formation of shear zones particularly when deformation affects metastable assemblages and mass transfer are involved.