Theoretical studies of interstitials in graphite

CNDO (Complete neglect of differential overlap) calculations are reported for a model of interstitial C and C₂ species between graphite planes, modelled by two C₁₆H₁₀ graphite like molecules. For a single C atom interstitial the optimum position is about 0.82 Å above the centre of a ring. The interstitial is bonded to one plane with only a weak interaction with the plane above. For the C₂ interstitial, the optimum position is with each C atom 0.83 Å above the ring in adjacent layers. Agreement with experimental values for the migration energy of an interstitial is not found, and may be achieved if deformation of the graphite layers is also taken into account.     

Effect of cation vacancies on the optical and dielectric properties of KSr2Nb5O15: A first-principles study

Using first-principles calculations, the effect of cation vacancies on the electronic structures and optical characters of KSr₂Nb₅O₁₅ (KSN) lead-free ferroelectrics are investigated. The calculated dielectric properties are demonstrated by the experimental results. The cation vacancies involve K⁺ vacancies (KSN-K), Sr²⁺ vacancies (KSN-Sr), and coexisting K⁺ and Sr²⁺ vacancies (KSN-K&Sr). When these cation vacancies exist in KSN, the unit cell volumes decrease, leading to phase transition from tetragonal to orthorhombic, and the cation vacancies show strong effects on the band gap of KSN, declining by 1.46%-9.46%. The optical properties including the static dielectric constants, refraction, and extinction coefficient of KSN-K, KSN-Sr, and KSN-K&Sr increase more than those of KSN without vacancies, but the reflectivity and loss function decrease. All structures with cation vacancies are mainly refractive in the 0-4 eV photon energy range and are reflective at 5-8 eV. The refractivity increases and reflectivity decreases after vacancies occur. KSN-Sr has the largest static dielectric constant while KSN-K&Sr has the smallest values. The dielectric constant can be adjusted in the range of 25% by controlling the cation vacancies. The calculated dielectric properties are in good agreement with the experimental results. The results pave the way to regulate the optical and dielectric properties of lead-free ferroelectrics by controlling different cation vacancies.     

Calculation of the expansion in a graphite crystal by Cn interstitials

Calculations are presented of the change of average interlayer distance in graphite, as a function of the concentration of C_n interstitials, n = 1, 2, 4 and 6. A model of a graphite crystal is considered, in which layers are taken as continuous elastic plates. A variational procedure is used to calculate the deformation of the layers. The deformation energy is calculated with a method due to Coulson and Senent. Interaction energy is expressed in terms of the interlayer potential functions of Drickamer et al., and Santos and Villagrá, the latter obtained from a Thomas-Fermi calculation. The deformation of the layers, and the self-energies of the interstitials are also calculated. The results are in fair agreement with experiment.     

Electronic properties of dehydrogenated nanodiamonds: A first-principles study

First-principles calculations using quantum-mechanical density functional theory (DFT) are carried out to study the geometric structure and electronic properties of dehydrogenated nanodiamonds with diameters varying from 0.8 nm to 1.6 nm. The results show that the electronic properties of dehydrogenated nanodiamond are quite different from those of bulk diamond or hydrogenated nanodiamond. Surface atoms play an important role in the electronic structure, especially the states near the Fermi level, for dehydrogenated nanodiamond. In addition, it has been revealed that the size-dependent feature in the electronic properties for dehydrogenated diamonds is also contributed by the surface effect, in addition to the quantum confinement effect.     

Dielectric and piezoelectric responses of nylon-7: A first-principles study

We have systematically investigated dielectric and piezoelectric responses of single polymeric thread and bulk crystal of nylon-7 using first-principles density functional theory based calculations based on pseudopotentials, plane-wave basis and a generalized gradient approximation (GGA) of exchange correlation energy of electrons. We estimate the strength of inter-chain interaction through the binding energies of the crystal and the thread. Vibrational spectrum of the bulk structure determined here is in good agreement with experimentally available IR and Raman spectra. We demonstrate that large dielectric and piezoelectric responses of nylon-7 arise primarily from the electronic contribution and contribution of phonons is relatively smaller. Our results show that the dielectric response is dependent on the specifics of structural order, suggesting a way of interpreting measured dielectric response of polymers in terms of their ordering.     

Tritium trapping by oxygen and lithium vacancies in Li4TiO4 from first-principles calculations

The Li₄TiO₄ ceramic is a promising solid-state tritium breeder material for the production of tritium fuel in the designs of fusion reactors. Tritium extraction efficiency is one of the key factors that determines the performance of the breeder material. Vacancies are known to trap tritium and adversely affect tritium extraction. Understandings of tritium-trapped defect configurations in Li₄TiO₄ are limited so far. In this work, the oxygen/lithium vacancy (V_O/Li) and the vacancy-tritium defect complex (V_O/Li + T) in Li₄TiO₄ are studied by first-principles density functional theory. The atomic configurations, formation energies and electronic structures of various charged defect species are obtained. We find that 2+ and 0 are the dominate charge states of V_O, 1- is the dominate charge state of V_Li, 1+ is the dominate charge state of the defect complex (V_O + T), and 0 is the dominate charge state of (V_Li + T). Tritium atoms trapped in V_O are bonded to Ti atoms, and those trapped in V_Li are bonded to O atoms.     

Laser-induced desorption of small molecules from oxide surfaces: A first-principles study

Recent efforts in the theoretical simulation of laser-induced desorption of small molecules from surfaces are summarized. As a representative example, photodesorption of CO molecules from a Cr₂O₃(0001) surface is investigated since detailed quantum state resolved experimental results are available for this system. In particular, vectorial properties such as the alignment of the desorbing species are considered. Furthermore, the influence of surface temperature as a control parameter is investigated, and lateral velocity distributions are calculated and compared with experimental results. All simulations presented in the present study are based on ab initio potential energy surfaces (PESs) for the electronic ground state as well as electronically excited states involved in the desorption process. These PESs provide the prerequisite for extensive high-dimensional quantum mechanical simulations of the dynamics of nuclear motion based on a stochastic wave packet scheme. These wave packet calculations allow for a detailed microscopic understanding of experimental results and provide a perspective for future experiments.     

Adsorption and dehydrogenation of methanol on alkali-cation-exchanged zeolite: A first-principles density functional study

The adsorption and dehydrogenation of methanol on an alkali-cation-exchanged zeolite model, M-zeolite (M = Na⁺, K⁺, Rb⁺, and Cs⁺), were studied using first-principles calculations based on density functional theory (DFT). The adsorption energies, geometric structures, and vibrational frequencies of the transition states were computed by full-geometry optimization with a 6MR (membered-ring) cluster model. We have calculated the transition states and adsorption complexes of the reactants, transition states, and products, as well as the corresponding activation barriers and adsorption energies of the numerous reactions involved in these processes. The interaction first leads to the formation of a methanol complex, where the methanol via the oxygen atom and the alkali metal cation of the 6MR. Then, a transition state involves the coordination of two hydrogen bonds. Finally, the adsorbed formaldehyde and hydrogen complex is formed. The calculated results are compared with the data obtained from previous experimental studies.     

基体(TDI-TMP-T313)与氧化剂(AP)相互作用的第一性原理研究

采用密度泛函理论（DFT）研究了氧化剂高氯酸铵（001）、（210）、（011）、（201）四种晶面的表面能并对其进行从头算分子动力学（AIMD）模拟,测试其稳定性。基体组分甲苯二异氰酸酯（TDI）、三羟基甲基丙烷（TMP）、三氟化硼三乙醇胺络合物（T313）在氧化剂晶面的吸附能同样由DFT进行计算。针对基体和氧化剂之间的相互作用进行了化学理论分析,最后选取吸附物和晶面相互作用最强的T313-AP（201）体系,模拟其分子电子结构并观察原子间的电荷转移情况。通过多种理论方法在分子尺度上揭示键合剂（T313）与氧化剂（AP）的作用机制,证实老化过程中产生的关键产物来源。     

Diffusion of a tritium interstitial in Li4TiO4 from first-principles calculations

Li₄TiO₄ is an attractive tritium breeding material due to its high lithium density for the application in fusion reactors. Diffusion in the crystal lattice is a vital step of the tritium release process. In this paper, we report the diffusion of a tritium interstitial in the Li₄TiO₄ crystal from first-principles calculations. The tritium interstitial sites, diffusion paths and activation energies of diffusion are obtained. We find the most favorable tritium diffusion path is along <001> of the Li₄TiO₄ crystal with an activation energy of 0.410 eV. The activation energy is 0.366 eV for tritium at 450 K after the vibrational correction is applied. The calculated activation energy is in good agreement with the experimental value of 0.370 eV.     

Possible origin of ferromagnetism in antiferromagnetic orthorhombic-YFeO3: A first-principles study

Rare-earth orthoferrites (RFeO₃) are well-known for the antiferromagnetic ground state. However, some of the recent experimental results suggest that the few members of RFeO₃ family possess ferromagnetism. In the present investigation we report the possible origin of ferromagnetism in antiferromagnetic YFeO₃ using density functional theory. For this purpose, we have considered pure as well as self-doping in YFeO₃ i.e. by considering the point defect at Y, Fe and O sites. Our finding suggests that the point defects in YFeO₃ results in the mixed-valence state of Fe, which may result in ferromagnetism through Zener double exchange mechanism.     

Intrinsic defects,Mo-related defects,and complexes in transition-metal carbide VC: A first-principles study

Intrinsic defects and Mo-related defects in vanadium carbide VC, as well as the defect complexes between vacancies and Mo defects were investigated by means of first-principles calculations within the framework of density functional theory. In addition, Mo diffusion in VC was also studied using LST/QST method. The formation energies of defects have clearly shown that except C vacancy (V_C) all other point defects are not energetic favorable compared to perfect VC. V_C can exist in the lattice forming nonstoichiometric carbide VC_x (x < 1), and also can stabilize the Mo-related defects (S_Mo-V, S_Mo-C, and T_Mo). Free Mo atoms have the strong tendency to enter the already formed V_V and occupy the lattice position of V atoms. Meanwhile, Mo atom in C lattice (S_Mo-C) and interstitial Mo (I_Mo) atom can also enter the V_V position stabilizing the lattice structure. S_Mo-C + V_V will transform into S_Mo-V + V_C and I_Mo + V_V will transform into S_Mo-V during optimization, and large binding energy makes Mo atom tend to exist in the interstitial position. From the perspective of energy, Mo atom tends to diffuse through the interstitial position.     

Antiferroelectricity of NaNbO3: Single-crystal experimental study and first-principles calculation

Similar to the canonical antiferroelectric (AFE) compound PbZrO₃ in Pb(Zr,Ti)O₃ solid-solutions, the presence of double hysteresis loops and that of electric field–induced phase transitions are important characteristics of NaNbO₃ AFE materials; yet the phase transition behavior in the latter system is typically irreversible with the related mechanisms not fully understood. Here, we explore the phase transition mechanism of ferroelectric and AFE phases in NaNbO₃ based on measurements of single crystals with different directions in conjunction with density-functional theory (DFT) calculations. The tilting and distortion behaviors of the [NbO₆] octahedra are explained by DFT, and the ion displacement in the lattice is traced. The tilting and distortion behaviors of the [NbO₆] octahedra with different orientations are compared. We confirm that the tilt and distortion of the [NbO₆] octahedra along the [1 1 1] direction is the main reason to improve the stability of AFE phase. This conclusion is verified by the experimental characterizations of large-size NaNbO₃ single crystals successfully obtained in this study.     

A first-principles study of nitrogen- and boron-assisted platinum adsorption on carbon nanotubes

We have performed first-principles calculations to investigate the origin of adsorption of platinum on nitrogen- and boron-doped carbon nanotubes (CNTs). Our calculation results reveal that both nitrogen- and boron-doped CNTs can assist the reactivity of platinum adsorption on the CNT surface, although the detailed mechanisms are very different. For nitrogen-doped CNTs, the enhanced adsorption results from activation of the nitrogen-neighboring carbon atoms due to the large electron affinity of nitrogen. In this case, the nitrogen atoms mediate the platinum adsorption enhancement on the CNT surface. In contrast, the enhanced platinum adsorption in boron-doped CNTs can be attributed to the strong hybridization between the platinum d orbital and boron p orbital. Our results explain the experimentally observed enhanced adsorption of platinum on nitrogen-doped CNTs and also suggest that boron-doped CNTs may be a better candidate for fuel cell applications.     

A study on ion-irradiation damage of graphite surface

Macroscopic damage of a graphite surface implanted with 150 keV Ar⁻ ions has been investigated by scanning electron microscopy. In pyrolytic graphite, cracks were formed at a dose of about 10¹⁴ ions/cm² and blisters at 10¹⁷ ions/cm²; the average length of cracks and the density of blisters were both increased by reducing the crystal perfection of original specimens. In kish graphite (an iron-melt single crystal), the crack formation was very scarce but a small number of blisters was produced. The elastic modulus calculated from the crack length is far below the shear modulus C₄₄ of unirradiated pyrolytic graphite. A comparison with chemical etching patterns implies that these cracks and blisters correspond to the grain boundaries and etch pits, respectively.     

A first-principles study of alkali-metal-decorated graphyne as oxygen-tolerant hydrogen storage media

One serious problem encountered in hydrogen storage based on metal-decorated porous materials is the oxidation of metal atoms as it is irreversible due to strong oxygen-binding and blocks the adsorption of molecular hydrogen. We study the adsorption of molecular oxygen on graphyne decorated with alkali (AM) and alkali earth metals (AEM) using first-principles calculations. For comparison, we also calculate the adsorption characteristics of metal atoms and subsequent molecular oxygen in pristine and boron-doped graphene. We find that the binding energy of molecular oxygen on AM–graphyne complexes, especially Li–graphyne complex, is much smaller than that on AM in graphene or comparable to that in boron-doped graphene. We show that the binding strength of molecular oxygen is mainly affected by the center of empty p- or d-band of AM or AEM on adsorbents and by the work-function of metal-adsorbent complexes. We investigate the effect of biaxial tensile strain as a means of controlling the binding strength of molecular oxygen.     

A neutron diffraction study of nano-crystalline graphite oxide

The structure of graphite oxide has been studied by neutron diffraction and is found to be nano-crystalline, the diffraction pattern being a mixture of Bragg and diffuse scattering. Three strong Bragg peaks were found in the structure factor from which the layer spacing was calculated and found to be consistent with different structural regions within the graphite oxide, specifically regions of graphitic-like flat hexagons and regions containing oxidized chain-like structures. The neutron scattering pair distribution function is compared to four previously published low energy Monte Carlo structural models of GO.     

The electronic and magnetic properties of functionalized silicene: a first-principles study

ABSTRACT: Based on first-principles calculations, we study the structural, electronic, and magnetic properties of two-dimensional silicene saturated with hydrogen and bromine atoms. It is found that the fully saturated silicene exhibits nonmagnetic semiconducting behavior, while half-saturation on only one side with hydrogen or bromine results in the localized and unpaired electrons of the unsaturated Si atoms, showing ferromagnetic semiconducting or half-metallic properties, respectively. Total energy calculations show that the half-hydrogenated silicene exhibits a ferromagnetic order, while the half-brominated one exhibits an antiferromagnetic behavior.     

An abnormal incorporation behavior of Th in Gd2Zr2O7: A first-principles study

A theoretical study of Th accommodation in Gd₂Zr₂O₇ has been performed by density functional theory. Our calculations show that although thorium has only one charge state of Th⁴⁺, it can be incorporated into both Gd³⁺ and Zr⁴⁺ sites in Gd₂Zr₂O₇, depending on the chemical environments. Th occupation at Gd³⁺ site results in charge redistribution and the excess electrons introduced by Th are transferred to the neighboring Zr ions. As compared with the pure state, Th-containing Gd₂Zr₂O₇ pyrochlores are probably more inclined to undergo order-disorder transformation and are less susceptible to radiation-induced amorphization.