Electronic, elastic and optical properties of zircon GdVO4 investigated from experiments and LSDA + U
The electronic structure, mechanical properties and optical properties of zircon-type GdVO4 were investigated by both local-spin density approximation (LSDA) band theory with Hubbard term of U and experiments. The calculated equilibrium parameters are in good agreement with experimental results and other reports. The band gap of GdVO4 calculated is 2.77 eV. The Mulliken analysis shows Gd–O bonds exhibits more ionic than V–O bonds. The elastic constants, the aggregate elastic moduli (B, G, E), and the Poisson’s ratio have been investigated from both calculation and experiment and they are in accordance with each other. The optical properties are also calculated, which shows GdVO4 has high optical isotropy.
Elastic stiffness coefficients of thenardite and their pressure and temperature dependence
The elastic stiffness coefficients, cij, of orthorhombic Na2SO4 thenardite (space group Fddd) were measured with an ultrasonic plane wave technique at ambient temperature as a function of hydrostatic pressure in the range of 0.1–70 MPa. The variation of the cij in the range of 1–5000 MPa was studied with density functional theory (DFT) based calculations. The experimental results and the DFT calculations were used to derive a force-field model, which was then employed to compute lattice parameters and elastic stiffness tensors of thenardite and of two other Na2SO4 polymorphs as functions of the temperature based on quasi-harmonic lattice dynamics. The structural parameters of the three polymorphs measured at high temperatures are reproduced to within 1.7% by the present calculations. Phases II (space group Pbnm) and III (Cmcm) appear to have significantly higher entropies than thenardite in agreement with their metastable formation at higher temperatures.
Synergistic effect of Ti and F co-doping on dehydrogenation properties of MgH2 from first-principles calculations
The energetic and electronic properties of MgH2 co-doped with Ti and F are investigated using first-principles calculations based on density functional theory. The calculation results show that incorporation of Ti combined with F atoms into MgH2 lattice is energetically favorable relative to single incorporation of Ti atom. After dehydrogenation, the co-doped Ti and F in MgH2 preferably generate TiH2 and MgF2, respectively. Comparatively, the combined effect of Ti and F in improving the dehydrogenation properties of MgH2 is superior to that of pure Ti. These results provide a reasonable explanation for experimental observations. Analysis of electronic structures suggests the enhanced dehydrogenation properties of doped MgH2 can be attributed to the weakened bonding interactions between Mg and H due to foreign species doping.
Density functional theory study of heterogeneous CO oxidation over an oxygen-enriched yttria-stabilized zirconia surface
The reaction mechanism of heterogeneous CO oxidation on yttria-stabilized zirconia (YSZ), frequently used as electrolyte in solid oxide fuel cell (SOFC) composite anodes, was investigated employing density functional theory (DFT). The results demonstrate the possibility for an Eley–Rideal type CO oxidation reaction on the electrolyte surface without the need for a metallic catalyst if the vacant sites of YSZ are filled by externally supplied oxygen, either by dissociative adsorption of gaseous O2 or via bulk oxygen atoms delivered by the SOFC cathode. Our results are consistent with the findings of recent experiments [J. Electrochem. Soc. 158 (2011) B5].
Stable structures and electronic properties of 6-atom noble metal clusters using density functional theory
The 6-atom clusters of group IB noble metals have been investigated theoretically using the density functional calculation with a plane-wave basis (CASTEP). We have calculated their optimized structures, relative cluster’s energies, atomic and bonding populations, spectra of the vibrational frequencies, energy gaps between the highest occupied and the lowest unoccupied molecular orbitals, and average polarizabilities per atom. The stable structures we found are planar triangular, pentagonal pyramid, and capped trigonal bipyramid. For the Cu6 and Ag6 cluster, the planar structure energetically competes with the pyramid structure for the ground state. According to the population analyses, the s–d orbital hybridization is explicitly shown to be in association with the corner atoms of the planar structure. We found that the vibrational spectra of the clusters are structural dependent. The average polarizabilities for the planar structure of the Cu6 and Ag6 cluster are quite different from their other stable isomers. In contrast, the polarizabilities are about the same for all stable gold hexamers. Our calculations benefit a reliable geometry identification of the 6-atom noble metal clusters.
Passivation of interfacial defects at III-V oxide interfaces
The electronic structure of gap states has been calculated in order to assign the interface states observed at III-V oxide interfaces. It is found that As-As dimers and Ga and As dangling bonds can give rise to gap states. The difficulty of passivating interface gap states in III-V oxide interfaces is attributed to an auto-compensation process of defect creation which is activated when an electron counting rule is not satisfied. It is pointed out that oxide deposition needs to avoid burying As dimer states from the free surface, and to avoid sub-surface oxidation during growth or annealing, in order to avoid defect states at the interface or in the subsurface semiconductor.
A First-Principle Study of B- and P-Doped Silicon Quantum Dots
Doping of silicon quantum dots (Si QDs) is important for realizing the potential applications of Si QDs in the fields of Si QDs-based all-Si tandem solar cells, thin-film transistors, and optoelectronic devices. Based on the first-principle calculations, structural and electronic properties of hydrogen terminated Si QDs doped with single Boron (B) or phosphorus (P) are investigated. It is found out that the structural distortion induced by impurity doping is related to the impurity characteristic, impurity position, and the QD size according to the structural analysis. The relative energetic stability of Si QDs with a single impurity in different locations has been discussed, too Furthermore, our calculations of the band structure and electronic densities of state (DOS) associated with the considered Si QDs show that impurity doping will introduce impurity states within the energy gap, and spin split occurs for some configurations. A detailed analysis of the influences of impurity position and QD size on the impurity levels has been made, too.
Tuning the magnetic and transport properties of metal adsorbed graphene by co-adsorption with 1,2-dichlorobenzene
A fundamental understanding of the properties of various metal/graphene nanostructures is of great importance for realising their potential applications in electronics and spintronics. The electronic and magnetic properties of three metal/graphene adducts (metal = Li, Co or Fe) are investigated using first-principles calculation. It is predicated that the metal/graphene adducts have strong affinity to aromatic molecule 1,2-dichlorobenzene (DCB), and the resultant DCB–metal/graphene sandwich structures are much more stable than the simple DCB/graphene adduct. Importantly, it is found that the adsorption of DCB slightly enhances the magnetic moment of the Co/graphene, but turns the Fe/graphene from magnetic to nonmagnetic. A detailed theoretical explanation of the different magnetic properties of the DCB/Co/graphene and DCB/Fe/graphene is achieved based on their different d-band splitting upon DCB adsorption. In addition, the transport property study indicates that the Fe/graphene is a better sensing material for DCB than the pristine graphene.
The structure, thermal properties and phase transformations of the cubic polymorph of magnesium tetrahydroborate
The structure of the cubic polymorph of magnesium tetrahydroborate (γ-Mg(BH4)2) has been determined in space group Ia-3d from a structural database of the isoelectronic compound SiO2; this has been corroborated by DFT calculations. The structure is found to concur with that recently determined by Filinchuk et al. (Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov and H. Hagemann, Angew. Chem. Int. Ed., 2011, DOI: 10.1002/anie.201100675). The phase transformations and subsequent decomposition of γ-Mg(BH4)2 on heating have been ascertained from variable-temperature synchrotron X-ray diffraction data combined with thermogravimetric and mass spectrometry measurements. At ~160 °C, conversion to a disordered variant of the β-Mg(BH4)2 phase (denoted as β′) is observed along with a further unidentified polymorph. There is evidence of amorphous phases during decomposition but there is no direct crystallographic indication of the existence of Mg(B12H12) or other intermediate Mg–B–H compounds. MgH2 and finally Mg are observed in the X-ray diffraction data after decomposition.
Applications of NMR Crystallography to Problems in Biomineralization: Refinement of the Crystal Structure and 31P Solid-State NMR Spectral Assignment of Octacalcium Phosphate
By combining X-ray crystallography, first-principles density functional theory calculations, and solid-state nuclear magnetic resonance spectroscopy, we have refined the crystal structure of octacalcium phosphate (OCP), reassigned its 31P NMR spectrum, and identified an extended hydrogen-bonding network that we propose is critical to the structural stability of OCP. Analogous water networks may be related to the critical role of the hydration state in determining the mechanical properties of bone, as OCP has long been proposed as a precursor phase in bone mineral formation. The approach that we have taken in this paper is broadly applicable to the characterization of crystalline materials in general, but particularly to those incorporating hydrogen that cannot be fully characterized using diffraction techniques.
87Sr Solid-State NMR as a Structurally Sensitive Tool for the Investigation of Materials: Antiosteoporotic Pharmaceuticals and Bioactive Glasses
Strontium is an element of fundamental importance in biomedical science. Indeed, it has been demonstrated that Sr2+ ions can promote bone growth and inhibit bone resorption. Thus, the oral administration of Sr-containing medications has been used clinically to prevent osteoporosis, and Sr-containing biomaterials have been developed for implant and tissue engineering applications. The bioavailability of strontium metal cations in the body and their kinetics of release from materials will depend on their local environment. It is thus crucial to be able to characterize, in detail, strontium environments in disordered phases such as bioactive glasses, to understand their structure and rationalize their properties. In this paper, we demonstrate that 87Sr NMR spectroscopy can serve as a valuable tool of investigation. First, the implementation of high-sensitivity 87Sr solid-state NMR experiments is presented using 87Sr-labeled strontium malonate (with DFS (double field sweep), QCPMG (quadrupolar Carr–Purcell–Meiboom–Gill), and WURST (wideband, uniform rate, and smooth truncation) excitation). Then, it is shown that GIPAW DFT (gauge including projector augmented wave density functional theory) calculations can accurately compute 87Sr NMR parameters. Last and most importantly, 87Sr NMR is used for the study of a (Ca,Sr)-silicate bioactive glass of limited Sr content (only ~9 wt %). The spectrum is interpreted using structural models of the glass, which are generated through molecular dynamics (MD) simulations and relaxed by DFT, before performing GIPAW calculations of 87Sr NMR parameters. Finally, changes in the 87Sr NMR spectrum after immersion of the glass in simulated body fluid (SBF) are reported and discussed.
Hydrous alumina/silica double-layer surface coating of TiO2 pigment
The hydrous alumina/silica double-layer surface coating of TiO2 pigment was prepared by precipitation method under two different conditions. High-resolution transmission electron microscopy (HRTEM), Energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrum (FTIR) as well as ζ-potential analysis were used to characterize the morphology, structure and surface electrokinetic behavior, respectively. The results show that hydrous alumina is continuously coated on the surface of silica-coated TiO2 particle with a compact layer at 60 °C, pH 4.0. There is a chemical interaction between the hydrous alumina layer and silica layer. An AlOSi bond was formed and a thin film of aluminosilicate was existed. A flocculent hydrous alumina layer was formed at 60 °C, pH 8.0. The XPS spectra of O1s show that the peak of 531.2 eV is assigned to AlO(OH) for the loose and flocculent morphology. The theoretical calculations reveal that the coated Al2O3·H2O on SiO layer combined with (−1 1 1) lattice plant of rutile TiO2 is more stable than the coated AlO(OH) and the discrepancy of total energy between them is about −1.303 eV, i.e., the sample obtained at 60 °C, pH 4.0 has a more stable thermodynamic property. In addition, the adsorption of H+ ion on AlO(OH) surface is easier than that on Al2O3·H2O surface, as the discrepancy of total energy between them decrease to −0.721 eV.
Single-Component Molecular Conductor [Cu(dmdt)2] with Three-Dimensionally Arranged Magnetic Moments Exhibiting a Coupled Electric and Magnetic Transition
Crystals of the single-component molecular conductor [Cu(dmdt)2] (dmdt = dimethyltetrathiafulvalenedithiolate) were prepared as a molecular system, with three-dimensionally arranged magnetic moments embedded in “sea” of π conduction electrons. [Cu(dmdt)2] had fairly large room-temperature conductivity (110 S cm–1) and exhibited weakly metallic behavior near room temperature. Below 265 K, the resistivity (R) increased very slowly with decreasing temperature and then increased rapidly, indicating a transition from a highly conducting state to an insulating state near 95 K. The magnetic susceptibility showed Curie–Weiss behavior at 100–300 K (C = 0.375 emu/mol, Θ = 180 K). The Curie constant and the high-temperature resistivity behavior indicate that conduction electrons and three-dimensionally arranged magnetic moments coexist in the crystal. The ESR intensity increased down to about 95 K. The ESR signal was broadened and decreased abruptly near 95 K, suggesting that electric and antiferromagnetic transitions occurred simultaneously near 95 K. The crystal structure was determined down to 13 K. To examine the stability of the twisted conformation of Cu complex with dithiolate ligands, the dihedral angle dependence of the conformational energy of an isolated M(L)2n- molecule was calculated, which revealed the dihedral angle dependence on the ligand (L) and the oxidation state of the molecule (n). High-pressure four-probe resistivity measurements were performed at 3.3–9.3 GPa using a diamond anvil cell. The small resistivity increase observed at 3.3 GPa below 60 K suggested that the insulating transition observed at ambient pressure near 95 K was essentially suppressed at 3.3 GPa. The intermolecular magnetic interactions were examined on the basis of simple mean field theory of antiferromagnetic transition and the calculated intermolecular overlap integrals of the singly occupied molecular orbital (SOMO) of Cu(dmdt)2.
Ab initio DFT study of urea adsorption and decomposition on the ZnO (10-10) surface
The mechanisms of urea decomposition into isocyanic acid (HNCO) and ammonia (NH3) in gas-phase and on the ZnO surface have been investigated by using density functional theory. In gas-phase, urea decomposes into HNCO and NH3 in one step, which is a concert reaction. However, on the ZnO surface urea is found to decompose gradually, in which urea molecule first adsorbs on the ZnO surface, followed by the NH and the NC bonds breaking, and eventually rebinds to form NH3 and HNCO. The presence of ZnO surface decreases the energy barrier of urea decomposition. And as the intense interaction between the surface and urea, urea decomposition is exothermic by 48.0 kcal/mol on the surface, which is endothermic by 22.0 kcal/mol in the gas-phase.
Two-temperature thermodynamic and kinetic properties of transition metals irradiated by femtosecond lasers
We consider the thermodynamic and kinetic properties of Nickel as an example of transition metal in two-temperature state (Te≫Ti,) produced by femtosecond laser heating. Our physical model includes essential processes induced in metals by ultrafast laser energy absorption. Specifically, the electron-ion collision frequency was obtained from recent high-temperature measurements of electrical conductivity and electron-electron screened Coulomb scattering was calculated by taking into account s-s and s-d collisions. In addition, chemical potential, energy, heat capacity, and pressure were obtained from first-principles density functional theory calculations. This model was implemented in two-temperature hydrodynamic code (2T-HD) and combined with molecular dynamics (MD) to determine strength of molten Ni at high strain rates ∼ 108-109s−1 under conditions of femtosecond laser ablation experiments. The simulated ablation threshold, which depends on material strength, was found to be in good agreement with our experimental measurements reported here. The combined 2T-HD/MD modeling explains the surprisingly high experimental energy density necessary to initiate ablation in Ni (the experimental crater depth in Ni is several times smaller than in Al and Au, while ablation threshold energies are similar).
Site preference of refractory elements in γ′-Ni3Al alloyed with Ru
The influence of ruthenium (Ru) on the partitioning behavior of W, Re and Cr in γ′-Ni3Al has been studied using the Dmol3 method based on the density functional theory. The calculated results show that W, Re and Cr have a preference for the Ni site in γ′-Ni3Al alloyed with Ru. When Ru substitutes the central Ni atom, the site preference for W, Re and Cr varies. Furthermore, an electronic structure analysis of the alloy in terms of the Mulliken population and partial density of states was performed to elucidate the alloying mechanism of Ru in γ′-Ni3Al. The results show that the strengthening effect of Ru in the alloy arises from a reduction in the binding energy of Ru as well as p-orbital hybridization between Ru and the host atoms.
Potential dependent and structural selectivity of the oxygen reduction reaction on nitrogen-doped carbon nanotubes: a density functional theory study
Nitrogen-doped carbon nanotubes (NCNTs) are attractive for electrocatalytic applications in fuel cells due to their low cost and high electrocatalytic activity. By using density functional theory calculations, the electrocatalytic mechanisms of the oxygen reduction reaction (ORR) under electrochemical conditions are studied at graphite-like N groups (NG) and pyridine-like N groups (NP) of NCNTs, in which the effect of electrode potentials on the activation energy (Ea) and reaction energy (Er) is taken into account. The ORR occurs at both NG and NP defect sites via two different four-electron OOH and two-electron H2O2 mechanisms. At the lower potential region, both mechanisms are simultaneously responsible for the reaction at NG and NP defect sites; while at higher potentials, the four-electron mechanism becomes dominant and the ORR at NP defect sites is more energetically favorable than that at NG defect sites.
Energetics and kinetics of carbonate orientational ordering in vaterite calcium carbonate
Vaterite is a less stable anhydrous crystalline calcium carbonate than calcite or aragonite and, thus, a rare mineral in geologic settings. However, vaterite is commonly found in biological environments. The mechanisms of crystal nucleation, transformation, and stabilization of vaterite in host materials remain unresolved. Understanding these issues may lead to answer some fundamental questions such as carbonate formation in geological systems and the intriguing occurrence of vaterite in biological systems. This requires an accurate knowledge of the crystal structure of vaterite and its order-disorder transformation. This study employs molecular-dynamics simulations to understand the thermodynamic stability of vaterite and kinetics of the orientational ordering of the carbonate ions. The results show that the potential energy change from disordered to ordered vaterite is about −11 kJ/mol, which significantly changes the relative stabilities of vaterite with respect to other anhydrous calcium carbonate polymorphs, including amorphous calcium carbonate. The heat capacity of vaterite is estimated to be 102.1 ± 0.4 J/(K·mol), comparable to an experimental result of 91.5 ± 3.8 J/(K·mol). The molecular-dynamics simulations also show similar energies for vaterite with different stacking structures, suggesting possible stacking disordering along the  axis. Cyclic high-temperature simulated-annealing molecular-dynamics simulations show that the CO3 orientational disorder-order transition is thermally activated. The calculated activation energy for the transition is 94 ± 10 kJ/mol with a pre-exponential factor of ~1.6 × 1013 s−1. A good linear fit of the logarithmic transition rate to inverse temperature (the Arrhenius plot) indicates that the transition is controlled by a single activation process that is related to a cooperative rotational motion of CO3 groups in vaterite.