Lithium Dihydroborate: First-Principles Structure Prediction of LiBH2 ß work presented at EUGM in Brussels
We report a first-principles structure prediction of the LiBH2, which structures are modeled by using four formula units per unit cell without symmetry restrictions. The computational methodology combines a simulated annealing approach and density functional total energy calculations for crystalline solid structures. The predicted lowest energy structure shows the formation of linear anionic chains, ∞1[BH2], enthalpy of formation at 0 K equal to −90.07 kJ/mol. Ring structures, in particular with butterfly and planar square topologies, are found to be stable but well above the ground sta, te by 20.26 and 12.92 kJ/mol, respectively. All convergent structures fall in the symmetry families monoclinic, tetragonal, and orthorhombic. For the representative structures of each family group, simulated X-ray diffraction patterns and infrared spectra are reported.
Surface composition of clean and oxidized Pd75Ag25(100) from photoelectron spectroscopy and density functional theory calculations
High resolution photoelectron spectroscopy and density functional theory calcu, lations have been used to study the composition of clean and oxidized Pd75Ag25(100). The results for the clean surface confirm earlier reports of surface segregation by Wouda et al. (1998), where the top most layers are rich in Ag. The Pd 3d core level component from the surface region is observed at higher binding energies than the contribution from the bulk which is found to be a signature of Pd embedded in Ag. Low energy electron diffraction and scanning tunneling microscopy measurements reveal that oxidation of the Pd75Ag25(100) surface results in a-O structure similar to the one reported for Pd(100). The calculations suggest that the stable structure is a PdO(101) monolayer supported on a (100) surface rich in Ag at the interface to the stoichiometric alloy. The calculated core level shifts for the oxidized surface are in good agreement with the experimental observations.
First-principles study of structural and electronic properties of SrLiSb under high pressure
Using the first-principles plane wave pseudopotential method, the structural and electronic properties of intermetallic compound SrLiSb have been studied within generalized gradient approximation in the frame of density functional theory. The calculations of lattice parameters are in well agreement with the available experimental data. The geometry optimization results indicated the compressibility of SrLiSb is anisotropic under high pressure. The energy band structure and density of states of SrLiSb were also calculated, indicating that SrLiSb has an electronic phase transition from direct-gap semiconductor to indirect-gap semiconductor at approximate 8 GPa.
Theoretic insights into the Ag doping in monolayer and bilayer ZnO armchair nanoribbons: edge effect and position-dependent properties
One-dimensional nanoribbons, thickness of which is smaller than width, exhibit special confinement and edge effects far from uniform in the cross-section. By means of density functional theory, we investigated the influence of the edges and doping positions on the electronic structure of Ag-doped ZnO armchair nanoribbons (ZnOANRs). Ag doping in monolayer and bilayer ZnOANRs (m- and b-ZnOANRs) have all been examined. The results indicated that there is no significant difference on the stability of Ag doping in different positions of both m- and b-ZnOANRs, but exhibits very different electronic properties directly related to the different doping positions. The depth of the acceptor states has essential relationship with the interaction between host O 2p and Ag 4d states in the acceptor. Ag substituting Zn atoms at the inner region of m-ZnOANRs could create shallow acceptors with small hole effective masses, benefit for p-type conduction. Ag doping in the inner region of b-ZnOANRs would create shallow acceptors but larger hole effective masses. The difficulty of hole mobility could be improved by increasing the Ag doping concentration in b-ZnOANRs. The discussion about the mechanism and the suggestion about the achievement in the experiment are interesting and timely.
Grain boundary sliding in pure and segregated bicrystals: a molecular dynamics and first principles study
Sliding behaviors of Σ9(221) grain boundary bicrystals have been investigated in pure metals (Al, Ag, Au, Cu, Pt and Co) and in segregated metals (Cu segregated by Al, Ag, Au, Pt and Co) by molecular dynamics simulations and first-principles calculations. The grain boundary energy, the atomic size and the electronegativity of the segregated elements were not critical for the occurrence of grain boundary sliding. On the other hand, the sliding rate increased as the minimum charge density decreased at the bond critical point. This was the case for both pure grain boundary models and segregated grain boundary models. Therefore, it seems that the sliding rate depends on atomic movement at sites with minimum charge density, irrespective of the elements involved and of the presence of segregated atoms.
Theoretical study of the origin of the enhanced visible light photocatalytic activity of N-doped CsTaWO6: Charge compensation effects modulated by N and other defects
First-principles calculations were used to investigate the origin of the enhanced visible light photocatalytic activity of N-doped CsTaWO6. The studies of the interactions of N and other defects found three kinds of charge compensation forms might be better. When NH-codoping is in CsTaWO6, the H atom acted as a charge donor to compensate the hole state caused by N-doping and induced the band gap narrowing of about 0.507 eV. For a higher N-doping concentration, a particular N–N cluster structure was formed. The electron transition energy from N–N π⁎ states to conduction band minimum decreased by 1.627 eV. When oxygen vacancy existed in the lattice, two electrons were transferred to compensate for two adjacent N acceptors and the band gap narrowed about 0.874 eV. The thermodynamics calculations indicated the formations of N and other defects were mutually promoted.
Comparative theoretical studies of high pressure effect on polymorph I of 2,2′,4,4′,6,6′-hexanitroazobenzene crystal
The effect of high pressure on the geometric and electronic structures of polymorph I of crystalline 2,2′,4,4′,6,6′-Hexanitroazobenzene (HNAB) has been comparatively investigated with the conventional and dispersion-corrected density functional theory (DFT and DFT-D). Two different exchange–correlation functionals, the local density approximation with the Ceperley-Alder exchange–correlation potential (CA-PZ), and the generalized gradient approximation with Perdew and Wang functional 91 (PW91) were employed. The lattice constants (a, b, and c), the energy gap between the highest occupied and the lowest unoccupied crystal orbital, and the bond lengths and bond angles of HNAB molecule, obtained from different functionals and methods, decrease regularly with the increasing pressure within the range of 0–60 GPa and 170–200 GPa, but not in 70–160 GPa, where abrupt changes happen. When applying the same method, the differences between the results of different functionals are larger for DFT than for DFT-D method. As for the same functional, the agreement between the normal PW91 and the dispersion-corrected one is better than those of CA-PZ. The unit cell volume decreases gradually, the crystal density and the unit cell total energy increase gradually as the pressure increases from 0 to 200 GPa, regardless of which methods and functionals are used.
Theoretical prediction of the structural, elastic, electronic and thermodynamic properties of V3M (M = Si, Ge and Sn) compounds
Density functional theory (DFT), is used in our calculations to study the V3M (M = Si, Ge and Sn) compounds, we are found that V3Sn compound is mechanically unstable because of a negative C44 = −19.41 GPa. For each of these compounds considered, the lowest energy structure is found to have the lowest N(Ef) value. Also there is a strong interaction between V and V, the interaction between M (M = Si, Ge, Sn) and V (M and M) is negative, not including Si [Sn]. In phonon density of states PDOS, the element contributions varies from lighter (high frequency) to heaviest (low frequency).
Optical isotropy in structurally anisotropic halide scintillators: Ab initio study
The present study explores the structural, electronic, and optical properties of XSrI3(X=K, Rb, and Cs) compounds within the framework of density functional theory. The ground state properties are calculated using the pseudopotential method with the inclusion of van der Waals interactions, which we find inevitable in reproducing the experimental structural properties of the above mentioned compounds with layered crystal structure. The electronic and optical properties are calculated using the full-potential linearized augmented plane wave method and the band structures are plotted with various functionals and we find the newly developed Tran and Blaha modified Becke-Johnson potential to improve the band gap significantly. From the band structures of these compounds, it is clearly seen that I-p states dominate the valence band. The optical properties such as complex dielectric function, refractive index, absorption spectra, and electron energy loss spectra are calculated which clearly reveal the optically isotropic nature of these materials though being structurally anisotropic, which is the key requirement for ceramic scintillators. The present study suggests that among the three compounds studied, CsSrI3 can act as a fast scintillating compound, which is well explained from the band structure calculations.
Assignment of Metal–Ligand Modes in Pt(II) Diimine Complexes Relevant to Solar Energy Conversion
This work describes a comprehensive assignment of the vibrational spectra of the platinum(II) diimine bisthiolate and chloride complexes as a prototype structure for a diversity of Pt(II) diimine chromophores. The dynamics and energy dissipation pathways in excited states of light harvesting molecules relies largely on the coupling between the high frequency and the low frequency modes. As such, the assignment of the vibrational spectrum of the chromophore is of utmost importance, especially in the low-frequency region, below 500 cm–1, where the key metal–ligand framework modes occur. This region is experimentally difficult to access with infrared spectroscopy and hence frequently remains elusive. However, this region is easily accessible with Raman and inelastic neutron scattering (INS) spectroscopies. Accordingly, a combination of inelastic neutron scattering and Raman spectroscopy with the aid of computational results from periodic-DFT and the mode visualizations, as well as isotopic substitution, allowed for an identification of the modes that contain significant contributions from Pt–Cl, Pt–S, and Pt–N stretch modes. The results also demonstrate that it is not possible to assign transition energies to “pure”, localized modes in the low frequency region, as a consequence of the anticipated severe coupling that occurs among the skeletal modes. The use of INS has proved invaluable in identifying and assigning the modes in the lowest frequency region, and overall the results will be of assistance in analyzing the structure of the electronic excited state in the families of chromophores containing a Pt(diimine) core.
The electronic and optical properties of carbon-doped SrTiO3: Density functional characterization
The electronic properties and optical activities of C-doped cubic SrTiO3 in perovskite structure are studied by first-principles calculation. The calculated formation energies show that the substitutional C impurity is preferentially occupied at the Ti site. For C@O, the C impurity introduces spin-polarized gap states, and the associated electron excitations among the valence band, the conduction band and the gap states produce various visible-light absorption thresholds. For C@Ti, some C gap states of s-character appear near the bottom of the conduction band, which result in the lowered optical transition energy and thus the visible light absorption as observed in the experiment.
Strain-induced improvements on linear and nonlinear optical properties of SrB4O7 crystal
Although the high nonlinearity, strontium tetraborate crystal SrB4O7 is angular non-phasematched in UV SHG process due to its low birefringence. In this Letter, we revealed that its birefringence can be significantly enhanced by uniaxial strain based on the first principles computations. The birefringence is thirteen and sixteen times larger than those of unstrained crystal at -10% a-axial compressive and 10%a-axial tensile strain, respectively. The compressive strain also effectively improve the static second-order coefficients and shift the optical absorption edge towards the UV side, which would shed light on the modulations of UV/VUV nonlinear optical crystals by directionally external stress.
First-principle Studies on Enhanced Optical Stability of BaMgAl10O17:Eu2+Phosphor by SiN Doping
Using density functional theory, we studied band structure, density of states, optical properties and Mulliken population of the pure and SiN doped BaMgAl10O17:Eu2+ (BAM:Eu2+) phosphors. Calculation results showed that the bands of BAM:Eu2+ were of low band energy dispersion, indicating large joint density of states, hence high performance of optical absorption and luminescence. BAM:Eu2+ showed stronger absorption intensity while Eu2+ occupied the BR sites instead of the mO sites. The concentration of Eu2+ at BR sites increased while that at mO sites decreased after Si—N doping. The influence of the variation of Eu2+distribution on the spectra was stronger than the influence of the decrease of Eu2+PDOS when SiN concentration was lower than 0.25, therefore the absorption and luminescence intensity of BAM:Eu2+ were enhanced. Mulliken population of Si—N bond was higher than Al—O bond, while that of Eu—N bond was higher than Eu—O bond as well, indicating that Si—N bonds and Eu—N bonds possessed higher covalence than Al—O bonds and Eu—N bonds respectively. The existence of Si—N bonds and Eu—N bonds enhanced the local covalence of Eu2+, hence the optical stability of BAM:Eu2+.
First-Principles Studies on Magnetic Stability of SrC and BaC in Rocksalt, Zincblende, and Nickel Arsenide Phases Under Pressure
Using the first-principles method based on the density functional theory, we investigated the magnetic stability of sp half-metal ferromagnets SrC and BaC in rocksalt (RS), zincblende (ZB), and nickel arsenide (NA) structures under external pressure. The magnetic moments, total energy of magnetic and nonmagnetic phases, and lattice constants are calculated as a function of the applied pressure. The calculations show the occurrence of pressure-induced magnetic phase transitions which are mainly resulted from the band widening of anion p states. It is also confirmed that for both SrC and BaC, the rocksalt structure is the most stable phase among the three phases.
Electronic and Optical Properties of KNbO3, NaNbO3 and K0.5Na0.5NbO3 in Paraelectric Cubic Phase: a comparative First-principles Study
The structural, electronic and optical properties of KNbO3 (KN), NaNbO3 (NN) and K0.5Na0.5NbO3 (KNN) in paraelectric cubic phase were calculated employing the plane-wave pseudopotential method based on density functional theory (DFT). The calculated electronic structures of the three crystals show similar features in the valence bands and the lower conduction bands. However, the structures in higher conduction bands differ markedly due to the effect of Na and K atoms. The calculated optical properties reveal that the features of optical spectrum at low energy are dominated by the transitions from O 2p valence bands to Nb 4dconduction bands and those at high energy are related to the transitions to K 4s4pand/or Na 3s3p states. Moreover, the optical constants of KNN are approximately the average of KN and NN at high energy. Therefore, the optical properties of KNNin high energy region can probably be altered by changing the ratio of Na/K.
Effect of Pressure on Structural, Electronic and Elastic Properties of Cubic (Pm3m) SnTiO3 Using First Principle Calculation
The electronic band structure, density of state and elastic properties of lead-free perovskite oxide SnTiO3 (ST) were investigated by employing first principles calculation using the Density Functional Theory (DFT) within local density approximation (LDA). The energy band gap was calculated from the separation between the Ti 3d (conduction band) and the maximum of O 2p (valence band). This gives an indirect band gap of 2.36 eV. The elastic constants and their pressure dependence were calculated up to 30 GPa and the independent elastic constants (C11, C12, and C44), bulk modules, B were obtained and analyzed. The results showed that SnTiO3 have a mechanical stability in cubic phase (Pm3m).
First Principle Study of Dynamical Properties of a New Perovskite Material Based on GeTiO3
The dynamical properties of a new perovskite GeTiO3 materials have been investigated by using first principle calculation based on Density Functional Theory (DFT) within the gradient generalized approximation (GGA). All calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) computer code. The calculations include the structural parameter, Born effective charges, and phonon dispersion. The calculated Born effective charges and phonon dispersion have been analyzed and the possibility of ferroelectric feature in GeTiO3 compounds has been discussed. From the analysis, the calculated Born effective charge ZGe and ZTi showed large anomaly compared to the nominal charge which contributed to the large atomic displacement. The calculated phonon dispersion showed the most unstable mode was at G point and the unstable modes were dominated by Ge branch. The dynamical properties and ferroelectric properties in GeTiO3 are discussed and compared with the ferroelectric feature in PbTiO3.
The Role of the Distal Histidine in H2O2 Activation and Heme Protection in both Peroxidase and Globin Functions
The distal histidine mutations of dehaloperoxidase-hemoglobin A (DHP A) to aspartate (H55D) and asparagine (H55N) have been prepared to study the role played by the distal histidine in both activation and protection against oxidation by radicals in heme proteins. The H55D and H55N mutants of DHP A have ~6-fold and ~11-fold lower peroxidase activities than wild type enzyme toward the oxidation of 2,4,6-trichlorophenol (TCP) to yield 2,6-dichloroquinone (DCQ) in the presence of H2O2. The origin of the lower rate constants may be the solvent-exposed conformations of distal D55 and N55, which would have the dual effect of destabilizing the binding of H2O2 to the heme iron, and of removing the acid–base catalyst necessary for the heterolytic O–O bond cleavage of heme-bound H2O2(i.e., compound 0). The partial peroxidase activity of H55D can be explained if one considers that there are two conformations of the distal aspartate (open and closed) by analogy with the distal histidine. We hypothesize that the distal aspartate has an active conformation in the distal pocket (closed). Although the open form is observed in the low-temperature X-ray crystal structure of ferric H55D, the closed form is observed in the FTIR spectrum of the carbonmonoxy form of the H55D mutant. Consistent with this model, the H55D mutant also shows inhibition of TCP oxidation by 4-bromophenol (4-BP). Consistent with the protection hypothesis, compound ES, the tyrosyl radical-containing ferryl intermediate observed in WT DHP A, was not observed in H55D.
Crystal Structure and Electronic Properties of a Piroxicam Derivative: A Combined X-Ray Analysis and Quantum Mechanical Studies
A piroxicam derivative, 4-acetamidobenzenesulfonate-2-methyl-N-(2-pyridal)-2H-1,2-benzothiazine-1,1-dioxide (2), has been synthesized and structurally characterized by X-ray analysis. The electronic structure of (2) was calculated at the DFT level using the hybrid exchange–correlation functional BLYP. The optimized molecular geometry of (2) corresponds closely to that obtained from the X-ray structure analysis. Intermolecular N–H···O and C–H···O hydrogen bonds connect the molecules in (2) forming a two-dimensional supramolecular frameworks in (011) plane, which are further linked via π···π interactions to generate a three dimensional architecture. The relative contribution of different interactions to Hirshfeld surface indicates that the H···H and O···H contacts can account for about 76 % of the total Hirshfeld surface area in (2). The HOMO–LUMO energy gap of 2.77 eV indicates a high kinetic stability of (2).
Magnetism of triangular nanoflakes with different compositions and edge terminations
Since the discovery of the giant magnetoresistance effect, extensive research has been devoted to finding new materials for spintronic devices. The hotly pursued nanostructure-based magnetic materials are potential candidates for such applications. Among them, graphene triangular nanoflakes (G-TNFs), due to their special magnetic configurations, can serve as building blocks for design of new C-based magnetic materials. This motivates the present study to systematically investigate how magnetism of the TNFs changes with their edge termination, composition, and atomic distribution. Using density functional theory, we show that the F-terminated G-TNFs have similar magnetic behavior to the H-terminated G-TNFs. Besides the edge terminations, partially hydrogenation of interior C atoms in the G-TNFs breaks the conjugate π orbitals and thus leads to extra net magnetic moment. The IV-group binary SiC-TNFs resemble the G-TNFs in magnetic properties, while the III–V group binary BN- and AlN-TNFs are different although they also have honeycomb structures. The different magnetic behaviors originate from the different occupations of p z atomic orbitals and the resulting change of conjugate π molecular orbitals. This study provides physical insight on tuning the magnetic behavior of TNFs through controlling their composition, size, and edge termination.
Chiral structures and tunable magnetic moments in 3d transition metal doped Pt6 clusters
The structural, electronic, and magnetic properties of transition metal doped platinum clusters MPt6 (M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) are systematically studied by using the relativistic all-electron density functional theory with the generalized gradient approximation. Most of the doped clusters show larger binding energies than the pure Pt7 cluster, which indicates that the doping of the transition metal atom can stabilize the pure platinum cluster. The results of the highest occupied molecular orbital (HOMO)—lowest unoccupied molecular orbital (LUMO) gaps suggest that the doped clusters can have higher chemical activities than the pure Pt7 cluster. The magnetism calculations demonstrate that the variation range of the magnetic moments of the MPt6 clusters is from 0 μB to 7 μB, revealing that the MPt6 clusters have potential utility in designing new spintronic nanomaterials with tunable magnetic properties.
Study of Copolymerization Mechanism between Vinyl-POSS and Citronellal with Quantum Chemistry Program Based on DFT
Conventional pesticide applications repeatedly failed to adequately control mosquito and sandfly populations in desert areas, due to effects of intense heat, blowing sand, ultraviolet light and/or combinations of them under severe environmental conditions. The citronellal was copolymerized with vinyl-POSS to enhance the resistant to ultraviolet radiation and thermal stability. The polymerization process between vinyl-POSS and citronellal were simulated by using Dmol3 program of MS software based on DFT. The calculation results showed that the double bonds in vinyl-POSS were initiated easily by phenyl radical, at the same time some double bonds in citronellal were also initiated. After the initiation process, the copolymerization between vinyl-POSS initiated by phenyl radical and citronellal was firstly processed. When the double bonds in vinyl-POSS were run out, the self-polymerizations of citronellal were processed.
Synthesis and Optical Determination in Rhodamine-Based Chemosensors Toward Hg2+
A new dye chemosensor molecule toward Hg2+ detection based on rhodamine 6G was synthesized by the condensation reaction of compound 2 and 2-amino-5,6-dimethyl-benzimidazole. Chemosensor 1 showed highly selective functions toward Hg2+ recognition with compared to other examined metal ions. The chemical structures of all the intermediates and chemosensor 1 were characterized by 1H NMR, Mass Spectrometer, and elemental analysis. Upon the addition of Hg2+, chemosensor 1 exhibited a remarkable emission change from colorless to green fluorescence. Additionally, the absorption color change from colorless to light pink in UV-vis range was observed. However, other metal ions such as Cu2+, Ag2+, Co2+, Pb2+, Zn2+, Fe3+, Fe2+, and Mg2+ did not accompany any noticeable spectral change.
Structure, Defect Chemistry, and Lithium Transport Pathway of Lithium Transition Metal Pyrophosphates (Li2MP2O7, M: Mn, Fe, and Co): Atomistic Simulation Study ß Samsung Corporate R&D
Lithium transition metal pyrophosphate materials (Li2MP2O7, M: Mn, Fe, and Co) have been proposed as promising novel cathode materials for lithium ion batteries. Using atomistic simulation with empirical potential parameters, which has been validated on various cathode materials by Islam et al. [Phil. Trans. R. Soc. A2010,368, 3255–3267], these new pyrophosphates are investigated to elucidate structure, defect chemistry, and Li+ ion transport pathway. The core–shell model with empirical force fields reproduces the experimental unit-cell parameters, and formation energies of intrinsic defects (Frenkel and antisite) are calculated. From migration energy calculation, it is found that the pyrophosphates without partial occupation have a 2D Li+ ion pathway. Meanwhile, under the condition of partial occupancies of Li and transition metal atoms, the diffusion pathway of Li+ ions is a 3D network.
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On the nature of spin- and orbital-resolved Cu+, –NO charg, e transfer in the gas phase and at Cu(I) sites in zeolites
Electronic factors essential for NO activation by Cu(I) sites in zeolites are investigated within spin-resolved analysis of electron transfer channels (natural orbitals for chemical valence). NOCV analysis is performed for three DFT-optimized models of Cu(I)–NO site in ZSM-5: [CuNO]+, (T1)CuNO, and (M7)CuNO. NO as a non-innocent, open-shell ligand reveals significant differences between independent deformation density components for α and β spins. Four distinct components are identified: (i) unpaired electron donation from NO π‖* antibonding orbital to Cus,d; (ii) backdonation from copper d yz to π⊥* antibonding orbital; (iii) donation from occupied π‖ and Cu d xz to bonding region, and (iv) donation from nitrogen lone-pair to Cus,d. Channel (i), corresponding to one-electron bond, shows-up solely for spin majority and is effective only in the interaction of NO with naked Cu+. Channel (ii) dominates for models b and c: it strongly activates NO bond by populating antibonding π* orbital and weakens the N–O bond in contrast to channel (i), depopulating the antibonding orbital and strengthening N–O bond. This picture perfectly agrees with IR experiment: interaction with naked Cu+imposes small blue-shift of NO stretching frequency while it becomes strongly red-shifted for Cu(I) site in ZSM-5 due to enhanced backdonation.