Formation of shallow acceptors in ZnO doped by lithium with the addition of nitrogen

We performed first-principle total-energy calculations to investigate the mechanism for the realization of high quality p-type ZnO codoped with lithium and nitrogen. We find that the higher hole concentrations measured in the codoped ZnO is related to decreased ionization energy of acceptors and reduction of compensations. The dual acceptor N_O-Li_Zn complex proposed in experiments is unstable. While in the (Li_I-N_O)-Li_Zn complex, where acceptor Li_Zn binds to the passivated (Li_I-N_O) complex is stable and acts as a single acceptor. The activation energy of this complex is about 60 meV lower than that of Li_Zn in Li-monodoped ZnO. The formation of inactive (Li_I-N_O) complexes creates a fully occupied impurity band just above the valence band maximum of ZnO. Thus Li atoms binding to this complex is activated by the electrons from the complex state rather than from the host states, accounting for decreased activation energy. Besides, Li_I⁺ and N_O⁻ bind tightly through the Coulomb interaction. Such binding will suppress the amount of compensating donor Li_I and limit the compensation for the desired acceptor Li_Zn.     

Doping-induced spin-manipulation in complex trimer system Ca3Cu3(PO4)4: A density functional investigation

Doping induced spin-manipulation with magnetic (Ni) and non-magnetic (Mg) dopants constitutes the experimental attempts to obtain a singlet ground state system from the linear chain Heisenberg antiferromagnetic Cu-based d⁹ spin-1/2 trimer compound Ca³Cu³(PO⁴)⁴ with doublet ground state. The present study is a density-functional investigation of the effects of such doping on the spin-exchange mechanism and electronic structure of the parent compound. Site-selective doping with zero-spin dopants like Mg is proved to be more efficient than an integral spin dopant Ni in obtaining a spin-gap system with singlet ground state, as also observed in the experimental studies. Doping induced dimerized state is found to be the lowest in ground-state energy. Calculated spin exchange couplings along various possible pathways are observed to attain good agreement with earlier experimental results with suitable optimization of Coulomb repulsion (U) and exchange (J) parameters.     

Density functional theory study of AunMn(n=1-8) clusters

Equilibrium geometries, relative stabilities, and magnetic properties of small Au_nMn (n=1-8) clusters have been investigated using density functional theory at the PW91P86 level. It is found that Mn atoms in the ground state Au_nMn isomers tend to occupy the most highly coordinated position and the lowest energy structure of Au_nMn clusters with even n is similar to that of pure Au_n+1 clusters, except for n=2. The substitution of Au atom in Au_n+1 cluster by a Mn atom improves the stability of the host clusters. Maximum peaks are observed for Au_nMn clusters at n=2, 4 on the size dependence of second-order energy differences and fragmentation energies, implying that the two clusters possess relatively higher stability. The HOMO-LUMO energy gaps of the ground state Au_nMn clusters show a pronounced odd-even oscillation with the number of Au atoms, and the energy gap of Au₂Mn cluster is the biggest among all the clusters. The magnetism calculations indicate that the total magnetic moment of Au_nMn cluster, which has a very large magnetic moment in comparison to the pure Au_n+1 cluster, is mainly localized on Mn atom.     

Effects of Stone-Wales Defect Symmetry on the Electronic Structure and Transport Properties of Narrow Armchair Graphene Nanoribbon

We report a first principles calculation to investigate the electron transport properties of defected armchair graphene nanoribbon (AGNR) influenced by Stone-Wales (SW) defect. The SW defect is found to be able to effectively influence the electronic structure of the defected AGNRs, and their electron transport behaviors can exhibit prominent differences depending on the symmetry of the nanostructured morphology. Moreover, our simulations have revealed that the introducing of the SW defect could be favorable for the electron transport of the defective AGNR. Our investigation has confirmed the possibility of tuning the electron transport of graphene nanoribbon by introducing a topological defect, which could be helpful to extending the field of applications for graphene nanoribbon-based nanodevices.     

Band structure calculations on the layered compounds FeGa2S4 and NiGa2S4

For the compounds FeGa₂S₄ and NiGa₂S₄ band structure calculations have been performed by the ab initio plane wave pseudo-potential method. The valence charge density distribution points to an ionic type of chemical bonding between the transition metal atoms and the ligand atoms. Two models for the pseudo-potentials are used to calculate the band structures: (a) only s and p electrons and (b) also the d-shells of the transition metal atoms are included in the pseudo-potentials. The differences between these two cases of band structures are discussed. Energy gap formation peculiarities are analysed for both crystals. Zak's elementary energy band concept is demonstrated for the energy spectra of the considered crystals.     

Volume and heat of solution of hydrogen in rare earths from proton screening charges

In this work we determine some fundamental microscopic and macroscopic properties of rare earth-hydrogen (RE-H) systems. The behaviour of the electronic density variation of RE-H systems is obtained, using a program based on the density functional formalism. This information allows us to calculate the volume of solution of hydrogen in rare earths, as well as their heat of solution, and to compare with experimental results.     

Oxidation fracturing of the graphitic BN sheet

Graphitic BN sheets with well-defined structure are promising candidate materials for future applications in nanoelectronics and molecular devices. The local oxidation is regarded as an effective means to produce a regular nanostructure. However, the underlying fracturing mechanism of such system is unclear. Here we aim to resolve this issue by the ab initio method. we predict the equilibrium configuration and the oxidative cutting process by introducing an epoxy-like chain and an added oxygen atom placed nearby, respectively. The results show that the intermediate epoxy-like pair can be eventually broken up after a key structure formation of B₃O during the oxidative processes.     

Half-metallic properties of Ti2FeSi full-Heusler compound

In this study, the electronic structure and magnetic properties of novel half-metallic Ti₂FeSi full-Heusler compound with CuHg₂Ti-type structure were examined by density functional theory (DFT) calculations. The electronic band structures and density of states of the Ti₂FeSi compound show the spin-up electrons are metallic, but the spin-down bands are semiconductor with a gap of 0.45 eV, and the spin-flip gap is of 0.43 eV. Fe atom shows only a small magnetic moment and its magnetic moment is antiparallel to that of Ti atoms, which is indicative of ferrimagnetism in Ti₂FeSi compound. The Ti₂FeSi Heusler compound has a magnetic moment of 2 μ_B at the equilibrium lattice constant a=5.997 Å.     

Theoretical study of the transition metal ring functionalized tips of open-ended (5,5) boron nitride nanotube (BNNT)

The nanotube with open edges is an excellent candidate for designing efficient tip for atomistic scanning probes or field emission display (FED) devices. In the present work, we have studied the functionalization of an open-ended boron nitride nanotube (BNNT) with a series of transition metal rings and the effects on the properties of open-ended BNNT through density functional theory (DFT) calculations. The results show that the TM-BNNT complexes are energetically favorable. Moreover, it is found that the functionalization (a) significantly decreases the band gap of BNNT to different degrees, which might effectively modify the electronic properties of the open-ended BNNT; and (b) efficiently lowers the work function, which might improve the field emission properties. Our results might be helpful not only to design specific BNNT-based tips but also to further discuss the chemical vapor deposition (CVD) growth of BNNT on nanoparticles.     

Atomic-scale studies of native point defect and nonstoichiometry in silicon oxynitride

The native point defects and mechanism of accommodating deviations from stoichiometry of Si₂N₂O crystal have been investigated using atomistic simulation techniques. This work firstly provides a reliable classical interatomic potential model derived from density functional theory calculations. The force-field parameters well reproduce the crystal structure, elastic stiffness, and dielectric constants of Si₂N₂O. It is expected that the force-field parameters are useful in future investigations on Si₂N₂O by molecular dynamic simulation. The calculated formation energies for native defects suggest that intrinsic disorder in stoichiometric Si₂N₂O is dominated by antisites and a degree of oxygen Frenkel defect may also exist in this system. In nonstoichiometric Si₂N₂O, the calculated reaction energies indicate that excess SiO₂ or Si₃N₄ is most likely accommodated by the formation of antisite in the lattice. And we also find that SiO₂ excess is energetically more favorable than Si₃N₄ surplus in Si₂N₂O.     

First-principle study on electronic structure and magnetic properties of molecular-based antiferromagnet: (2-amino-5-chloropyridinium)2CuBr4

The electronic structure and magnetic properties of the (2-amino-5-chloropyridinium)₂CuBr₄ compound were studied using the full potential augmented plane wave plus local-orbitals method (FP-APW+lo) within density functional theory. The Cu atoms are the magnetic centers, magnetic moments originate mainly from the Cu 3d and Br 4p states, leading to a total magnetic moment of 1.00 μ_B per molecule. There is an important hybridization between the Cu 3d and Br 4p states, which causes the magnetic interactions between the Cu centers to pass through the Br p-orbitals near the Cu atoms. According to the self-consistent total energies, it was found that in the ground state there exist antiferromagnetic interactions for both intraplanar and interplanar magnetic exchange, but the latter is much weaker than the former.     

Numerical evidence of Luttinger liquid to Fermi liquid transformation in lithium doped trans-polyacetylene chain

We report on the first direct numerical evidence of doping-induced transformation of Tomonaga-Luttinger liquid to Fermi liquid in quasi-one-dimensional lithium doped trans-polyacetylene chain. Using density functional theoretical calculation, an analysis of density of states near the Fermi energy reveals a power-law scaling factor of Tomonaga-Luttinger liquid at low dopant concentration in the metallic regime. As soon as the doping level reaches 0.0763e/C, normal power-law scaling factor of Fermi liquid has been realized as a special case of Luttinger liquid in one dimension. The variation of density-density correlation is consistent with the present theoretical prediction.     

Electron correlation effects on magnetic Compton profiles of nickel in the GW approximation

The magnetic Compton profiles (MCPs) of 3d transition metal nickel are calculated by the GW approximation with FLAPW basis sets on the LSDA. The 3d valence band width narrowing, which is ascribed to the dynamical screening effects, contributes to eliminate an anomalous peak at 0.7 a.u. in the 〈110〉 direction, and also to diminish the excessive estimation of the Umklapp effects, in the LSDA MCPs. These are in good agreement with experimental observation.     

The influence of oxygen vacancies on the electronic and magnetic properties of perovskite-like SrFeO3-x

The electronic, magnetic properties and lattice relaxations of oxygen-deficient cubic strontium ferrite, SrFeO_2.875, in ferromagnetic configuration are studied by means of the density functional theory using LCAO basis (SIESTA code) calculations. It is shown that Fe and Sr atoms are displaced from oxygen vacancies while oxygen anions are attracted to the vacancies. The DOS distributions, magnetic moments and atomic effective charges are analyzed in comparison with vacancy free SrFeO₃; these parameters are found to change weakly with appearance of oxygen vacancies, in contrast to conventional ionic picture. Some strengthening of Fe-O covalent bonds in the vicinity of the oxygen vacancy is found. The formation energy of oxygen vacancies and divacancies are evaluated.     

Ab initio study of electronic structures of BaMoO4 crystals containing an interstitial oxygen atom

The electronic structures of the perfect BaMoO₄ and BaMoO₄ crystals containing an interstitial oxygen atom situated at an appropriate position with the total energy being the lowest are studied within the framework of the density functional theory with the lattice structure optimized. The calculated results reveal that the interstitial oxygen atom situated at two different interstitial sites would combine with formal lattice oxygen ions forming molecular ions in two different ways, and the interstitial oxygen atom would cause visible range absorption band peaked at about 320 nm.     

Binary and ternary metal carbides based upon Ti, Zr and Hf

Ab initio electronic structure calculations are employed to study the behavior of ternary alloy carbides based upon the metals Ti, Zr and Hf. Significant variations of the bulk modulus are found. The relative importance of each of the metals in influencing the cohesive properties of both binary and ternary metal carbides is discussed. A miscibility concept of ternary systems relative to the binary counterparts is considered at length and where it is argued that miscibility of the equilibrated or stable system is far different than for the metastable system as would be the case, for example, in a pressure synthesized alloy. From comparisons of two extremes of stable and metastable miscibility, speculation is made of extreme conditions as to how the ternary alloys can be achieved.     

First-principles study of small aluminum clusters: Oxygen adsorptions, oxidation and phase stability

It is important to understand the properties of individual nanometals before we can exploit full potential of their applications, for example, as energetic materials, enhancing additives, or catalysts. Here, we present a density functional theory study of the structure and properties of clean Al₁₃ clusters, oxygen adsorptions on the cluster surface, and the completely oxidized clusters. The relative stability of various phases at various oxygen pressures and temperatures is investigated based on the so-called “atomistic thermodynamics”, which was previously employed for studying metals. The effect of temperature and oxygen pressure on the phase stability is taken into account via the oxygen chemical potential and reflected in the (P, T) phase diagram. Our results show that only intact and completely oxidized clusters are thermodynamically stable, and that the O adsorption phases are never thermodynamically stable. Also, our results show that the Al₁₃ clusters are extremely easy to get oxidized. The present study provides valuable insight into the basic behaviors of small Al clusters in the presence of oxygen and a theoretical basis for exploring practical applications of these clusters.     

Crystal-chemistry from XPS analysis of carbide-derived Mn+1AXn (n=1) nano-laminate compounds

The M_n+1AX_n layered carbide/nitride-derived phases, where M is an early transition metal, A is an A-group element and X is N or C, have an unusual combination of mechanical, electrical and thermal properties. The surface and crystal-chemistries of several recently synthesized n=1 members have been investigated by X-ray photoelectron spectroscopy. The results show that the constituent species are characterized by low binding energies, sometimes exceptionally so. The C 1s energies are in the lower end of the range for carbides, at 281.1-282.0 eV. The M-species—Ti, V, Nb and Hf—have binding energies at or below those corresponding to the elemental metallic state. The binding energies of the A-species in apparent planar coordination—In, Ge, As and Al—are quite exceptional, being 0.5-2 eV below those corresponding to the elemental state. Those results suggest that screening of the A-group species is derived from out-of-plane interactions, while the XPS signatures of the species associated with the MX blocks are reminiscent of those obtained from the relevant carbide phases.     

Synthesis and cation distribution of copper-substituted spinel-related lithium ferrite

Single-phased Cu²⁺-substituted spinel-related Li_0.5Fe_2.5O₄ was synthesized by sintering a mixture of Cu²⁺-substituted corundum-related α-Fe₂O₃ and Li₂CO₃ at 700 °C which is ∼325-400 °C lower than the temperature at which the material is prepared by the conventional ceramic methods. X-ray powder diffraction, X-ray photoelectron spectroscopy, Mössbauer spectroscopy and magnetic measurements were used to characterize the material. In contrast to high-temperature synthetic routes, the present one leads to a Cu⁺-and Fe²⁺-cation free material, thereby optimizing its technological value. Rietveld refinement of the XRD data favors a structural model in which Cu²⁺ substitutes for both Fe³⁺ and Li⁺ at the octahedral sites. Mössbauer and magnetic data are consistent with this model if spin thermal reversal and/or spin canting are taken into account for the later.