Chemical bonding and pseudogap formation in D022- and L12-structure (V, Ti)Al3

To obtain a rigorous definition of the chemical bonds in binary transition-metal aluminides, topological analyses were performed for VAl₃ and TiAl₃ in the D0₂₂ and L1₂ structures. The analyses were based on the valence charge densities calculated with the ab initio density functional theory. To better understand the formation mechanism of the pseudogap in these compounds, the band structure, the density of states (DOS) and the band decomposed charge density (BDCD) were calculated. The topological analyses reveal that the interactions between the (V, Ti) and Al atoms are all pure shared-shell interactions, the bonds are covalent and clearly have π-bond character. The study of the band structure, DOS and BDCD shows that the formation of the pseudogap is due to the crystal field energy splitting of the (V, Ti)-3d orbitals combined with the inter-unit-cell orbital interaction.     

Electronic and pressure dependent vibrational properties of silicide Sr(Ni0.5Si0.5)2

We report a detailed ab initio study of the structural, electronic, and volume dependent elastic and lattice dynamical properties of Sr(Ni_0.5Si_0.5)₂. The calculations have been carried out within the local density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. The phonon dispersion curves along the high-symmetry directions and phonon frequencies with their Grüneisen parameters at the Brillouin zone center are computed by using density functional perturbation theory while the elastic constants are calculated in metric-tensor formulation. The band structure, and partial densities of states and Fermi surface topology are also discussed.     

Anomalous Hall effect and perpendicular magnetic anisotropy in Sm28Fe72 and Sm32Fe68 thin films

Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films of 100 nm thickness were grown using DC magnetron sputter deposition and their structure, magnetization, electrical and Hall resistance characteristics were investigated. An increase in electrical resistivity from 4.75×10⁻⁶ to 5.62×10⁻⁶ Ω m and from 2.26×10⁻⁶ to 2.84×10⁻⁶ Ω m for Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films, respectively, with decrease in temperature from 300 to 40 K is attributed to the strain induced anisotropy that dominates at lower temperatures. The positive extraordinary Hall coefficients (R_S) are observed for both films at 300 and 80 K. The existence of hysteresis indicates that Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films possess perpendicular anisotropy at 300 K. Hysteresis loop becomes narrow at 80 K for both Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films. Magnetization measurements at 300 K exhibiting small coercive field values of 31 and 49 Oe for Sm₂₈Fe₇₂ and Sm₃₂Fe₆₈ films, respectively, confirm the existence of perpendicular anisotropy at 300 K.     

Phase stability and electronic structure of Si2Sb2Te5 phase-change material

On the basis of an ab initio computational study, the present work provide a full understanding on the atomic arrangements, phase stability as well as electronic structure of Si₂Sb₂Te₅, a newly synthesized phase-change material. The results show that Si₂Sb₂Te₅ tends to decompose into Si₁Sb₂Te₄ or Si₁Sb₄Te₇ or Sb₂Te₃, therefore, a nano-composite containing Si₁Sb₂Te₄, Si₁Sb₄Te₇ and Sb₂Te₃ may be self-generated from Si₂Sb₂Te₅. Hence Si₂Sb₂Te₅ based nano-composite is the real structure when Si₂Sb₂Te₅ is used in electronic memory applications. The present results agree well with the recent experimental work.     

First principles study of electronic structure and optical properties of LiMTe2 (M=Al, Ga, In) crystals

The results of first principles calculations of the electronic band structure, density of states and frequency dependent dielectric functions of LiAlTe₂, LiGaTe₂ and LiInTe₂ chalcopyrite crystals are reported. The calculations have been carried out within the density functional theory using norm-conserving pseudopotentials and a plane-wave basis. The peculiarities of the imaginary part ε₂(ω) of the complex permittivity are discussed and interpreted on the basis of the obtained band spectra. Our calculations show that the Ga-containing compound is characterized by the largest optical anisotropy compared to Al- and In-containing compounds and, therefore, is the most promising candidate for nonlinear optical applications among considered crystals.     

Ab initio study of electronic and magnetic properties of the Fe3Zn intermetallic

First-principles calculations have been carried out to study the electronic structure and magnetic properties of the Fe₃Zn compound with the full-potential linearized augmented-plane wave (FLAPW) method. The results indicate a lower magnetostriction for Fe₃Zn as compared to Galfenol (Fe-Ga), as a result of a weaker spin-orbit coupling, which is due to a smaller magnetic moment induced on the Zn atom. With the Zn addition to Fe the bulk modulus and the cohesive energy (per atom) decrease, whereas the electronic specific heat coefficient γ has a substantial increase.     

Ab initio calculation of structural, electronic and phonon properties of ZrRu and ZrZn in B2 phase

The structural, electronic and dynamic properties of cesium chloride, ZrRu and ZrZn were studied by employing an ab initio pseudopotential method and a linear response scheme, within the generalized gradient approximation. The calculated lattice constant, bulk modulus and first-order pressure derivative of the bulk modulus were reported in B2 structure and compared with available experimental and other theoretical results. The electronic band structure, partial and total density of states were determined by using the Quantum-Espresso ab initio simulation package based on pseudopotential method. Phonon dispersion curves and density of states were calculated by employing a density functional perturbation theory.     

Atomic, electronic and thermodynamic properties of cubic and orthorhombic LaMnO3 surfaces

We studied in detail the atomic and electronic structure of the LaMnO₃ surfaces, in both cubic and orthorhombic phases, combining GGA-plane wave approach, as implemented into the VASP-4.6.19 computer code, with a slab model. These studies are complemented by a thermodynamic analysis of the surface stability at different gas pressures and temperatures. The obtained results are compared with similar studies for other ABO₃-perovskites.     

Calculations of physical properties of pure and doped crystals: Ab initio and semi-empirical methods in application to YAlO3:Ce and TiO2

In this paper we demonstrate that two independent methods of calculations (DFT based ab initio and semi-empirical crystal field theory) can be used to form a complementary picture of the optical and electronic properties of the doped host and impurity ion. The crystals considered in the present paper are: (i) YAlO₃:Ce³⁺ and (ii) two dominant phases of TiO₂—rutile and anatase. As an example, detailed calculations of the band structure and crystal field energy level scheme of YAlO₃:Ce³⁺ are reported. From the analysis of the band structure and density of states, the character of the YAlO₃ energetic bands and positions of the Ce impurity energy levels were established. It was also shown how the ab initio methods can be used for calculations of the structural properties of solids under elevated pressure. Taking the two dominant phases of TiO₂ as an example, it was demonstrated how the elastic properties can be extracted from the calculated unit cell’s volume at different pressures. Particular attention was paid to the microscopic effects of crystal field, which were evidenced by the pressure-induced changes of the structure and shape of distribution of the Ti 3d electrons density of states. It was demonstrated how the difference in crystal structure of the anatase and rutile phases leads to remarkable difference in microscopic crystal field effects, which was explained by different Ti-O distances in both phases. In addition, the pressure dependence of the band gaps for anatase and rutile was investigated. It was shown that the hydrostatic pressure leads to the band gap narrowing in anatase and band gap widening in rutile, with pressure coefficients +0.00681 eV/GPa for rutile and −0.0088 eV/GPa for anatase.     

Structural, elastic, and electronic properties of orthorhombic NH3BH3: An ab initio study

At the generalized gradient approximation (GGA), the elastic constants of the orthorhombic phase of NH₃BH₃ were calculated with plane-wave pseudo-potential method. Our calculation showed that the orthorhombic phase NH₃BH₃ is a loose and brittle material, as well as hard to be deformed, also we calculated the elastic anisotropies and the Debye temperatures from the elastic constants. And from the band structure and density of state (DOS), we concluded that NH₃BH₃ is a wide-gap semiconductor and the band gap is almost 6.0 eV. The bonds between N atoms and H atoms show a strong covalent characteristic, B atoms and H atoms form ironic bonds, and so as to the B-N bonds. Electrons from the B atoms are absorbed by the H atoms around the B atoms, and the H atoms display electronegativity.     

Experimental and theoretical study of silicon-doped Sb2Te3 thin films: Structure and phase stability

The influence of Si in Sb₂Te₃ on structure and phase stability was studied by experiments and ab initio calculations. With the increase of Si content in Sb₂Te₃ samples, the crystallization temperature increases and the crystalline grain size decreases. The incorporation of Si atoms into Sb₂Te₃ lattice is energetically unfavorable and hence Si atoms most probably accumulated in the boundaries of Sb₂Te₃ grains.     

Elastic properties, thermal expansion coefficients and electronic structures of Ti0.75X0.25C carbides

Elastic properties, thermal expansion coefficients and electronic structures of Ti_0.75X_0.25C carbides (X=W, Mo, Ta, Nb, V, Hf, Zr, Cr and Al) were systematically investigated using ab initio density functional theory (DFT) calculations. The calculated elastic moduli, electronic structures and thermal expansion coefficients α(T) of pure TiC are in good agreement with experimental data and other DFT calculations. Based on a phenomenological formula, the trends of elastic properties and ductile/brittle behavior of Ti_0.75X_0.25C were analyzed. It was found that alloying elements W, Mo, Ta, Nb, V and Hf can increase elastic moduli, while Zr, Cr and Al reduce moduli. The nearly free electron model and Debye approximation were applied in the evaluation of α(T). The anharmonic effect was taken into account by including volume-dependent elastic moduli and Debye temperature. Results show that alloying additions of 3d V, 4d Zr and Mo slightly reduce α(T), while 3d Cr increases α(T), Al, 4d Nb, 5d Hf and W almost keep α(T) unchanged in Ti_0.75X_0.25C at high temperatures. The electronic structures of Ti_0.75X_0.25C were calculated and analyzed, and the electronic density of states was used to interpret variations of elastic properties and ductile/brittle behavior induced by alloying additions.     

Electronic structure and Fermi surface of new K intercalated iron selenide superconductor KxFe2Se2

Using the ab initio FLAPW-GGA method we examine the electronic band structure, densities of states, and the Fermi surface topology for a very recently synthesized ThCr₂Si₂-type potassium intercalated iron selenide superconductor K_xFe₂Se₂. We found that the electronic state of the stoichiometric KFe₂Se₂ is far from that of the isostructural iron pnictide superconductors. Thus the main factor responsible for experimentally observed superconductivity for this material is the deficiency of potassium, i.e. the hole doping effect. On the other hand, based on the results obtained, we conclude that the tuning of the electronic system of the new K_xFe₂Se₂ superconductor in the presence of K vacancies is achieved by joint effect owing to structural relaxations and hole doping, where the structural factor is responsible for the modification of the band topology, whereas the doping level determines their filling.     

First-principles investigation of Mg2Ni phase and high/low temperature Mg2NiH4 complex hydrides

Using a density functional approach calculation, the structural, energetic and electronic properties of Mg₂Ni phase as well as its high/low temperature (HT/LT)-Mg₂NiH₄ complex hydrides are systematically investigated. The optimized structural parameters including lattice constants and atomic positions are very close to the experimental data determined from X-ray and neutron powder diffraction. A detailed study of the electronic structures including the energy band, density of states (DOS) and charge density distribution reveals the orbital hybridization and characteristics of bonding orbits within Mg₂Ni and its hydrides. Based on the calculated results of the reaction heat of hydrogenation, enthalpy of formation and energy cost to remove H atoms, it is found that the formation ability of LT-Mg₂NiH₄ is higher than that of the HT phase during the hydrogenation of Mg₂Ni alloy; moreover, LT-Mg₂NiH₄ has a relatively higher structural stability than HT phase, which is also well explained through the DOS and the charge distributions of HT/LT-Mg₂NiH₄ phases.     

Theory of interface structure, energetics, and electronic properties of embedded Si/a-SiO2 nanocrystals

We examine the interrelation of the structural and bonding alterations, when Si nanocrystals are embedded in amorphous silicon dioxide, with the electronic properties of the resulting nanocomposite system. Monte Carlo simulations using a valence force-field model obtain the equilibrium structure of the interface, and investigate its energetics, stability and disorder as a function of the nanocrystal size. It is found that when the size is smaller than 2 nm, the embedded nanocrystals get heavily distorted. First-principles calculations of such small nanocrystals reveal a drastic reduction of the energy gap compared to the free-standing case. The origin of this pinning is attributed to the structural deformations, while oxygen states at the interface seem to play a minor role.     

Simulation of elasto optical properties of K2SO4 crystals

The electronic optical spectra of the mechanically free and stressed crystals of potassium sulfate, K₂SO₄, in the orthorhombic phase Pnma have been calculated by the Cambridge Serial Total Energy Package (CASTEP) code. On the basis of these calculations, the components of stress elasto optical tensors based on the changes of refractive index n (π_im) and birefringence Δn () (i, k, m=1, 2, …, 6) have been obtained for the indices i, k ,m=1, 2, 3. Absolute magnitudes of the calculated tensor π_im are probably underestimated because the magnitudes of the calculated elastic stiffness tensor c_rm are found to be overestimated about two times. Features of the spectral dependences n(E) and k(E) of refractive and absorption indices of the mechanically free and stressed potassium sulfate crystals have been analyzed.     

Ab initio simulations on the atomic and electronic structure of single-walled BN nanotubes and nanoarches

To simulate the perfect single-walled boron nitride nanotubes and nanoarches with armchair- and zigzag-type chiralities and uniform diameter of ∼5 nm, we have constructed their one-dimensional (1D) periodic models. In this study, we have compared the calculated properties of nanotubes with those for both hexagonal and cubic phases of bulk: bond lengths, binding energies per B-N bond, effective atomic charges as well as parameters of total and projected one-electron densities of states. For both phases of BN bulk, we have additionally verified their lattice constants. In the density functional theory (DFT), calculations performed using formalism of the localized Gaussian-type atomic functions as implemented in the CRYSTAL-06 code we have applied Hamiltonians containing either PWGGA or hybrid (DFT+HF) B3PW exchange-correlation functionals. After calculation of Hessian matrix for the optimized structures of BN bulk (both phases) and nanotubes (both chiralities) using the CRYSTAL code we have estimated their normal phonon modes within the harmonic approximation. Applying both atomistic and continuum models we have calculated the elastic energies and moduli for SW BN nanoarches. Our calculations clearly show a reproducibility of the atomic structure, effective charges and total energy, as well as phonon and elastic properties when using either PWGGA or hybrid B3PW Hamiltonians. On other hand, there is a high sensitivity of the discrete energy spectra parameters (including band gap) to the choice of the first principles approach (the hybrid method reproduce them noticeably better).     

Structural, electronic, magnetic and elastic properties of tetragonal layered diselenide KCo2Se2 from first principles calculations

The structural, elastic, magnetic and electronic properties of the layered tetragonal phase KCo₂Se₂ have been examined in details by means of the first-principles calculations and analyzed in comparison with the isostructural KFe₂Se₂ as the parent phase for the newest group of ternary superconducting iron-chalcogenide materials. Our data show that KCo₂Se₂ should be characterized as a quasi-two-dimensional ferromagnetic metal with highly anisotropic inter-atomic bonding owing to mixed ionic, covalent, and metallic contributions inside [Co₂Se₂] blocks, and with ionic bonding between the adjacent [Co₂Se₂] blocks and K sheets. This material should behave in a brittle manner, adopt enhanced elastic anisotropy rather in compressibility than in shear, and should show very low hardness.     

Structural, magnetic and electronic structure studies of Mn doped TiO2 thin films

We report structural, magnetic and electronic structure study of Mn doped TiO₂ thin films grown using pulsed laser deposition method. The films were characterized using X-ray diffraction (XRD), dc magnetization, X-ray magnetic circular dichroism (XMCD) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase related to Mn metal cluster or any oxide phase of Mn. Magnetization studies reveal that both the films (3% and 5% Mn doped TiO₂) exhibit room temperature ferromagnetism and saturation magnetization increases with increase in concentration of Mn doping. The spectral features of XMCD at Mn L_3,2 edge show that Mn²⁺ ions contribute to the ferromagnetism. NEXAFS spectra measured at O K edge show a strong hybridization between Mn, Ti 3d and O 2p orbitals. NEXAFS spectra measured at Mn and Ti L_3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO₂ films.     

Elastic, electronic, and optical properties of hypothetical SnNNi3 and CuNNi3 in comparison with superconducting ZnNNi3

The elastic, electronic, and optical properties of MNNi₃ (M=Zn, Sn, and Cu) have been calculated using the plane-wave ultrasoft pseudopotential technique, which is based on the first-principle density functional theory (DFT) with generalized gradient approximation (GGA). The optimized lattice parameters, independent elastic constants (C₁₁, C₁₂, and C₄₄), bulk modulus B, compressibility K, shear modulus G, and Poisson's ratio υ, as well as the band structures, total and atom projected densities of states and finally the optical properties of MNNi₃ have been evaluated and discussed. The electronic band structures of the two hypothetical compounds show metallic behavior just like the superconducting ZnNNi₃. Using band structures, the origin of features that appear in different optical properties of all the three compounds has been discussed. The large reflectivity of the predicted compounds in the low energy region might be useful in good candidate materials for coating to avoid solar heating.