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Metal catalysts play an important role in the catalytic electrochemical processes and optimization of their performance is usually achieved through alloying with other metal atoms. Doping with interstitial hydrogen atoms is a special but effective way to regulate the electronic structure of host catalysts. Herein we demonstrate the intermixing of Pd and Rh atoms during the hydrogen-doping process of Pd@Rh core-shell nanocubes, forming an alloyed surface in Pd@Rh-H. The catalysts show enhanced performance in electrocatalytic methanol oxidation, as compared to commercial Pt/C and are even better than PdH@Rh core-shell nanocubes. The small structural differences between the two hydride catalysts are revealed by X-ray electron spectroscopy and pair distribution function analysis of electron diffraction. The theoretical calculation results show that Rh in Pd@Rh-H contains more negative charges than Rh in PdH@Rh, indicating more effective charge transfer in Pd@Rh-H. The d-band center (ed) of the Rh site in Pd@Rh-H shifts up, and the synergy between Rh and Pd optimizes the binding energy of CO and OH, inducing preferential catalytic activity. Our work provides guidance for the synthesis of high-performance catalysts by doping with interstitial atoms, which may provide a new strategy to fine-tune the electronic structure of other bimetallic nanoparticles.

Ammonia generation was studied in the reaction between water and nitrogen-containing iron at 323 K and atmospheric pressure. Similar to metallic Fe, the interstitial compound Fe3N reduced water through Fe oxidation to produce hydrogen gas, while the N combined with atomic hydrogen to produce ammonia as a byproduct. The addition of carbon dioxide to this system accelerated the reaction with concomitant consumption of carbon dioxide. The promoted ammonia production upon addition of carbon dioxide can be attributed to the generation of atomic hydrogen from the redox reaction of carbonic acid and Fe, as well as removal of used Fe from the reaction system through the formation of a soluble carbonato complex. When carbonate was added to the reaction system, the production rates of ammonia and hydrogen increased further. The results here confirmed that ammonia can be synthesized from iron nitride under mild conditions by utilizing carbon dioxide.

We build the theory of diffusion for FCC binary interstitial alloy under pressure based on the statistical moment method, where there are the analytic expressions of the jumping frequency of interstitial atom, the effective jumping length, the correlation factor, the diffusion coefficient, and the activated energy. In limit cases, we can obtain the diffusion theory for FCC metal A under pressure.

Nanohybrids have shown considerable promise for hydrogen storage due to their enhanced capacity and fast kinetics. Hollow nanostructures have attracted considerable attention in diverse contexts and recently have been envisaged as nanocontainers for hydrogen storage. In the current work a two-scale novel hollow nanohybrid is synthesized, wherein the outer Ni ‘capsule’ plays a catalytic role in the breakdown of hydrogen molecule into atomic form and the inner Mg hollow structure enhances the hydrogen storage via a multi-mode storage philosophy developed recently. The structure of the dual-phase nanohybrid is elucidated using transmission electron microscopy and selected area diffraction patterns and characterized for hydrogen storage via pressure-composition-isotherms. The system shows good hydrogen storage capacity (∼5 wt% at 10 bar & 180 °C); coupled with fast kinetics. The hydrogen is stored in an additional form than that reported earlier (interstitial compound).

The mean nearest neighbor distance between two atoms, the Helmholtz free energy and characteristic quantities for elastic deformation such as elastic moduli E, G, K, and elastic constants C11, C12, C44 for binary interstitial alloys with BCC structure under pressure are derived from the statistical moment method. The numerical calculations for interstitial alloy FeSi are performed by combining the Mie-Lennard-Jones potential and the model atomic potential. Our calculated results are compared with other calculations and the experimental data.

A statistical moment method has been applied to study the melting of defective face-centered cubic (FCC) interstitial alloy γ-FeC under pressure up to 100 GPa. The theory of melting is based on the absolute stability limiting condition of the crystalline state. The relationship between the pressure, temperature, and concentration of equilibrium vacancies at the melting point of alloy γ-FeC is discussed and evaluated. The numerical results in limit cases are in good agreement with experiments and calculations by other authors, allowing reliable prediction of the melting features of alloy γ-FeC at high pressures.

Taking the model of interstitial alloy AB with a body-centred cubic structure and the condition of absolute stability for the crystalline state, we derive analytic expression for the temperature of the limit of absolute stability for the crystalline state, the melting temperature, and the equation for the melting curve of this alloy using the statistical moment method. The results allow us to determine the melting temperature of alloy AB under pressure as well as at zero pressure. In limit cases, we obtain the melting theory of main metal A with a body-centred cubic structure. The theoretical results are numerically applied for alloys FeH, FeSi and FeC using different potentials.

From the model of interstitial alloy AB with BCC structure and the condition of absolute stability for crystalline state we derive analytic expression for the temperature of absolute stability for crystalline state, the melting temperature and the equation of melting curve of this alloy by the way of applying the statistical moment method. The obtained results allow us to determine the melting temperature of alloy AB at zero pressure and under pressure. In limit cases, we obtain the melting theory of A main metal with BCC structure. The theoretical results are numerically applied for alloys FeH and FeC with using different potentials.

The statistical moment method is used to study the melting of defective interstitial AB alloys, where A is the main element and B is an interstitial atom, with a body-centered-cubic (BCC) structure. The melting temperature of the AB alloy with defects is obtained from the temperature of absolute stability for the crystalline state and the equilibrium vacancy concentration. Numerical calculations are performed for the interstitial alloys TaSi and WSi. Our calculated results are in good agreement with other calculations.

In this study, the mean nearest neighbor distance between two atoms, the Helmholtz free energy and characteristic quantities for elastic deformation such as elastic moduli E, G, K and elastic constants C11, C12, C44 for binary interstitial alloys with FCC structure under pressure are derived with the statistical moment method. The numerical calculations for interstitial alloy AGC were performed by combining the Mie-Lennard-Jones potential and the Morse potential. Our calculated results were compared with other calculations and the experimental data.

The objective of this study is to determine the analytic expressions of the Helmholtz free energy, the equilibrium vacancy concentration, the melting temperature, the jumps of volume, enthalpy the mean nearest neighbor distance and entropy at melting point, the Debye temperature for the BCC defective, the limiting temperature of absolute stability for the crystalline state, and for the perfect binary interstitial alloy. The results obtained from the expressions are combined with the statistical moment method, the limiting condition of the absolute stability at the crystalline state, the Clausius–Clapeyron equation, the Debye model and the Gruneisen equation. Our numerical calculations of obtained theoretical results were carried out for alloy WSi under high temperature and pressure. Our calculated melting curve and relation between the melting temperature and the silicon concentration for WSi are in good agreement with other calculations. Our calculations for the jumps of volume, enthalpy and entropy, and the Debye temperature for WSi predict and orient experimental results in the future.

The article discusses previously obtained results of experimental studies of multi-component alloys, in which all components are elements of substitution. It adduces the arguments that the formation of diffusion pairs in the liquid state of the alloy is the reason that causes the very possibility of the decomposition of multi-component substitution alloys. It is shown that in substitution-intеrstitial alloys such diffusion pairs are formed only between substitution atoms, the interstitial atoms do not participate in this process in view of their very high diffusion rate at high temperatures. Electron microscopic data obtained in the article show how the ternary Fe-22% Cr-1.1% N alloy decomposes when the temperature of its heating is reduced from the liquid state to the aging temperature.

This work relates to the field of thermophysical parameters of refractory interstitial alloys. The isochoric heat capacity of cubic titanium carbide TiCx has been calculated within the Debye approximation in the carbon concentration range x = 0.70–0.97 at room temperature (300 K) and at liquid nitrogen temperature (80 K) through the Debye temperature established on the basis of neutron diffraction analysis data. It has been found out that at room temperature with decrease of carbon concentration the heat capacity significantly increases from 29.40 J/mol·K to 34.20 J/mol·K, and at T = 80 K – from 3.08 J/mol·K to 8.20 J/mol·K. The work analyzes the literature data and gives the results of the evaluation of the high-temperature dependence of the heat capacity СV of the cubic titanium carbide TiC0.97 based on the data of neutron structural analysis. It has been proposed to amend in the Neumann–Kopp formula to describe the high-temperature dependence of the titanium carbide heat capacity. After the amendment, the Neumann–Kopp formula describes the results of well-known experiments on the high-temperature dependence of the heat capacity of the titanium carbide TiCx. The proposed formula takes into account the degree of thermal excitation (a quantized number) that increases in steps with increasing temperature.The results allow us to predict the thermodynamic characteristics of titanium carbide in the temperature range of 300–3000 K and can be useful for materials scientists.

The analytic expressions for the thermodynamic and elastic quantities such as the mean nearest neighbor distance, the free energy, the isothermal compressibility, the thermal expansion coefficient, the heat capacities at constant volume and at constant pressure, the Young modulus, the bulk modulus, the rigidity modulus, and the elastic constants of binary interstitial alloy with body-centered cubic (BCC) structure, and the small concentration of interstitial atoms (below 5%) are derived by the statistical moment method. The theoretical results are applied to interstitial alloy FeC in the interval of temperature from 100 to 1000 K and in the interval of interstitial atom concentration from 0 to 5%. In special cases, we obtain the thermodynamic quantities of main metal Fe with BCC structure. Our calculated results for some thermodynamic and elastic quantities of main metal Fe and alloy FeC are compared with experiments.

Grain boundary (GB) segregation in nanocrystalline alloys can cause reduction of GB energy, which leads to thermodynamic stabilization of nanostructures. This effect has been modelled intensively. However, the previous modelling works were limited to substitutional alloy systems. In this work, thermodynamics of nanocrystalline binary interstitial alloy systems was modelled based on a two-sublattice model proposed by Hillert [M. Hillert, et al. Acta Chem. Scand., 24 (1970) 3618] and an atomic configuration for nanocrystalline systems proposed by Trelewicz and Schuh [J.R. Trelewicz, et al. Physical Review B, 79 (2009) 094112]. The modelling calculations agree with the reported experimental data, indicating that the current thermodynamic model is capable of accounting for the alloying effect in the nanocrystalline binary interstitial alloys.

From the model of interstitial alloy AB with FCC structure and the condition of absolute stability for crystalline state we derive analytic expression for the temperature of absolute stability for crystalline state, the melting temperature and the equation of melting curve of this alloy by the way of applying the statistical moment method. The obtained results allow us to determine the melting temperature of alloy AB at zero pressure and under pressure. In limit cases, we obtain the melting theory of main metal A with FCC structure. The theoretical results are numerically applied for alloys AuSi and AgSi.

Metal nitrides have great potential for electrochemical energy storage, but are relatively scarcely investigated. Herein, a novel metal nitride, Co3N, is prepared by nitridation using a Co3O4 precursor heated at 500 °C for 2 h in flowing NH3, and characterized by using X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS) methods. Besides, the electrochemical properties of Co3N are investigated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). The as-prepared Co3N demonstrates remarkable electrochemical performance with a high specific capacitance of 112.3 F g−1 at a current density of 0.5 A g−1, good rate capability (72.4%, from 0.5 to 5 A g−1) and superior cycling stability (109.7% retention over 10 000 cycles at a current density of 2 A g−1). Finally, a Co3N//activated carbon (AC) asymmetric supercapacitor (ASC) is successfully assembled by using Co3N and AC as the positive electrode and negative electrode, respectively. The as-fabricated ASC device can achieve a maximum energy density of 12.1 W h kg−1 at a power density of 204.6 W kg−1, which shows that the Co3N material should be a potential electrode material for supercapacitors.

The diffusion of interstitial oxygen (O) in fcc aluminum (Al) has been studied using first-principles and the diffusion coefficient has been calculated. Whereas interstitial atoms in fcc systems are typically found to hop directly between interstitial centres, the diffusion pathway for interstitial O in fcc Al was calculated to have a curved minimum energy pathway with an energy barrier of 0.95 eV. The barrier was found to be off-centre of a neighboring octahedral site. Also unlike the majority of fcc metals, O prefers to sit in the tetrahedral interstitial site as opposed to the octahedral site. The calculated O diffusion coefficient is on the same order of magnitude of the diffusion coefficient of O in other fcc metals. The preferred interstitial site, diffusion pathway and vacancy binding energy were found to be related to the bond length of O with neighboring Al atoms.

The self-interstitial carbon is the key to radiation damage in graphite moderator nuclear reactor, so an understanding of its behavior is essential for plant safety and maximized reactor lifetime. The density functional theory is applied on four different graphite unit cells, starting from of 64 carbon atoms up to 256 carbon atoms, using AIMPRO code to obtain the energetic, athermal and mechanical properties of carbon interstitial in graphite. This study presents first principles calculations of the energy of formation that prove its high barrier to athermal diffusion (1.1 eV) and the consequent large critical shear stress (39 eV–50 eV) necessary to shear graphite planes in its presence. Also, for the first time, the gamma surface of graphite in two dimensions is calculated and found to yield the critical shear stress for perfect graphite. Finally, in contrast to the extensive literature describing the interstitial of carbon in graphite as spiro interstitial, in this work the ground state of interstitial carbon is found to be canted spiro interstitial.

In this paper, the effects of hydrogen interstitial defect on the structural stability of two kinds of MAX materials (Ti 3 SiC 2 and Ti 3 AlC 2 ) were investigated by first-principles calculations. The results indicated that the hydrogen interstitial energetically prefers to reside at the 2Ti3Si site for Ti 3 SiC 2 and 3TiAl site for Ti 3 AlC 2 , respectively, and the latter has much lower formation energy. Both of these MAX phases are slightly hardened and the elastic anisotropy is reduced appreciably after the introduction of hydrogen interstitial. The hydrogen interstitial in Ti 3 SiC 2 and Ti 3 AlC 2 leads to an electronic localization effect on the Si/Al atom and the effect is more remarkable in Ti 3 AlC 2 . The interlayer bonding strength of Ti 3 AlC 2 is more weakened by hydrogen interstitials than that of Ti 3 SiC 2 . As a result, the interatomic bonding between Si/Al and Ti atom layers is deteriorated and their structural stabilities degrade subsequently.

Complex investigations have been carried out of the radiation damage of nickel alloys microalloyed with interstitial carbon and boron impurities under cascadeless electron and cascade neutron irradiation. Boron’s contribution to the resistivity has been found and the binding energy of a vacancy with boron atom has been determined to be 0.2 eV. The main mechanisms of radiation-induced structural and phase changes have been revealed.

The effect of the volume size factor of solute atoms and their clusters on the one-dimensional motion of interstitial clusters was studied by computer simulations. Solute atoms were placed on the migration path of the interstitial clusters, and the change in the migration energy of the interstitial clusters was obtained by the static method. The volume size factor of Au atoms in a Ni-Au binary alloy was systematically changed from −30% to +100%. When the volume size factor of the solute atoms was larger or smaller than Ni atoms, the migration energy of the interstitial clusters increased. The average migration energy required for the interstitial clusters to pass through one, two, three, and four solute atoms (volume size factor: +63.7%) was 0.57, 1.28, 2.09, and 2.47 eV, respectively. If the solute atom clusters grow and their density is sufficiently high, the jump frequency of the interstitial clusters is so low that the interstitial clusters cannot move through the solute atom clusters under an irradiation temperature of 573 K.

The bonding properties of the twin boundary in polysynthetic twinned &gamma, TiAl crystal and the effect of interstitial alloy elements on it are investigated by first principles. Among the three different kinds of interface relationships in the &gamma, /&gamma, interface, the proportion of true twin boundaries is the highest because it has the lowest interfacial energy, the reason for which is discussed by local energy and three-center bond. The presence of the interstitial atoms C, N, H, and O induces the competition for domination between their affinity to host atoms and three-center bonds, which eventually influences the values of unstable stacking fault energy (USFE) and intrinsic stacking fault energy (ISFE). The relative importance of different bonding with different alloy elements is clarified based on the analysis of local energy combined with Electron Localization Function (ELF) and Quantum Theory of Atoms in Molecules (QTAIM) schemes.

We present the results of ab initio modeling of structure of dilute Ti-Fe, a typical representative of quenched Ti-based transition-metal alloys. We have demonstrated that beyond the solubility limit this alloy cannot be described in common terms of substitutional and interstitial alloys. Instead, very stable local clusters are formed in both low-temperature hcp and high-temperature bcc phases of alloys, with almost identical local structures. This gives an example of geometrically frustrated state and explains unusual concentration behavior of M\"ossbauer spectra discovered long ago for this system., Comment: 11 pages 5 figures accepted in J. Phys.: Materials

The fundamental understanding and precise tuning of interatomic s,p-d orbital hybridization are critical to the design of interstitial alloy catalysts containing light constituent atoms. Here, we present a joint theoretical and experimental study that reveals the importance of distribution, concentration and ordering of interstitial boron atoms in Pd-B alloy’s near-surface electronic structure and catalytic performance. Our theoretical results demonstrate that the sub-surface location of interstitial boron atoms with a Pd:B atomic ratio of 2:1 is necessary to ensure an appropriate degree of interatomic s,p-d orbital hybridization, and thereby an optimal surface electronic structure for the hydrogen evolution reaction (HER). We experimentally achieve this with highly-crystalline intermetallic Pd2B. Due to its optimal electronic structure and ordered arrangement of interstitial boron atoms, the intermetallic Pd2B exhibits Pt-like catalytic activity for HER and has excellent catalytic stability for over seven days.

Hexagonal e-iron nitride and e-iron carbonitride phases are formed on nitriding and nitrocarburizing of iron and steel surfaces and can exist in broad compositional ranges. Long-range nitrogen ordering and magnetic properties for e-iron nitrides and their dependence on composition have been the focus of several studies. So far, limited attention has been paid to the carbonitrides. In the current work, the effects of substitution of nitrogen by carbon on the interstitial ordering and magnetic properties in Fe2(C,N)1-z are explored using neutron diffraction, Mossbauer spectroscopy and vibrating sample magnetometry. Neutron diffraction patterns showed 001 and 301 superstructure reflections, confirming a previously proposed structural model in space group P 3 ¯ 1 m (compared to P6322 for the pure nitrides). On partial substitution of nitrogen by carbon in e-iron nitride the Curie temperature, the saturation magnetization and the hyperfine fields of the iron atoms are increased, while isomer shifts are decreased. The effects on the a and c lattice parameters indicate a change in interstitial ordering, which is related to more favorable interactions between a nitrogen and carbon atom than among nitrogen atoms. This interaction leads to additional interstitial (short-range) ordering and a decrease in the c lattice parameter, while the a lattice parameter is largely unaffected.

A single crystal of a new layered telluride KFeCuTe2 has been successfully synthesized by the self-flux method, which is formed by intercalating K in the parent telluride Fe1+xCuTe2. This new compound crystallizes in the space group I4/mmm with the interstitial Fe in pristine Fe1+xCuTe2 completely removed after K intercalation. X-ray photoelectron spectroscopy (XPS) shows that after intercalation, the valence of Fe switched from a mixture of +2 and +3 to the single trivalent one. The KFeCuTe2 is a Mott semiconductor (Eg = 1.06 eV) with a larger resistivity than that of Fe1+xCuTe2 due to the absence of electron doping from interstitial Fe atoms. Most importantly, the magnetic susceptibility shows the antiferromagnetic transition in KFeCuTe2 at 60 K, instead of the spin-glass behavior in Fe1+xCuTe2, indicating the crucial role of interstitial Fe in breaking the long-range magnetic ordering. Our work provides a new compound to study the effect of interstitial Fe on the crystal and magnetic structure in layered tellurides.

The formation energies of hydrogen (H) defects at tetrahedral and octahedral interstitial sites in a pyrochlore-type yttrium–titanium oxide (Y 2 Ti 2 O 7 ) were calculated using density functional theory. The results were compared with the corresponding energies of H and helium (He) defects in iron (Fe) with a body-centered cubic (bcc) crystal structure and Y 2 Ti 2 O 7 interstitial and substitutional sites in the matrix. The formation energy of a H defect at a tetrahedral site in Y 2 Ti 2 O 7 (2.87 eV), which is lower than that at an octahedral site (3.01 eV), was larger than the corresponding energies (0.11, 0.24, and 0.41 eV) at tetrahedral interstitial, octahedral interstitial, and substitutional sites in Fe, respectively. This result indicates that H atoms in nanostructured ferritic alloys, constructed by only bcc Fe containing nanometer-sized Y 2 Ti 2 O 7 nanofeatures, prefer to occupy tetrahedral interstitial sites in Fe, whereas He atoms prefer to occupy octahedral interstitial sites in Y 2 Ti 2 O 7 . The electron-density distribution and density of states of interstitial H in Y 2 Ti 2 O 7 reveal an essentially attractive chemical interaction resulting from charge transfers from neighboring O 2− anions and surrounding Y 3+ and Ti 4+ cations, whereas those of interstitial He reveal a repulsive interaction. Therefore, the doping H and He atoms prefer to occupy smaller tetrahedral and larger octahedral sites, respectively.

Electrochemical reduction of CO to value-added products holds promise for storage of energy from renewable sources. Copper can convert CO into multi-carbon (C2+ ) products during CO electroreduction. However, developing a Cu electrocatalyst with a high selectivity for CO reduction and desirable production rates for C2+ products remains challenging. Herein, highly lattice-disordered Cu3 N with abundant twin structures as a precursor electrocatalyst is examined for CO reduction. Through in situ activation during the CO reduction reaction (CORR) and concomitant release of nitrogen, the obtained metallic Cu° catalyst particles inherit the lattice dislocations present in the parent Cu3 N lattice. The de-nitrified catalyst delivers an unprecedented C2+ Faradaic efficiency of over 90% at a current density of 727 mA cm-2 in a flow cell system. Using a membrane electrode assembly (MEA) electrolyzer with a solid-state electrolyte (SSE), a 17.4 vol% ethylene stream and liquid streams with concentration of 1.45 m and 230 × 10-3 m C2+ products at the outlet of the cathode and SSE-containment layer are obtained.

This work investigated the superelastic response of the low-modulus porous β type Ti-35Nb-2Ta-3Zr scaffolds with different pore dimensions fabricated by laser powder bed fusion. The superelastic behavior was enhanced with increasing the pore size and stress-induced phase transformation, which correspondingly led to stress-induced α" [110]-type I twin martensitic transformation and ω formation adjacent to β matrix/twins. The resultant interstitial compound phase structure facilitated the β → α" and β → ω transition, which was triggered by interfacial stress/strain concentration and high-density dislocations. Substantial high-angle grain boundaries (HAGBs) accumulated high-intensity Schimd factor and crystallographic texture after being deformed. Moreover, a lower Young’s modulus was obtained when the pore size and stress increased.

The analytic expressions of the free energy, the mean nearest neighbor distance between two atoms, the elastic moduli such as the Young modulus E, the bulk modulus K, the rigidity modulus G and the elastic constants C11, C12, C44 for interstitial alloy AB with BCC structure under pressure are derived from the statistical moment method. The elastic deformations of main metal A is special case of elastic deformation for interstitial alloy AB. The theoretical results are applied to alloy FeC under pressure. The numerical results for this alloy are compared with the numerical results for main metal Fe and experiments.

Overall water splitting driven by a low voltage is crucial for practical H2 evolution, but it is challenging. Herein, anion-modulation of 3D Ni-V-based transition metal interstitial compound (TMIC) heterojunctions supported on nickel foam (Ni3 N-VN/NF and Ni2 P-VP2 /NF) as coupled hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts for efficient overall water splitting is demonstrated. The heterointerface in Ni3 N-VN has a suitable H* absorption energy, being favorable for enhancing HER activity with onset overpotential (ηonset ) of zero and Tafel slope of 37 mV dec-1 in 1 m KOH (close to that of Pt/C/NF). For the OER, the synergy of Ni2 P-VP2 with oxide species can give enhanced activity with ηonset of 220 mV and Tafel slope of 49 mV dec-1 . The good activity is ascribed to heterointerface for activating the intermediates, good conductivity of TMICs for electron-transfer, and porous structure facilitation of mass-transport. Additionally, the minimal mutual influence of Ni3 N-VN/NF and Ni2 P-VP2 /NF allows easy coupling for efficient overall water splitting with a low driving voltage (≥1.43 V), a voltage of 1.51 V at 10 mA cm-2 , and remarkable durability for 100 h. It can be driven by a solar cell (1.5 V), indicating its potential to store intermittent energy.

A series of statistical thermodynamic analyses were made since 1974 for different types of non-stoichiometric interstitial compounds MXx under simplifying a priori assumption of constant interaction energy E(X-X) between nearest neighbour interstitial atoms X within a homogeneity composition range of MXx at arbitrary temperature T [K]. Mode of distribution of X atoms in interstitial sites in MXx lattice is represented by number θ of available interstitial sites for occupation by X atoms per M atom and the value of θ is determined to fulfil the a priori assumption. Mode of atomic configuration would yield major contribution to entropy term ∆S that appears in conventional thermodynamic expression of Gibbs free energy of formation, ∆G, in form of T∆S. In the statistical thermodynamic formulation, contribution of tightly bound electron appearing in form of RT ln fX where fX refers to atomic partition function of X atom in the MXx lattice and R the universal gas constant. Judging from this mathematical form of the term, R ln fX is considered to represent entropic contribution from tightly bound electron to X atom in MXx lattice. In the published series of works on statistical thermodynamic analysis for non-stoichiometric interstitial compounds, calculated values for R ln fX were reported but they were not reviewed with serious attention because R ln fX was considered merely as a secondary factor compared to principal factor E(X-M) referring to interaction energy between X and M in MXx lattice that represents enthalpy ∆H in conventional thermodynamic term. In this review article, consideration is given exclusively to the factor R ln fX evaluated in statistical thermodynamic approach to non-stoichiometric interstitial compounds. info:eu-repo/semantics/publishedVersion

Titanium can dissolve unusually high concentrations of interstitial elements such as carbon and nitrogen to form a rich variety of compounds with beneficial structural and functional properties. A first-principles statistical mechanics study was performed to predict phase stability in the Ti-C and Ti-N binaries. Density functional theory calculations were combined with the cluster-expansion approach to determine ground-state carbon-vacancy and nitrogen-vacancy orderings over the octahedral sites of hcp and fcc Ti. A large number of vacancy-ordered rocksalt phases were found to be stable at low temperature. Monte Carlo simulations showed that the ordered rocksalts transform to a disordered rocksalt that can tolerate high vacancy concentrations at intermediate to high temperatures. Clear trends in phase stability, rooted in electronic structure, are revealed upon a comparison of the calculated Ti-C and Ti-N phase diagrams with the Ti-O phase diagrams from a previous study [N. S. H. Gunda et al., Phys. Rev. Mater. 2, 033604 (2018)].

Steels belong to one of the best established materials, however, the mechanisms of various phase transformations down to the nano length scale are still not fully clear. In this work, high-resolution transmission electron microscopy is combined with atomistic simulations to study the nanoscale carbide precipitation in a Fe–Cr–C alloy. We identify a cooperative growth mechanism that connects host lattice reconstruction and interstitial segregation at the growing interface front, which leads to a preferential growth of cementite (Fe3C) nanoprecipitates along a particular direction. This insight significantly improves our understanding of the mechanisms of nanoscale precipitation in interstitial alloys, and paves the way for engineering nanostructures to enhance the mechanical performance of alloys., The specifics of nanoscale precipitation in steels remain complex. Here the authors combine high-resolution microscopy and atomistic simulations to identify a cooperative growth mechanism leading to the preferential growth of cementite nanoprecipitates along a specific crystallographic direction.

Hydride precipitation in 25 nm and 40 nm epitaxial Nb-films was studied by Scanning Tunnelling Microscopy (STM) supported by X-ray diffraction (XRD) measurements. In combination, these methods yield information about the phase transition, the coherency state, the hydride precipitates’ density and size as well as their lateral distribution, at 293 K. For both film thicknesses, hydride formation was detected with STM; it can be easily missed by XRD. While the 25 nm film showed a coherent phase transition, the phase transition was incoherent for the 40 nm film. This is in good accordance with theory. The phase transition features are found to strongly depend on the coherency state: a large number of small hydrides appear in the coherent regime while a small number of large hydrides evolve in the incoherent regime.

The influence of a copper impurity on the spectrum of defects induced in p-Si crystals containing a low oxygen concentration by irradiation with electrons with an energy of 5 MeV at room temperature is studied by deep-level transient spectroscopy. It is found that interstitial carbon atoms (Ci) which are the dominant defects in irradiated samples free of copper are unobservable immediately after irradiation, if the concentration of mobile interstitial copper atoms (Cui) is higher than the concentration of radiation defects. This phenomenon is attributed to the formation of {Cui, Ci} complexes, which do not introduce levels into the lower half of the band gap. It is shown that these complexes dissociate upon annealing at temperatures of 300–340 K and, thus, bring about the appearance of interstitial carbon.

One of the main aims of our work is to obtain general equations for the diffusion fluxes under strain that give the possibility for using these equations at low temperatures, as in this case the strain influence on the diffusion fluxes is manifested in maximal degree. Our approach takes into consideration that the strains can alter the surrounding atom configuration near the jumping atom and consequently the local magnitude of the activation barrier and a rate of atom jumps. The approach is derived under assumptions that the total energy depends on the pair distances only and the attempt frequencies are the same for all jumps. The rates of atom jumps in different directions define the flux density of the defects. Now we take into account that the strain tensor is different at the saddle point and at the rest atom position, that differentiates our approach from previous ones. As a result, general equations for the vacancy fluxes and impurity fluxes are obtained for fcc and bcc metals. These equations differ significantly from those obtained earlier.

Atomic-scale characterization of hydrogen and formation of niobium hydrides, using ultraviolet (wavelength=355 nm) picosecond laser-assisted local-electrode atom-probe tomography, was performed for ultrahigh purity niobium utilizing different laser pulse energies, 10 or 50 pJ/pulse or voltage pulsing. At 50 pJ/pulse, hydrogen atoms migrate onto the 110 and 111 poles as a result of stimulated surface diffusion, whereas they are immobile for 2, and NbH and the atomic concentration ratios H2/H, NbH/Nb, and Nb3+/Nb2+ were calculated with respect to the systematically changing laser pulse energies. The atomic concentration ratios H2/H and NbH/Nb are greater when decreasing the laser pulse energy than when increasing it, because the higher residual thermal energy after decreasing the laser pulse energy increases the mobility of H atoms by supplying sufficient thermal energy to form H2 or NbH.

The structural, electronic, and elastic properties of pristine and carbon-doped boron suboxide (B6O) are calculated using density functional theory. The results indicate that it is energetically preferable for a single carbon atom to substitute into an oxygen site rather than a boron site. The lattice parameters and cell volume increase to relieve the residual stress created by the carbon substitution. The interstitial position is not favorable for a single atom substitution. However, if two carbon atoms substitute for two neighboring oxygen atoms, then it becomes energetically favorable to dope an interstitial oxygen, boron, or carbon atom along the C–C chain. If the interstitial dopant is either boron or carbon, a local B4C-like structure with either a C–B–C or C–C–C chain is created within the boron suboxide unit cell. The resulting structure shows improvements in the bulk modulus at the expense of the shear and Young's moduli. The moduli further improve if an additional carbon is substituted within a polar or equatorial site of the neighboring B12 icosahedron. Based on these calculations, we conclude that carbon doping can either harden or soften B6O depending on the manner in which the substitutions are populated. Furthermore, as B6O samples are often oxygen deficient, C doping can occupy such sites and improve the elastic properties.

The interstitial chemical impurities hydrogen, oxygen, nitrogen, and carbon are important for niobium metal production and particularly for the optimization of niobium SRF technology. These atoms are present in refined sheets and can be absorbed into niobium during processing treatments, resulting in changes to the residual resistance and the performance of SRF cavities. A first-principles approach is taken to study the properties of carbon in niobium, and the results are compared and contrasted with the properties of the other interstitial impurities. The results indicate that C will likely form precipitates or atmospheres around defects rather than strongly bound complexes with other impurities. On the basis of the analysis of carbon and hydrogen near niobium lattice vacancies and small vacancy chains and clusters, the formation of extended carbon chains and hydrocarbons is not likely to occur. Association of carbon with hydrogen atoms can, however, occur through the strain fields created by interstitia...

Kinetic Monte Carlo and molecular dynamics simulations, with interatomic interaction described by an embedded atom method, have been performed to study the Snoek relaxation behavior in FeC alloy. The reorientation of elastic dipole under uniaxial cycle stress is modeled with kinetic Monte Carlo and the corresponding anelastic stain resulted by the reorientation is calculated from molecular dynamics. Internal friction is obtained directly in terms of energy definition. The results indicate the calculated Snoek relaxation profiles are in good agreement with those of experiments and linear point defect theory with characteristic temperature corresponding to Snoek peak being 320 ± 5 K at the stress frequency of 1 Hz. The fitted shape factors of elastic dipole from the concentration-dependent and orientation-dependent Snoek profiles are consistent, with the value of about 0.72. This method can realize microstructural evolution and kinetic process, which is promising for investigating the Snoek relaxation in concentrated interstitial alloys and complex alloy system with substitutional elements.

Using first-principles calculations based on density functional theory, we explore the interstitial boron doping effects on the geometrical and electronic structures of graphitic carbon nitrides (g-C 3 N 4 and g-C 4 N 3 ) and reveal the favorable interstitial boron doping configurations. By analyzing the formation energies of boron-doped systems, we show that boron atoms can be easily doped in g-C 4 N 3 rather than in g-C 3 N 4 . Boron doping significantly changes the electronic structure of g-C 4 N 3 from half-metallic to metallic system and from magnetism to non-magnetism after different degrees of doping. Analysis of the density of states and the spatial distribution of spin-polarized electron densities provides an insight into the mechanism underlying such changes. The conclusions obtained in this study are helpful for future studies of two dimensional carbon nitride or boron carbonitride materials.

Angle-dispersive X-ray diffraction and microfluorescence together with precession electron diffraction (PED) and scanning electron microscopy measurements on iron nitride prepared at 15 GPa and 1800 K from iron and sodium azide starting materials reveal synthesis of both hexagonal P6322 and trigonal P312 Fe3N1+x modifications (a = 4.745 (1) A, c = 4.403 (1) A, Z = 2). Nitrogen access to vacant interstitial sites, repulsions between nitrogen ions and metal nitride thermal stability are the factors relating iron nitride-phase relations to those of other early (Hf, Zr, Ti)-N and late (Ni-N) transition metal nitrides subjected to similar pressure and temperature conditions. Here, Fe3N1+x can accommodate pressure and x variability by situating nitrogen in a broader range of interstitial crystallographic sites in the intimately related hexagonal and trigonal crystal structures.