Calculated Enthalpies Of Formation Research Articles

New Insights into the Crystal Chemistry of FeB-Type Compounds: The Case of CeGe.

Several alkaline earth or rare earth binary monosilicides and -germanides possess complex bonding properties, such as polycation formation exceeding the scope of classical electron counting rules. In this study, we present characterization by powder and single-crystal diffraction and thermal analysis of CeGe, one of the few monogermanides crystallizing in the FeB-type structure. Comparative computational investigations for structure types experimentally observed for monogermanides and alternative structures with different structural motifs were performed to gain energetical insights into this family of compounds, underlining the preference for infinite germanium chains over other structural motifs. Formation enthalpy calculations and structural chemical analysis highlight the special position of FeB-type compounds among the monogermanides.

Pressure dependent structural, dynamical, mechanical and electronic properties of magnesium dicarbide

ABSTRACT Magnesium dicarbide (MgC2) at high pressure has been systematically investigated using the first-principle methods. Ten possible structures were studied and three of them (Cmcm, C2/m, P-3m1) were found to be dynamically and mechanically stable under the given pressure. The calculated formation enthalpies indicated that the Cmcm structure was thermodynamically unstable while the C2/m and P-3m1 structures were thermodynamically stable. The interesting thing was that the P-3m1 structure of MgC2 could also be as potential hard materials owing to their Vickers hardness closing to 40 GPa with increasing pressure. The Cmcm and C2/m structures presented ductile behaviour while P-3m1 structure displayed brittle character. The elastic anisotropy of Young’s modulus E and shear modulus G became smaller with the increasing pressure, and the P-3m1 structure possessed a lower anisotropy in E and G than Cmcm and C2/m structures. The Cmcm and C2/m structures of MgC2 had metallic characteristic; however, the P-3m1 structure of MgC2 was semiconductor with an indirect band gap.

Insights into Cation-disorder effect on stability, electronic structure and defect properties of Zn-IV-nitrides: The case of ZnGeN2

Structural, electronic and intrinsic defect properties of ZnGeN 2 in different space groups are systematically studied using first-principles calculations. Accessibility of cation-disordered structure is verified by formation enthalpy calculations and the rather low formation energies of cation antisite defects. Furthermore, the electronic structures and intrinsic defect properties of ZnGeN 2 semiconductors in all the space groups can be well regulated by cation-disorder. The band gap of ZnGeN 2 can be significantly reduced by cation-disordering. Cation-disordering is revealed to weaken Zn 3 d -N 2 p antibonding at the valence band maximum and enhance the bonding at the conduction band minimum. Electronic structures, as well as associated physical properties, are intimately related to the chemical environment of N atoms. The nonstoichiometrically coordinated N atoms produced by cation disordering show significant influence on the stability and electronic structure of ZnGeN 2 . Our predictive results may inspire further experimental and theoretical exploration on the potential applications of ZnGeN 2 in optoelectronics. • Accessibility of cation-disordered ZnGeN 2 is confirmed by formation enthalpy and defect formation energy calculations. • Zn-IV-N 2 exhibit good modulation in their electronic structures and intrinsic defect properties by cation-disorder. • Cation-disordering is revealed to weaken Zn 3 d -N 2 p antibonding at the valence band maximum and enhance the bonding at the conduction band minimum. • The electronic structures as well as associated physical properties are intimately related to the chemical environment of N atoms.

Prediction for high superconducting ternary hydrides below megabar pressure

The recent findings of high-temperature hydrides ushered a new era of superconductivity research under high pressure. However, the stable pressure for these remarkable hydrides remains extremely high. In this work, we performed the extensive simulations on a series of hydrides with the prototype structure of UH8 and UH7. Our results indicate several compounds possess superconducting critical temperature (T c) above liquid nitrogen temperature below 100 GPa, such as CeBeH8 and ThBeH8 that are dynamical stable with a T c of 201 K at 30 GPa and a T c of 98 K at 10 GPa, respectively. Further formation enthalpy calculations suggest that thermodynamical stable pressure of CeBeH8 and ThBeH8 compounds is above 50 GPa and 88 GPa with respect to binary compounds and solid elements. Moreover, we also found that ThBeH7 could be dynamically stable down to 20 GPa with a T c of 70 K. Our further simulations suggested this newly predicted ThBeH7 is thermodynamically stable above pressure of 33 GPa with respect to binary compounds and solid elements. The present results shed light on future design and discovery of high-temperature superconductor at moderate pressure.

Calibrating DFT Formation Enthalpy Calculations by Multifidelity Machine Learning.

The application of machine learning to predict materials properties measured by experiments are valuable yet difficult due to the limited amount of experimental data. In this work, we use a multifidelity random forest model to learn the experimental formation enthalpy of materials with prediction accuracy higher than the Perdew-Burke-Ernzerhof (PBE) functional with linear correction, PBEsol, and meta-generalized gradient approximation (meta-GGA) functionals (SCAN and r2SCAN), and it outperforms the hotly studied deep neural network-based representation learning and transfer learning. We then use the model to calibrate the DFT formation enthalpy in the Materials Project database and discover materials with underestimated stability. The multifidelity model is also used as a data-mining approach to find how DFT deviates from experiments by explaining the model output.

First-principles study of Co21W18 with pressure effect: The structural, mechanical, electronic properties and Debye temperature

A first-principles study on structural, mechanical, electronic properties and Debye temperature of Co21W18 have been presented based on density functional theory. The calculated formation enthalpy and elastic constants show that Co21W18 is thermodynamically stable and mechanically stable under high pressure, respectively. The calculated results of elastic constants and elastic modulus (B, G and E) reveal the potential application of Co21W18 compound in an extreme environment. The calculated density of states reveals the structural stability of Co21W18 under high pressure. The Debye temperature and sound velocities as a function of pressure are displayed.

The effects of Ti on the precipitation evolution and age-hardening behavior of Al–7Si–Mg alloy

The effect of Ti addition on the precipitation evolution for Al–7Si–Mg alloy was systematically investigated by combing high-resolution transmission electron microscopy (HRTEM) and first-principles calculations. At the early aging stage, the rod-shaped Al3Ti and ellipsoid Ti5Si3 phases precipitated firstly, and the stronger Si capture ability of them suppressed the precipitation of the Mg–Si phases (β'' and β′), leading to the deceleration of the aging hardening kinetics. With prolonging aging time, one Al atom in Al3Ti was easily replaced by Si atom to form a more stable Al2SiTi phase. It was demonstrated by Energy Dispersive Spectrometry (EDS) and the calculation of formation enthalpy. The accurate atomic structure of the Al2SiTi phase was also obtained, which provided a resonable speculation on the structure of Al3Ti two-dimensional compound (2DC) containing Si on the surface of TiB2. This shed light on a deep understanding of Si poisoning on grain refinement of Al–Si/Al–5Ti–B system. At the over-aged stage, the Mg–Si phases began to precipitate, preventing the decrease of the hardness. Finally, a new precipitation sequence in the Ti-rich Al–7Si–Mg alloy can be obtained: SSSS → Al3Ti + Ti5Si3 → Al2SiTi + Ti5Si3 → Al2SiTi + Ti5Si3 + β" → Al2SiTi + Ti5Si3 + β"+ β'.

Elastic and thermodynamic properties of Al4Si6La3 under pressure from first-principles calculation

The structure stability, mechanical properties and thermodynamic behaviors of Al4Si6La3 compound in a wide pressure range 0–100 ​GPa have been explored by performing first-principles calculations based on density functional theory. The calculated formation enthalpy demonstrates that the Al4Si6La3 compound at 0 ​GPa is thermodynamically stable and exhibits unstable with the increasing of pressure. The single crystal and polycrystalline elastic constants as a function of pressure are also calculated and discussed. Al4Si6La3 compound exhibits ductile nature at pressure up to 100 ​GPa by calculating Cauchy pressure, B/G ratio and Poisson's ratio. It is found that the Debye temperature and minimum thermal conductivity of Al4Si6La3 compound can be improved to some extent when the pressure is increasing. The analysis of electronic structures including charge density difference, Mulliken overlap population and density of state reveal that Al–Si and La–Si can form covalent bonds in Al4Si6La3 compound whereas Al–La form antibonding states at various pressures.

Theoretical insights on elastic anisotropy and thermal anisotropy of TM5Al3C (TM= Zr, Hf, and Ta) carbides

Structural, elastic, and thermal properties of the hexagonal TM5Al3C (TM = Zr, Hf, and Ta) carbides were investigated using first-principles calculations based on density functional theory (DFT). The calculated formation enthalpies and phonon dispersions indicate that these carbides are thermodynamically and dynamically stable. The calculated hardness values of TM5Al3C carbides are less than 20 GPa. The obtained Poisson's ratio, GH/BH, and Cauchy pressure indicate that these carbides are brittle. Elastic anisotropy indexes, three-dimensional (3D) surface construction, and two-dimensional (2D) planar projection were used to quantify the elastic anisotropy of these carbides, and the elastic anisotropy is in the order of Zr5Al3C > Ta5Al3C > Hf5Al3C. Finally, sound velocity, Debye temperature, and thermal conductivity were calculated based on the elastic constants and moduli. The calculated sound velocities and Debye temperatures of these carbides are found to be anisotropic. The minimum thermal conductivity has the same anisotropy order as elastic modulus.

The role of solvents in the formation of methylammonium lead triiodide perovskite

Metal halide perovskites (MHPs) are gaining increasing attention as low-cost, high-performance semiconductors for optoelectronics. In particular, their solution processing is compatible with the large-scale manufacturing of thin-film devices, including solar cells and light-emitting diodes. Understanding the coordination chemistry in precursor-solvent solution and atomistic mechanisms of film formation is of great importance for optimizing the optoelectronic properties of the final films. Using the methylammonium lead triiodide (MAPbI3) as an example, we study the complex evolution of the molecular species from the solution to the initial stage of the crystallization by using a combination of density functional theory (DFT) calculations and ab-initio molecular dynamics (AIMD) simulations. We focus on the widely employed solvents DMSO and DMF, analyze the structures and energies of the iodoplumbate complexes in the form of simple complex of [PbImLn]2−m and polymeric iodoplumbates of ([PbImLn]2−m)x. Based on the calculated formation enthalpies, we propose reaction schemes of MAPbI3 formation in DMSO, DMF and DMF-DMSO binary solvent and explain the advantages of the binary solvent. We highlight the important role of NH…O hydrogen bonds in the formation of iodoplumbates monomers. Our calculations indicate unbalanced reaction energies at several elementary reaction steps in either DMF (formation of [PbI4Ln]2− being highly favourable) or DMSO (formation of [PbI5Ln]3− being retarded). Mixing a small amount of DMSO in DMF gives rise to a better balance in the energies and, therefore, potentially better equilibria in the overall crystallization process and better quality of the final perovskite films.

Physical properties of chromium-doped barium titanate: Effects of chromium incorporation

This study utilizes the density functional theory to investigate the physical properties of BaTi1-xCrxO3 (x = 0, 0.005, 0.025, and 0.05) in order to analyze the doping effects of Cr. Since, doping has a substantial impact on a material's characteristics; this work aims to explore the physical properties of BaTiO3 by incorporating Cr of various concentrations to fully comprehend this system. The lattice parameters of BaTiO3 are increased with increased Cr incorporation. Besides, the formation enthalpy calculations and elastic constants confirm the thermodynamic and mechanical stability of all the doped phases. The brittle nature of undoped BaTiO3 is noticed, which gets increasingly brittle as Cr is added incrementally. Most importantly, the machinability of the pure phase is found to be slightly better than that of doped phases. The incremental doping level exhibits slightly increasing tendency of elastic anisotropy in BaTi1-xCrxO3. The semiconducting behavior of BaTiO3 is expressed as it has band gap in its electronic band structure, which supports the previous experimental and theoretical reports. Finally, the band structure and density of states reflect the decreasing affinity of band gap with increased doping.

Unraveling the effects of metal incorporation in cubic perovskite SrCoO3 by partially replacing Co atoms

This work employs density functional theory to calculate the structural, electronic, and mechanical properties of SrCo1-xMxO3 (M = Cr, V, and Nb; x = 0 and 0.1), to explore the effects of doping elements. The thermodynamic and mechanical stability of the studied phases are manifested by the calculated formation enthalpy and elastic constants, respectively. The lager ionic radious of doping elements than that of Co enlarges the lattice parameters of SrCoO3. The indication of metallic behavior of pure and doped phases is justified by the band structure and density of states. The densely populated valence band carriers near the Fermi level are mostly responsible for the conducting nature of the systems under study. The present calculations demonstrate the ductile nature of pure and doped phases of SrCoO3, among which the pure phase is more ductile than that of other phases. Similarly, this phase also exhibits the highest anisotropic nature among all the studied phases, as evidenced by several anisotropy indices.

Experimental investigation and thermodynamic modeling of the Mo–Co–B ternary system

Among the ternary borides, the Mo–Co–B system is of great interest because of its excellent hardness, toughness, and stability performance. Eight samples with 60 at.% Co were designed to investigate the isothermal section of Mo–Co–B system at 1073 K in the Co-rich portion. Scanning electron microscopy, energy-dispersion spectroscopy, electron probe microanalysis, differential scanning calorimetry, and X-ray diffraction were used to investigate the phase equilibria of the samples. The formation enthalpies of the ternary borides were obtained by first-principles calculations to serve as key information for thermodynamic assessment. By coupling the reviewed experimental data from the literature, the presently determined phase equilibria, and the calculated formation enthalpies of the compounds, the thermodynamic parameters for the Mo–Co–B ternary system were optimized and used to calculate the isothermal sections, vertical section, and liquidus projection of the system. Comprehensive comparisons showed that the calculated results are in reasonable agreement with the reported phase diagram and thermodynamic data.

Quantum molecular dynamics investigations of protactinium (V) fluoro and oxofluoro complexes in solution

Abstract Quantum Molecular Dynamics simulations of protactinium (V) fluoro and oxofluoro complexes in solution were undertaken using density functional theory with generalised gradient approximation (and the ABINIT software package). The complexes were studied in the gas phase (at 0 K) and in solution (at 298 K) with water molecules as the solvent. We characterised all of the systems, taking into account their structures, dissociation energies, bond lengths and formation enthalpies in solution, and explained their relative stabilities. At ambient temperature, the hydrated species PaF 5, P a F 4 + $Pa{F}_{4}^{+}$ , P a O F 5 2 − $PaO{F}_{5}^{2-}$ , P a O F 4 − $PaO{F}_{4}^{-}$ and PaOF 3 were found to be the most stable (and to exhibit similar stabilities). The calculated formation enthalpies of the complexes are in close agreement with measurements made elsewhere.

Effect of co-alloying elements on the structural stability, elastic, ductility and thermodynamic properties of TiAl intermetallic compound

The considerable lack of room temperature ductility and toughness is widely recognized as an important limitation for the commercialization of titanium aluminides in industrial engineering. To meet this target, we optimize a new polycrystalline alloy based on Gamma-titanium aluminide co-alloying with Mo–Cr, Mo–Nb and Mo–Re atom pairs that simultaneously satisfy the physical criteria of workability using the density functional theory within the framework of the generalized gradient approximation. The formation enthalpy calculations showed that the Ti–Ti sites remain the preferential sites of Mo–Cr and Mo–Nb atomic defects, while the Mo–Re elements tend to occupy Ti–Al sites. The elastic properties systematically suggest that TiAl–Mo–Nb system is further resistant to tensile deformation along c-axis and a-axis (b-axis). In addition, the co-alloying Mo–Re atom pair exhibit improved mechanical performance compared to the other systems. These trends are confirmed by analyzing the electronic structure. Using quasi-harmonic approximation, thermodynamic properties were also predicted by including the description of the temperature. The results of co-alloying with molybdenum and rhenium atoms confirm effectively their usability in turbine blades and engine-valves made of Gamma-TiAl alloys.

Crystal Structures, Phase Stabilities, Electronic Properties, and Hardness of Yttrium Borides: New Insight from First-Principles Calculations.

Yttrium borides have attracted much attention for their superconducting and high chemical stability. However, the fundamental knowledge of the mechanical properties and hardness of yttrium borides is rather lack. Here, we performed a systematic investigation on yttrium borides with various stoichiometries based on an unbiased CALYPSO structure search method and first-principles calculations. A new YB6 compound with R3m hexagonal structure is observed, which is more stable than the experimental synthesized Pm3̅m phase. The calculated formation enthalpy, elastic constants, and phonon dispersions distinctly show that the R3m-YB6 is energetically, mechanically, and dynamically stable. The density of states and electronic band structure reveal that all the stable yttrium borides are metallic. Based on a semiempirical method, the Vicker hardness of R3m-YB6 is expectant to be 37.0 GPa, indicating it is a potential ultrahard metal. Our results provide insights for future synthesis and design of new type functional materials.

First-principles study of the stability, electronic and mechanical properties of Cr2N and CrN with the variation of Ni doping concentration

First-principles approach was applied to investigate the stability, electronic and mechanical properties of Cr2-xNixN (x = 0, 0.083, 0.167,0.250, 0.333) and Cr1-xNixN (x = 0,0.125,0.25,0.375, 0.5). The calculated formation enthalpy and mechanical stability results show that Cr2-xNixN and Cr1-xNixN are all stable. The bulk, shear and Young's modulus results indicate that different variation trend is observed in Cr2-xNixN and Cr1-xNixN with the increase of x. Base on Pugh and Pettifor criteria, Cr2N belongs to the brittle area and the ductility of Cr2-xNixN increases with the increment of x, obtain the maximum results when x = 0.333. However, CrN, which belongs to the ductile area, alloying with Ni decreases its ductility and increases its brittleness, reach the maximum brittleness when x = 0.5. The charge density difference study reveals that the doped Ni atom affects the interaction between Cr and N in Cr2-xNixN and Cr1-xNixN differently. Furthermore, the stress-strain curve of Cr2N, Cr1.833Ni0.167N, and Cr1.667Ni0.333N under shear and tensile deformation shows that the ultimate stress of Cr2N is decreased and its ductility increased. Nevertheless, the stress-strain curve of CrN, Cr0.75Ni0.25N, and Cr0.5Ni0.5N under shear and tensile deformation indicates that the strength of CrN can be enhanced and its deformation process is significantly changed when x = 0.25.

Effect of Mo concentration on structural, mechanical, electronic and magnetic properties of Fe2B: a first-principles study

ABSTRACT First-principles calculations were performed to study the structural, mechanical, electronic and magnetic properties of Fe(8−x)Mo x B4 (x = 0, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 7, 8). Results show that the Mo atoms would enter the Fe2B lattice by replacing the Fe atoms and cause a change in the crystal structure. According to the calculated formation enthalpy, all borides are thermodynamically stable; moreover, Mo would reduce the stability of Fe2B when the Mo atomic number is less than four in the unit cell. On the whole, with the increase of Mo content, the Young’s moduli and shear moduli decrease firstly and then increase, while the bulk moduli get bigger. Although the hardness of Fe2B would decline with increasing Mo content, Mo is an excellent candidate for improving the ductility of Fe2B on the basis of the G/B and Poisson’s ratio ν values. The charge density plots indicate that the integral strength of the linear boron chains in [002] orientation is reduced in Mo-alloyed Fe2B. The toughness improvement of Fe2B would be attributed to the evolutions of B–B bonds, Fe/Mo–B bonds and Fe/Mo–Fe bonds, which may induce the deflection of the Fe2B cleavage plane. Besides, the magnetic moments of these borides monotonically decrease with the increase in Mo content.

Electronic and optical properties of ASc2S4 (A = Ca, Sr) compounds

This study investigates the ASc2S4 (A = Ca, Sr) compounds having an AB2O4-type structure of orthorhombic crystal nature. The calculated formation enthalpies reveal the thermodynamic stability of these compounds. In addition, the mechanical and dynamical stabilities are found as well. The detailed electronic properties are studied using Perdew-Burke-Ernzerhof functional within the Generalized Gradient Approximation (GGA-PBE) and DFT + U methods. The ASc2S4 compounds are found to be semiconductor due the electronic band structures. The band gaps are determined as 1.11 eV and 1.25 eV using the GGA-PBE method, and 1.54 eV and 1.64 eV using the DFT + U method for CaSc2S4 and SrSc2S4, respectively. The orbital contributions to the partial density of states are investigated in detail. Furthermore, the optical properties of these compounds are obtained using the Bethe-Salpeter Equation (BSE) to consider the electron-hole interactions. The optical properties of the ASc2S4 compounds such as refractive indexes, extinction coefficients, absorption coefficients and loss functions are determined. This study fills the gap in the literature concerning the compounds that have S atoms instead of O atoms in AB2O4, while also revealing the electronic and optical properties of ASc2S4 (A = Ca, Sr) compounds in detail.

Insights into structural stability, electronic structure, and elastic and thermodynamic properties of A15-type Mo3X (X = Si, Ge, and Sn) compounds based on first-principles predictions

In this study, we performed first-principles calculations based on density functional theory to investigate the structure, electronic properties, and elastic and thermodynamic performance of A15-type Mo3X (X = Si, Ge, and Sn) molybdenum-based compounds. The calculated formation enthalpies indicate that all of these Mo3X compounds exhibit thermodynamic stability. The electronic properties of Mo3X, including the band structures, density of states, and electron density difference, indicate that these compounds are metallic. Moreover, the Mo–Mo bonds are covalent whereas the Mo-X bonds are mixed ionic–covalent. The elastic properties of Mo3X compounds were predicted based on the single-crystal and polycrystal elastic moduli. In addition, the anisotropy of elasticity was determined using the elastic anisotropy indexes, three-dimensional surface constructions, and two-dimensional projections. Finally, we explored the thermodynamic parameters, including the Debye temperatures and minimum thermal conductivities. The order of the minimum thermal conductivity was determined as: Mo3Si > Mo3Ge > Mo3Sn.