Ab initio study of the structural,electronic and elastic properties of AgSbTe2, AgSbSe2, Pr3AlC,Ce3AlC,Ce3AlN,La3AlC and La3AlN compounds

In this paper, we study the structural, electronic and elastic properties of the ternary AgSbTe₂, AgSbSe₂, Pr₃AlC, Ce₃AlC, Ce₃AlN, La₃AlC and La₃AlN compounds using the full-potential linearized augmented plane wave (FP-LAPW) scheme and the pseudopotential plane wave (PP-PW) scheme in the frame of generalized gradient approximation (GGA). Results are given for the lattice parameters, bulk modulus, and its pressure derivative. The calculated lattice parameters are in good agreement with experimental results. We have determined the full set of first-order elastic constants, shear modulus, Young's modulus and Poisson's ratio of these compounds. Also, we have presented the results of the band structure, densities of states, it is found that this compounds metallic behavior, and a negative gap Г→R for Pr₃AlC. The analysis charge densities show that bonding is of covalent–ionic and ionic nature for AgSbSe₂ and AgSbTe₂ compounds.     

Photoemission study of UFe2 and UPt3 using synchrotron radiation

The U 5f density of states in UPt₃ and UFe₂ has been determined experimentally using synchrotron radiation. Cross section variations were exploited to separate the valence band contributions. By means of difference spectra, the variations in the U 5f density at the Fermi level have been measured.     

Densities of phonon and electron states in V3Si and Cr3Si crystals

The regularization method has been used to solve the non-correct problem of finding out the phonon density of states from the temperature dependence of phonon heat capacity for two single crystals with A15 structure (the superconductor V₃Si and the non-superconductor Cr₃Si. The solution found agrees with the results of neutrongraphical and tunnel study. The electron density of states obtained in a narrow energy range near the Fermi level as the regularized solution of the reverse problem (from the temperature of the electron spin susceptibility) has at the Fermi energy the sharp maximum for V₃Si, but the flat minimum for Cr₃Si.     

First-principles study on electronic structure and elastic properties of Ti2SC

We have performed first-principles study on electronic structure and elastic properties of Ti₂SC. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of this compound. The five independent elastic constants were derived and the bulk modulus, Young's modulus, shear modulus, and Poisson's ratio were determined. The high bulk modulus and hardness was found to be originated from the strong Ti 3d-S 2p hybridization. Such strong MA bonding is unusual in the MAX phases studied so far. Ti₂SC is elastically stable and exhibits highly elastic isotropy.     

Electronic structure and structural stability of LaAl2 and LaAl3—A comparative study

The large structural stability regime of LaAl₂ and LaAl₃ as a function of pressure is investigated by the band structure calculations using the FP-LAPW method. An earlier experimental study has revealed that there is no structural phase transition at ∼35 and ∼30 GPa for LaAl₂ and LaAl₃, respectively. Our calculations indicate that in the density of states curve of LaAl₂, the Fermi level (E_F) lies in a slope between bonding maxima and antibonding minima. At high pressures the E_F moves slightly towards the valley, but this shifting does not affect its structural stability. In LaAl₃, the E_F falls in a flat region in the density of states and does not move even up to 33 GPa. The band dispersion curves for both the compounds show movement of bands under the influence of pressure. Some of them cross the Fermi level leading to so called Lifshitz transitions. However, it is seen that these electronic changes do not manifest into any volume anomaly in LaAl₃ under pressure. Our study clearly shows that the density of states behavior for LaAl₂ and LaAl₃ satisfies the Yamashita-Asano criterion for structural stability. The theoretical equations of state, bulk modulus and its pressure derivative values are compared with the experimental values.     

Th3Ni2B2N3 a possible new superconducting compound: insights from electronic band structure

La₃Ni₂B₂N₃, which is similar to YNi₂B₂C and related borocarbides, was earlier studied by the electronic structure calculations [D.J. Singh, W.E. Pickett, Phys. Rev. B 51 (1995) 8668.], and was predicted to be a 3-D metal. In search of new compounds of the borocarbide and related families to get higher T_C, we have studied the compound Th₃Ni₂B₂N₃, by the first principles full potential electronic structure calculations by the linear augmented plane wave method. We get similar band structure for Th₃Ni₂B₂N₃ as found for La₃Ni₂B₂N₃, and the various atom-split component density of states show similar properties. The total electron density of states at Fermi level has been increased to about 92 states per Ry per f.u. as compared to 57 states per Ry per f.u. in La₃Ni₂B₂N₃. The main increase is due to the increased hybridization of the 5f states as seen by the more prominent low energy tail in the Th-component density of states. Based on this enhancement we predict Th₃Ni₂B₂N₃ to be a high temperature superconductor with a T_c in excess of 30 K.     

Valence and core state modifications in Pt3Ti

XPS data for the valence band, the Pt 4? states, and the Ti 2p states are presented for the intermetallic Pt₃Ti. Relative to the Pt valence band, the Pt₃Ti band shows a decrease in the density of states just below the Fermi level and a shift of the centroid to higher (binding) energy. Ti 2p and Pt 4? binding energies showed relatively large shifts with respect to the pure metals. These changes in the valence band density of states and core level binding energies are interpreted as arising from hybridization of the d-orbitals in both metals to form strong intermetallic bonds.     

Magnetic susceptibility of NbSe3 and TaSe3

The magnetic susceptibility of NbSe₃ shows a decrease beginning slightly above its upper charge density wave transition (CDW) of 144 K, but no change within our resolution near the 59 K transition. The change in the density of states at the Fermi level due to the upper transition is 0.14 states-eV/Nb. TaSe₃ on the other hand has a temperature independent susceptibility. In some cases the trichalcogenides are contaminated with their corresponding dichalcogenide. Such contamination can be observed by susceptibility measurements in the case of 2HTaSe₂ but not of 2HNbSe₂. We also report an anomaly in the susceptibility of 4HaNbSe₂, which suggests a CDW transition at 45 K.     

First-principles study on structural and electronic properties of AlNSix heterosheet

Under generalized gradient approximation (GGA), the structural and electronic properties of AlN and Si sheets, hydrogen terminated AlN and Si nanoribbons with hexagonal morphology and 2, 4, 6 zigzag chains across the ribbon width and the hexagonally bonded heterosheets AlNSi_x (x=2, 4, and 6) consisting of hexagonal networks of AlN (h-AlN) strips and silicene sheets with zigzag shaped borders have been investigated using the first-principles projector-augmented wave (PAW) formalism within the density function theory (DFT) framework. The AlN sheet is an indirect semiconductor with a band gap of 2.56 eV, while the Si sheet has a metallic character since the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) meet at one k point from Γ to Z. In the semiconductor 6-ZAlNNR, for example, the states of LUCB and HOVB at zone boundary Z are edge states whose charges are localized at edge Al and N atoms, respectively. In metallic 6-ZSiNR, a flat edge state is formed at the Fermi level E_F near the zone boundary Z because its charges are localized at edge Si atoms. The hybridizations between the edge states of h-AlN strips and silicene sheets result in the appearance of border states in the zigzag borders of heterosheets AlNSi_x whose charges are localized at two atoms of the borders with either bonding or antibonding π character.     

First-principles investigation of pressure-induced changes in structural and electronic properties of Y 2C3 superconductor

The structural and electronic properties of Y ₂C₃ superconductor under different external pressures were calculated by employing the first-principles method. This shows that the lattice constants as well as the lengths of C-C dimers decrease with the pressure. Results of band structure calculations indicate that the Fermi level advances to the bonding zone with an increase in pressure; meantime, the valence and conduction bands intersect more deeply with the Fermi level. Moreover, the Fermi level is found to shift from the valley bottom of the density of states (DOS) curve to the shoulder, which means an increase in N(E_F), and therefore the critical temperature, T_c. The calculations verify that the critical temperature is directly related to the electronic structure.     

Impedance spectroscopy study and ground state electronic properties of In(Mg1/2Ti1/2)O3

The complex perovskite oxide In(Mg_1/2Ti_1/2)O₃ (IMT) is synthesized by a solid state reaction technique. The X-ray diffraction of the sample at 30 °C shows a monoclinic phase. The dielectric properties of the sample are investigated in the temperature range from 143 to 373 K and in the frequency range from 580 Hz to 1 MHz using impedance spectroscopy. An analysis of the dielectric constant ε′ and loss tangent (tan δ) with frequency is performed assuming a distribution of relaxation times. The Cole-Cole model is used to explain the relaxation mechanism in IMT. The scaling behavior of imaginary part of electric modulus (M″) shows that the relaxation describes the same mechanism at various temperatures. The electronic structure and hence the ground state properties of IMT is studied by X-ray photoemission spectroscopy (XPS). The valence band XPS spectrum is compared with the electronic structure calculation. The electronic structure calculation indicates that the In-5s orbital introduces a significant density of states at the Fermi level, which is responsible for a high value of conductivity in IMT.     

Energy bands of and spinwave dispersion in Pd3Fe

The results of a band calculation carried out for ferromagnetic Pd₃Fe suggest that the anisotropy observed in the spinwave dispersion curves may be explained by the particular shapes of the upspin and downspin Fermi surfaces and the symmetries of the corresponding one-electron states. The specific number of outer electrons per unit cell seems important in producing a Fermi level in a trough in the density of states so that only few states are involved. Qualitative considerations along the same lines seem capable of explaining differences in the spinwave dispersion of Pt₃Mn compared with that of Pd₃Fe.     

Magnetic and electronic properties of the antiferromagnetic YbFe4Al8 compound

The magnetic, electrical and electronic properties of the tetragonal ternary YbFe₄Al₈ compound have been investigated. This compound was supposed to be an antiferromagnetic superconductor due to the negative magnetization signal appearing at a low field of the field cooling mode, however, based on the measurements of the temperature dependence of magnetization and resistivity we do not confirm the presence of superconductivity in this material and we ascribe the negative magnetization to the complicated non-collinear magnetic structure. A switch to the antiferromagnetic order at about 150 K has been visible both on the M(T) and ρ(T) curves. The valence state of the Yb ions has been studied by X-ray photoemission spectroscopy. The valence band spectrum at the Fermi level exhibits the domination of the hybridized Yb(4f) and Fe(3d) states.     

Structure electronique des oxydes de cobalt CoO et Co3O4

The total density of occupied states in the valence band of CoO and Co₃O₄ is determined by XPS and UPS. From variations of excitation probability of the bands, the 4 e V wide O2_p band is shown to be located around 5 eV for both oxides, while structures obtained from photoionisation of the localized 3d band spread over 10 eV range below the Fermi level overlapping with O2_p band. The 3d peaks located at binding energy <3 eV correspond to the calculated energy of the d_n ?1 manifold final state in the octahedral and tetrahedral crystal field of CoO and Co₃O₄. The 3d levels at higher binding energy are shown to occur from configuration interaction in both final and initial states. These last peaks are higher in intensity for CoO relative to Co₃O₄. A superior limit for the width of the 3d initial band in a one electron energy diagram is given to be <3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV from shake-up and Auger energy confirms the Mott insulator nature of CoO.     

Ab initio electronic structure,bonding and optical properties of two relatively new ceramics Hf2Al3C4 and Hf3Al3C5: A comparative study

The structural, electronic and optical properties of the ternary carbides Hf₂Al₃C₄ and Hf₃Al₃C₅ are studied via first principles orthogonalized linear combination of atomic orbitals (OLCAO) method. Results on crystal structure, interatomic bonding, band structure, total and partial density of states (DOS), localization index (LI), effective charge (Q*), bond order (BO), dielectric function (ε), optical conductivity (σ) and electron energy loss function are presented and discussed in detail. The band structure plots show the conducting nature of Hf₂Al₃C₄ and Hf₃Al₃C₅ carbides. DOS results disclose that the total number of states at Fermi level N(E_F) are 1.89 and 2.38 states/(eV unit cell) for Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively. The Q* calculations show an average charge transfer of 0.723 and 0.711 electrons from Hf and 0.809 and 0.807 electrons from Al to C sites in Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively. The BO results provide the dominating role of Al–C bonds with BO value of 6.62 (BO%?=?59%) and 6.66 (BO%?=?49%) for Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively and are considered responsible for the crystals cohesion. The LI results reflect the presence of highly delocalized states in the vicinity of the Fermi level. The dielectric function plots of the real (?₁(?ω)) and imaginary (?₂(?ω)) parts show the anisotropic behavior of Hf₂Al₃C₄ and Hf₃Al₃C₅. The results on optical conductivity (σ) support the trends observed in dielectric functions. The electron energy loss functions reveal the presence of sharp peaks both in ab-plane and along c-axis around 20?eV in Hf₂Al₃C₄ and Hf₃Al₃C₅ ternary carbides.     

Bulk vs. surface effects in ARPES experiment from La2/3Sr1/3MnO3 thin films

Experiments directly probing the electronic states using angle-resolved photoemission (ARPES) were carried out on La_2/3Sr_1/3MnO₃ in order to elucidate its electronic properties. ARPES is a surface sensitive technique where bulk and surface states are usually both present. We present high-resolution ARPES studies in the (1 0 0) and (1 1 0) mirror planes and compare them with simulated ARPES based on GGA + U band structure calculations. In the (1 1 0) mirror plane we identify surface umklapps accounted by surface reconstruction which couple to bulk electronic states. As predicted by the simulated spectra there is additional spectral intensity at the Fermi level detected in ARPES data due to k_⊥-broadening effects in the photoemission final states. We demonstrate that this additional spectral intensity is a convenient spectral marker for determination of the k_F Fermi momenta.     

Magnetic and electronic structure studies of the perovskite oxide Nd2/3Sr1/3MnO3 from the first principle calculation

Generalized gradient approximation (GGA) and GGA + U (U denotes on-site Coulomb interactions) methods are applied to investigate the magnetic and electronic structures of the perovskite oxide Nd_2/3Sr_1/3MnO₃. Under GGA the compound prefers ferrimagnetic ordering in which Nd sublattice is spin-antiparallel to Mn sublattice. Nd 4f states cross over the Fermi level under GGA, leading the ferrimagnetic Nd_2/3Sr_1/3MnO₃ to a metallic character. The on-site Coulomb interactions should be included to emphasize the localized feature of Nd 4f states. Under GGA + U, the spins of Nd and Mn sublattices tend to be parallel in the ground state, and fully spin-polarized Mn 3d electrons yield a half-metallic band structure for the ferromagnetic Nd_2/3Sr_1/3MnO₃. The ferromagnetic coupling between Nd and Mn sublattices is ascribed to the super-exchange interaction between Nd 4f and Mn 3d (t2g) electrons via O 2p electrons.     

Thermoelectric properties and electronic structure of Te-doped CoSb3 compounds

Co₄Sb_12−xTe_x compounds were prepared by mechanical alloying combined with cold isostatic pressing, and the effects of Te doping on the thermoelectric properties were studied. The electronic structure of Te-doped and undoped CoSb₃ compounds has been calculated using the first-principles plane-wave pseudo-potential based on density functional theory. The experimental and calculated results show that the value of the solution limit x of Te in Co₄Sb_12−xTe_x compounds is between 0.5 and 0.7. The Fermi surface of CoSb₃ is located between the conduction band and the valence band, and its electrical resistivity decreases with increasing temperature. The density of states is mainly composed of Co 3d and Sb 5p electrons for intrinsic CoSb₃.The Fermi surface of Te-doped compounds moves to the conduction band and its electrical resistivity increases with increasing temperature, exhibiting n-type degenerated semiconductor character. Under the conditions of the experiment, the maximum value 2.67 mW/m K² of the power factor for Co₄Sb_11.7Te_0.3 is obtained at 600 K; this is about 14 times higher than that of CoSb₃.     

Silicon L2,3VV Auger lineshape and oxygen chemisorption study of Pd4Si

The SiL_2,3VV Auger Lineshape for Pd₄Si was measured and found to be in good agreement with the self-fold of the Si partial density of states model calculated by Riley et al. Oxygen chemisorption altered both the Auger lineshape and the HeI photoemission spectrum, especially near the Fermi energy.     

Conduction mechanism in polycrystalline Na1−xSrxNbO3 niobium bronzes

Electrical conductivity and thermoelectric power of polycrystalline Na_1?xSr_xNbO₃ have been measured between 5 and 300 K for various compositions. The results suggest that the density of states is finite at the Fermi level, whilst the carrier mobility is activated. The observed behavior is attributed to an Anderson localization of states at the Fermi level due to disorder resulting from either a random distribution of Sr²⁺ and Na⁺ ions or from the polycrystalline character of the samples or from both effects.