First-principles Calculations Research Articles

Effect of Yb3+ on the thermophysical and mechanical properties of gadolinium zirconate ceramics: First-principles calculations and experimental study

Thermal barrier coating materials require high phase stability, low thermal conductivity, and a thermal expansion coefficient that matches the bonding layer. Cation doping is a highly efficient method for enhancing the thermophysical and mechanical characteristics of materials used in thermal barrier coatings. The thermophysical and mechanical properties, as well as the phase stability at high temperatures of ceramic materials doped with Yb3+ in gadolinium zirconate, are examined by utilizing first principles calculations and solid-state reaction methods. As the Yb3+ content increases, the grain average particle size of gadolinium zirconate ceramics first decreases and then increases. Given that a smaller grain size results in more grain boundaries, thereby reducing the thermal conductivity of the material, it can be inferred that Yb0.20 ((Gd0.8Yb0.2)2Zr2O7) with the smallest grain size exhibits lower thermal conductivity. As the Yb3+ content increases, both the calculated and experiment results suggest the Young’s modulus of gadolinium zirconate initially declines to its minimum value when the Yb3+ content is 0.5 and subsequently experiences a little increase. Furthermore, as the concentration of Yb3+ increases, the material initially experiences a drop in both hardness and fracture toughness, followed by a subsequent increase. Both the calculation and experimental results indicate that the thermal conductivity of gadolinium zirconate initially decreases and then increases. Among the different compositions, Gd1.5Yb0.5Zr2O7 demonstrates the minimum thermal conductivity, measuring 1.029 W/(m⸱K) according to the Clark model and 1.152 W/(m⸱K) according to the Cahill model. At a temperature of 1273 K, the experimental results demonstrated that (Gd0.8Yb0.2)2Zr2O7 has the lowest thermal conductivity of 1.415 W/(m·K). Furthermore, both the calculated and experimental thermal expansion coefficients of the material decrease first until the Yb3+ concentration is 0.5 and then increase slightly when the Yb3+ content increases. Moreover, gadolinium zirconate ceramics have demonstrated a consistent single-phase fluorite structure, excellent stability at high temperatures, and remarkable structural integrity during repeated heating and cooling cycles. The calculation results exhibit strong congruence with the experimental data. The objective of this work is to improve the thermophysical and mechanical properties of gadolinium zirconate ceramics for their application as materials for thermal barrier coatings.

Exploring charge transfer and schottky barrier modulation at monolayer Ge2Sb2Te5-metal interfaces

Monolayer Ge2Sb2Te5exhibits great potential in non-volatile memory technology due to its excellent electronic properties and phase-change characteristics, while the fundamental nature of Ge2Sb2Te5-metal contacts has not been well understood yet. Here, we provide a comprehensiveab initiostudy of the electronic properties between monolayer Ge2Sb2Te5and Pt, Pd, Au, Cu, Cr, Ag, and W contacts based on first-principles calculations. We find that the strong interaction interfaces formed between monolayer Ge2Sb2Te5and Pt, Pd, Cr, and W contacts show chemical bonding and strong charge transfer. In contrast, no apparent chemical bonding and weak charge transfer are observed in the weak interaction interfaces formed with Au, Cu, and Ag. Additionally, our study reveals the presence of a pronounced Fermi level pinning effect between monolayer Ge2Sb2Te5and metals, with pinning factors ofSn=0.325andSp=0.350. By increasing the interlayer distance, an effective transition fromn-type Ohmic contact ton-type Schottky contact is facilitated because the band edge of Ge2Sb2Te5is shifted upwards. Our study not only provides a theoretical basis for selecting suitable metal electrodes in Ge2Sb2Te5-based devices but also holds significant implications for understanding Schottky barrier height modulation between semiconductors and metals.

Lattice dynamics and vibrational properties of scheelite-type alkali-metal perrhenates.

The present work provides insight into the structural, vibrational, and elastic properties of scheelite-type alkali-metal perrhenates AReO4 (A = Na, K, Rb, Cs) from first-principles calculations. Sodium, potassium, and rubidium perrhenates are isostructural and crystallize in a tetragonal structure whereas caesium perrhenate crystallizes in an orthorhombic structure. All phonon frequencies and their corresponding mode assignments were estimated, through the linear response method within density-functional-perturbation theory. The phonon density of states highlights the predominant participation of the oxygen anions and both the A-type and rhenium (Re) cations in the low-frequency range. In contrast, oxygen and Re atoms exhibit higher and moderate contributions to the remaining phonon frequency spectrum. Considerable splitting of the longitudinal and traverse optic modes was observed. Elastic constants and phonon dispersion calculations confirm the mechanical and dynamical stability of all the studied AReO4 compounds. A redshift was observed due in the frequency of the phonons following the sequence Na→Cs. The calculated low bulk modulus (ranging from 28.36 GPa to 14.15 GPa) and shear modulus values indicate a low resistance to deformation of the perrhenates. The values of these moduli decrease in the order of Na→Cs which is associated with the increase in the ionic radius of the anion. The response to pressure was found to be anisotropic. This characteristic and the ductile nature of the alkali-metal perrhenates, were confirmed through an elastic analysis.&#xD.

The thermoelectric properties of the two-dimensional Nowotny-Juza material NaBeX (X = P, As, and Sb)

The two-dimensional β-type Nowotny-Juza phase-filled tetrahedral compound has excellent electrical conductivity, low thermal conductivity and outstanding thermoelectric properties. This study investigated the stability and electronic properties of NaBeX (X = P, As, and Sb) through first-principles calculations. The results demonstrate that these materials are structurally stable and are direct bandgap semiconductors with bandgaps of 0.6542 eV, 0.6549 eV, and 0.6686 eV, respectively. The electrical and thermal transport properties of NaBeX are investigated by combining the deformation potential theory and Boltzmann transport theory. Subsequently, the thermoelectric properties of NaBeX are evaluated at temperatures ranging from 400 to 800 K. The n-type NaBeX materials exhibit large ZT values of 2.48, 2.62, and 1.72. The results of the present study indicate that n-type NaBeX has the potential to be used as a thermoelectric candidate material at temperatures ranging from 400 to 800 K.

Efficient and emission-reduced recovery of high-purity copper from waste enameled copper wires

The escalating demand for refined copper intensifies research into the recycling of copper-based solid waste, exemplified by recovery of copper from waste enameled copper wires (WECWs). Despite its potential, the pyrolysis process for WECWs is hindered by embrittlement, diminishing both recycling yield and quality. Our study pioneers the elucidation of the embrittlement mechanism during WECWs pyrolysis through atom probe tomography and first-principles calculations, revealing that hydrogen, a byproduct of paint film pyrolysis, accumulates at copper grain boundaries, inducing embrittlement by reducing adhesion energy. We introduce two innovative strategies to mitigate hydrogen-induced embrittlement: vacuum pyrolysis and enhanced N2 flow. These approaches significantly reduced hydrogen content from 11.1 % to about 5 % and increased elongation from 2.65 % to 15 %. Moreover, pyrolysis efficiencies of 92.79 % and 91.4 % were achieved, respectively, at temperatures 50 °C lower than conventional methods. Our findings provide crucial theoretical insights into embrittlement and offer effective recovery strategies for copper from solid waste.

Codoped germanene with 3p and 4p elements elements.

The relentless need for new materials to be used in electronic devices has opened new research directions in materials science. One of them involves using two-dimensional materials, among which there is current interest in using germanene. The heteroatom doping of germanene has been proposed as a possible approach to fine-tuning its electronic properties. However, this procedure is complicated because locating the dopants with a specific arrangement is challenging, thus achieving reproducibility. To avoid this problem, we propose the codoping of germanene to understand if dopants prefer to be agglomerated as observed for graphene or if they prefer to adopt a random disposition. Herein, we employed first-principles calculations to study 21 codoped germanene systems with one 3p (Al, Si, P, and S) and one 4p (Ga, As, and Se) element. Our results indicate that in the cases of AlP, AlS, GaP, GaS, GaAs, and GaSe codoped germanene, the dopants show a tendency to be located in specific lattice positions. The ortho disposition of dopants is preferred for AlP, AlS, GaP and GaS codoped germanene and their 4p counterparts GaAs and GaSe codoped germanene, and the materials showed interesting electronic propertiesmaking them suitable to develop germanene-based electronic materials. We utilized the M06-L, HSE06 methods accompanied by the 6-31G* basis sets to perform periodic boundary conditions calculations as implemented in Gaussian 09. The unit cells were sampled employing 100k-points for geometry optimizations and 2000k-points for electronic properties The ultrafine grid was employed. Results were visualized employing Gaussview 5.0.1. In addition to this, we performed B3LYP-D3 periodic calculations as implemented in CRYSTAL17.

Charge Scaling in Classical Force Fields for Lithium Ions in Polymers.

Polymer electrolytes are of interest for applications in energy storage. Molecular simulations of ion transport in polymer electrolytes have been widely used to study the conductivity in these materials. Such simulations have generally relied on classical force fields. A peculiar feature of such force fields has been that in the particular case of lithium ions (Li+), their charge must be scaled down by approximately 20% to achieve agreement with experimental measurements of ion diffusivity. In this work, we present first-principles calculations that serve to justify the charge-scaling factor and van der Waals interaction parameters for Li+ diffusion in poly(ethylene glycol) (PEO) with bistriflimide (TFSI-) counterions. Our results indicate that a scaling factor of 0.79 provides good agreement with DFT calculations over a relatively wide range of Li+ concentrations and temperatures, consistent with past reports where that factor was adjusted by trial and error. We also show that such a scaling factor leads to diffusivities that are in quantitative agreement with experimental measurements.

The stability, electronic and optical properties of nonmetal doped g-GaN: A first-principles calculation

Doping is an effective strategy to modulate the electronic performance of materials by forming new chemical bonds and relaxing the neighboring bonds, which may change catalytic performance of materials. Herein, we demonstrate the effects of a series of nonmetal (NM) dopants on the electronic properties and photocatalytic activity of g-GaN monolayer using first-principle calculations. NM dopants prefer to substitute N atom under Ga-rich condition. C, O and F doped specimens are highly stable under both Ga-rich and N-rich conditions. NM dopants induce the generation of impurity levels, contributing to reduce the electronic transition energies. S, Se and Te doped specimens increase by about 11, 8 and 4 times for absorption intensity in the region of visible light, respectively. Remarkably, S, Cl, Se, Br, Te and I dopants can effectively decrease the recombination rate of photogenerated electrons and holes of the g-GaN in photocatalytic reaction. H, B, C Si, P and As doped system can induce more active sites. Remarkably, halogen dopants could increase the both redox and reduction ability of g-GaN monolayer. Thus, NM dopants can effectively tune redox potential of g-GaN monolayer and improve its photocatalytic performance.

Precisely regulated crystalline phase of TiO2@composite G doped with Al3+: For the preparation of highly conductive, corrosion resistant conductive coatings

The development of conductive materials is crucial to the upgrading of modern science and technology industries. In recent years, the research on conductive materials synthesized based on TiO2 has mainly focused on improving the conductivity and stability of the materials. And this study, based on the high conductivity of the material, innovatively proposes a method to precisely regulate the crystal phase and morphology of TiO2 by CH3COOH and temperature together. Meanwhile, the heterostructure was successfully built by doping Al3+ into the TiO2 lattice, and a new conductive pathway (Al-O-C) was constructed. Finally, the composite G was prepared as an Al3+-doped TiO2 composite G (Al-T/G) functional material, and it was verified by first-principles calculations (DFT). The experimental results show that the Al-T/G material not only has the low surface resistance and high whiteness of TiO2, but also is endowed with more excellent resistivity (0.203 Ω·cm) and protection efficiency (92.45 %). Meanwhile, compared with the conventional antimony-doped tin dioxide (ATO) conductive materials, the Al-T/G functional materials prepared in this paper are basically harmless to the environment in the experimental process while ensuring high conductivity. Most importantly, this paper explores the bonding mechanism of Al3+-doped TiO2 conductive materials, which provides new insights and ideas in the field of dopant-modified TiO2 conductive materials.

Intercalated Architecture of the Ca2A2Z5 Monolayer with High Electron Mobilities and High Power Conversion Efficiencies.

The exploration of novel two-dimensional (2D) materials with a direct band gap and high mobility has attracted huge attention due to their potential application in electronic and optoelectronic devices. Here, we propose a feasible way to construct multiatomic monolayer Ca2A2Z5 (A = Al and Ga and Z = S, Se, and Te) by first-principles calculations. Our results indicated that the energies of α1-phase Ca2A2Z5 are slightly lower than those of experimentally synthesized α3-phase-like Ca2A2Z5 monolayers with excellent structural stability. Moreover, the α1- and α3-phase Ca2A2Z5 monolayers possess not only direct band gaps but also high electron mobilities (up to ∼103 cm2 V-1 s-1), demonstrating an intriguing range of visible light absorption. Importantly, α1- and α3-phase Ca2Ga2Se5 monolayers are good donor materials, and the corresponding Ca2Ga2Se5/ZrSe2 type-II heterostructures exhibit desirable power conversion efficiencies of 22.4% and 22.9%, respectively. Our findings provide a feasible way to explore new 2D materials and offer several Ca2A2Z5 candidate monolayers for the application of high-performance solar cells.

Pressure-induced magnetic and topological transitions in non-centrosymmetric MnIn2Te4.

In recent years, the study of magnetic topological materials, with their variety of exotic physics, has significantly flourished. In this work, we predict the interplay of magnetism and topology in the non-centrosymmetric ternary manganese compound MnIn$_2$Te$_4$ under external hydrostatic pressure, using first-principles calculations and symmetry analyses. At ambient pressure, the ground state of the system is an antiferromagnetic insulator. With the application of a small hydrostatic pressure ($\sim$0.50 GPa), it undergoes a magnetic transition, and the ferromagnetic state becomes energetically favorable. At $\sim$2.92 GPa, the ferromagnetic system undergoes a transition into a Weyl semimetallic phase, which hosts multiple Weyl points in the bulk. The presence of non-trivial Weyl points have been verified by Wilson bands computations and the presence of characteristic surface Fermi arcs. Remarkably, we discover that the number of Weyl points in this system can be controlled by pressure and that these manifest in an anomalous Hall conductivity (AHC). In addition to proposing a new candidate magnetic topological material, our work demonstrates that pressure can be an effective way to induce and control topological phases, as well as AHC, in magnetic materials. These properties may allow our proposed material to be used as a novel pressure-controlled Hall switch.

Coexistence of multiple magnetic interactions in oxygen-deficient V2O5 nanoparticles

This paper reports on the spin glass-like coexistence of competing magnetic orders in oxygen-deficient V2O5 nanoparticles with a broad size distribution. X-ray photoelectron spectroscopy yields the surface chemical stoichiometry of nearly V2O4.65 due to significant defect density. Temperature-dependent electrical conductivity and thermopower measurements demonstrate a polaronic conduction mechanism with a hopping energy of about 0.112 eV. The V2O5-δ sample exhibits strong field as well as temperature-dependent magnetic behaviour when measured with a SQUID magnetometer, showing positive magnetic susceptibility across the temperature range of 2-350 K. Field-cooled and zero-field-cooled data indicate hysteresis, suggesting glassy behaviour. The formation of small polarons due to oxygen vacancy defects, compensated by V4+ charge defects, results in Magneto-Electronic Phase Separation (MEPS) and various magnetic exchanges, as predicted by first-principle calculations. This is evidenced by the strong hybridisation of V orbitals in the vicinity of vacant oxygen site. An increase in V4+ defects shows an antiferromagnetic (AFM) component. The magnetic diversity in undoped V2O4.9 originates from defect density and their random distribution, leading to MEPS. This involves localised spins in polarons and ferromagnetic (FM) clusters on a paramagnetic (PM) background, while V4+ dimers induce AFM interactions. Electron paramagnetic resonance spectra measured at different temperatures indicate a dominant paramagnetic signal at a g-value of 1.97 due to oxygen defects, with a broad FM resonance-like hump. Both signals diminish with increasing temperature. Neutron diffraction data rules out long-range magnetic ordering, reflecting the composition as V2O4.886. Despite the FM hysteresis, no long-range order is observed in neutron diffraction data, consistent with the polaron cluster-like FM with MEPS nature. This detailed study shall advance the understanding of the diverse magnetic behaviour observed in undoped non-magnetic systems.

Insights into effects of Fe doping on phase stability, martensitic transformation, and magnetic properties in Ni-Mn-Ti-Fe all-d-metal Heusler alloys

In this work, the effects of Fe doping on phase stability, martensitic transformation, and magnetic properties of Ni50Mn37.5-xTi12.5Fex (x = 3.125, 6.25, 9.375) all-d-metal Heusler alloys were systematically investigated by first-principles calculations. The results indicate a tendency for doped Fe atoms to aggregate within the Ni50Mn37.5-xTi12.5Fex alloys. As Fe concentration increases, a gradual reduction in both lattice constants and phase stability of austenite and martensite is observed. In the absence or presence of minimal Fe doping, both austenite and martensite exhibit antiferromagnetic behavior. However, at x = 9.375, a transition to a ferromagnetic state is observed in the austenite phase. This transition is attributed to the activation of the ferromagnetic coupling effect in the austenite phase induced by Fe doping in the Ni-Mn-Ti alloy. In contrast, the martensite phase maintains its antiferromagnetic characteristics throughout the doping range. A comprehensive analysis of the electronic structure elucidates the underlying physical mechanisms responsible for the martensitic transformation and magnetic property changes.

Phonon Dynamics in MnPSe3 Studied by Raman Spectroscopy and First-Principles Calculations

Phonon Dynamics in MnPSe₃ Studied by Raman Spectroscopy and First-Principles Calculations

Investigation on Be, Mg, Ca, B, Ga, C, Si, and Ge atoms doped on Al (111) surface and their effect on oxygen adsorption: A first-principle calculation

Oxygen corrosion of the aluminum alloy has a substantial impact on the durability of engineering materials and equipment. To better understand how various alloy elements affect oxygen corrosion, we used first-principle DFT method to study the role of each given dopant atom (Be, Mg, Ca, B, Ga, C, Si, and Ge) in the formation of surface oxide film on the Al (111) surface, in the case replacing a surface Al atom. Our calculations show that the electron gain or loss abilities of these alloy elements differ from that of Al metal, with Be, B, C, and Si atoms tending to penetrate into the surface, Mg, Ca, and Ga atoms protruding, and Ge atom just embedded into the Al (111) surfaces. Additionally, the type of doping element at the Al (111) surface can greatly affect the O2 absorption, predicting that Ca-doped surface absorb the least amount of oxygen, followed by Si-doped surface, with Be-, Mg-, B-, Ga-, C-, and Ge-doped surfaces showing higher O2 absorption. Based on adsorption energy (Eads) and partial density of states (PDOS) analysis, we conclude that the doped Al (111) surfaces can make O2 adsorption different owing to the difference in electronic properties of the doping atoms. These insights draw a greater knowledge of the role of alloy elements in the oxygen corrosion of Al alloys, enabling guidance for designing more corrosion-resistant materials.

Superconductivity in Monolayer Carbon Allotropes with High Thermal Stability.

Intrinsic superconductivity is rarely discovered in sp2-hybridized monolayer carbon allotropes. Here we design a carbon monolayer configured of pentagon, heptagon, and hexagon rings with p2 plane group symmetry. Full-sp2 hybridization is proposed to favor thermal metastability on a low Gibbs free energy. The extremely small thermal expansion coefficient is predicted to the turn negative value to positive with elevating temperature. Carbon polygon structures remain intact at a high thermal temperature of 3,000 K. The high specific surface area is found to approach 2,700 m2/g, with O2-adsorption being advantageous over pristine graphene. We reveal electronic Fermi surfaces mediated by phonon modes of carbon out-of-plane vibrations. By calculating the Eliashberg equation, we evaluate intrinsic superconductivity with a large electron-phonon coupling coefficient. The superconducting transition temperature is estimated to reach 20 K under a high logarithmic average frequency. These first-principles calculations shall stimulate experimentalists' interest in exploring low-dimensional carbon superconductors with gas sensitivity.

The effect of solute atom concentration on the generalized planar fault energy and twinnability of basal plane in magnesium alloys

The influence of 39 solute atoms under uniform distribution on the generalized plane fault energy (GPFE) and twinnability of the basal plane in Mg solid solutions at various concentrations has been investigated through first-principles calculations. Firstly, it is revealed that the interaction between solute atoms and the GPFs on basal plane of Mg extends beyond the fault layer to encompass multiple atomic layers. Moreover, it is observed that as the distance from the fault layer increases, the interaction energy is decreased. And the increase in the total interaction energy between the solute atoms and the deformed intrinsic stacking fault (SF) I2 results in a corresponding enhancement of their interaction with other key points of GPF, namely UI2, T2, and UT2, establishing a roughly linear relationship. Finally, based on this interaction, we employ a uniform distribution model to investigate the influence of solute atom concentration on the twinnability of basal plane in Mg solid solution alloys. It is observed that with an increase in solute atom concentration, several conventional elements such as K, S, Sr, Zr, As, as well as most rare earth (RE) elements including Y, Pr, La, Nd, Eu, Gd, Dy, Ce, Tb, Ho, etc., gradually enhanced the twinnability of Mg solid solution alloys. Conversely, certain elements like B, P, Cs, Se, Er, Tl, etc., were found to diminish the twinnability of Mg alloys. This study can provide valuable insights for designing high-strength and toughness Mg alloys.

Diffusion behavior and mechanical properties of the Mg-Er binary system

The Mg-Er binary system, as a new lightweight alloy material system with important research value and prospects for development and application, has attracted much attention from researchers in the field of materials science. Studies have shown that the addition of the rare earth element Er can improve the mechanical strength and heat resistance of magnesium alloys. However, the specific mechanism of the interaction between Mg and Er and its effect on the mechanical properties still need to be studied in depth. A combination of microscopic morphology characterization and nanoindentation experiments were employed to investigate the binary diffusion behavior and mechanical properties of the Mg-Er system between 748 K and 823 K in this research. The results indicate that the growth constant of the diffusion layer increases with temperature, with Mg24Er5 exhibiting a higher growth constant than the sum of Mg2Er and MgEr. Notably, the Mg2Er phase has the lowest growth activation energy (127.81 kJ/mol), which suggests that it is formed first during the initial growth stage due to its relatively low energy barrier. Moreover, the interdiffusion coefficient of Mg24Er5 was found to be the highest. By calculating diffusion activation energy, this sequence of formation (Mg2Er→Mg24Er5→MgEr) is in agreement with the trend of growth activation energy. Additionally, nanoindentation experiments and first-principles calculations were utilized to derive the mechanical properties of the Mg-Er binary system. The investigations unveil that Mg2Er boasts the highest hardness and Young’s modulus. These findings have significant implications for understanding the correlation between structure and properties for the Mg-Er alloy system.

Thermodynamic assessment of the Co-Cr-Fe-Ni quaternary system and diffusion study of its fcc phase

As a core of the high entropy alloys, the Co-Cr-Fe-Ni system has been widely investigated. In the present work, the thermodynamics of the Co-Cr-Fe-Ni system and the atomic mobilities of its fcc phase have been evaluated by means of the CALPHAD approach. First-principles calculations were performed to obtain the total energies for the end-member compounds of the σ phase in the Co-Cr-Fe-Ni system. Combining with the experimental data and thermodynamic modeling of the sub-systems from the literature, a set of self-consistent thermodynamic parameters were derived and extrapolated to obtain a thermodynamic description of the Co-Cr-Fe-Ni quaternary system. In order to verify the accuracy of the model parameters, the phase equilibria of a series of the CoCrxFeNi alloys with different Cr contents were determined using DSC, BSE and XRD analysis. Subsequently, based on the diffusion experimental data, the atomic mobilities of the fcc Cr-Fe-Ni alloys were reassessed using the DICTRA software. A mobility database for the fcc Co-Cr-Fe-Ni quaternary system was constructed by directly extrapolating the atomic mobilities of all sub-systems, and comprehensive comparisons prove the consistency between the present assessments and the experiments. In addition to the direct extrapolation approach, extra four-body interaction parameters concerning all four components were added and assessed. The results demonstrate that the extra interaction contributions are ignorable, so that the direct extrapolation from the sub-systems to the quaternary system is feasible in the fcc Co-Cr-Fe-Ni quaternary system.

First-principles calculations of optical and positron annihilation properties of NV center in 3C-SiC

We studied the optical and positron annihilation properties of NCVSi in 3C-SiC using first-principles calculations. We found that the charge-state transition and the spin-conserving optical transition levels are very close to each other. In order to distinguish between the NV center and intrinsic defect, we calculate in detail the defect formation energies, temperature-dependent positron lifetimes, and electron-positron momentum distribution of these defects in 3C-SiC. We suggest that using positrons technique alone is insufficiently sensitive in identifying the charge-states of NCVSi, necessitating the use of additional characterization methods to address this issue.