Electronic structure and elastic moduli of the simple cubic fullerite C24—A new allotropic carbon modification

The energy band structure, equation of state, density of states, and elastic moduli of a new allotropic carbon modification, namely, fullerite C₂₄ with a simple cubic lattice (known previously as cubic graphite), are calculated by the full-potential linearized augmented-plane-wave (FLAPW) method with geometry optimization for the first time. The dependence of the total energy on the lattice constant exhibits a minimum for a ₀ = 0.60546 nm. In this case, the lengths of the C-C bonds between fullerene molecules, the lengths of the 6,6-bonds shared by hexagons, and the lengths of the 4,6-bonds shared by a square and a hexagon are equal to 0.1614, 0.1503, and 0.1637 nm, respectively. An analysis of the energy band structure and the density of states demonstrates that the simple cubic fullerite C₂₄ is a direct-band-gap insulator or a semiconductor with a band gap of 1.6 eV. The calculated bulk modulus B ₀ = 196 GPa and the elastic moduli C ₁₁ = 338 GPa, C ₁₂ = 139 GPa, and C ₄₄ = 30 GPa indicate that the fullerite under investigation is a mechanically stable material. The inference is made that the simple cubic fullerite C₂₄ is a new diamond-like molecular zeolite with a unique combination of properties, such as the porosity and nonpolarizability, on the one hand, and the mechanical strength, chemical inertness, and high thermal conductivity, on the other hand. The simple cubic fullerite C₂₄ can be considered a promising low-dielectric-constant (low-k) material (?₀ < 5.7) for use in fabricating interconnections and substrates intended for integrated circuits and nanoelectronics.     

Self-consistent calculation of the electron structure and X-ray spectra of chromium nitride

The energy band structure of cubic chromium nitride is calculated by the self-consistent method of associated plane waves for a broad energy range. Self-consistency led to overlapping of the p-band of nitrogen and the d-band of chromium and to the appearance of an energy discontinuity in the region of unbounded states. The total and local partial densities of the states are calculated. With allowance for the probability of transition, the K₅- and L₁₁₁-emission bands of chromium, the K-band of nitrogen, and the K-edge of absorption of chromium in chromium nitride are calculated in a dipole approximation. The possibility of calculating the absorption edge with allowance for the effect of shell holes is discussed. Statisfactory agreement is obtained with experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 72–78, August, 1985.     

Crystal growth and the electronic structure of Tl3PbCl5

Total and partial densities of states of constituent atoms of two tetragonal phases of Tl₃PbCl₅ (space groups P4₁2₁2 and P4₁) have been calculated using the full potential linearized augmented plane wave (FP-LAPW) method and Korringa-Kohn-Rostoker method within coherent potential approximation (KKR-CPA). The results obtained reveal the similarity of occupations of the valence band and the conduction band in the both tetragonal phases of Tl₃PbCl₅. The FP-LAPW and KKR-CPA data indicate that the valence band of Tl₃PbCl₅ is dominated by contributions of the Cl 3p-like states, which contribute mainly to the top and the central portion of the valence band with also significant contributions throughout the whole valence-band region. Further, the bottom of the valence band of Tl₃PbCl₅ is composed mainly of the Tl 6s-like states, while the bottom of the conduction band is dominated by contributions of the empty Pb 6p-like states. The KKR-CPA results allow to assume that the width of the valence band increases somewhat while band gap, E_g, decreases when changing the crystal structure from P4₁2₁2 to P4₁. The X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion-irradiated surfaces of a Tl₃PbCl₅ monocrystal grown by the Bridgman-Stockbarger method have been measured.     

Electronic structure of crystal-forming fullerenes C2n,fulsicenes Si n C n,and their crystals—Fulsicenites

A number of crystal-forming polyhedral clusters X _n Y _n from which zeolite-like covalent crystals can be constructed and synthesized through copolymerization by faces are proposed. A series of the smallest sized crystal-forming carbon clusters C_2n (where n = 10, 12, 14, 16, 18, 24, 36, 60) and silicon carbide clusters Si _n C _n (where n = 12, 16, 18, 24, 36, 60) are constructed. Their optimized geometries, electronic structures, charge transfers, band gaps, cohesive energies, and electron density maps are calculated using the spinrestricted Hartree-Fock method in the 6–31G(d) basis set. The Si₁₂C₁₂ and Si₂₄C₂₄ clusters, referred to as the fulsicenes, are used to construct possible crystals with rock salt (Si₁₂C₁₂), simple cubic (Si₂₄C₂₄), body-centered cubic (Si₁₂C₁₂), face-centered cubic (Si₂₄C₂₄), and hyperdiamond (Si₁₂C₁₂) lattices, which are termed fulsicenites. Their X-ray diffraction patterns are calculated.     

First-principles FP-LAPW calculations and X-ray spectroscopy studies of the electronic structure of Zr3V3O and Zr3V3O0.6 oxides

Electronic properties of Zr₃V₃O oxide, a very promising hydrogen-storage material, were studied both from theoretical and experimental points of view employing the full potential linearized augmented plane wave (FP-LAPW) method as well as X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). Total and partial densities of states of the constituting atoms of Zr₃V₃O have been derived from the FP-LAPW calculations. These data indicate that, the O 2p-like states are the dominant contributors in the bottom of the valence band, whilst the top of the valence band and the bottom of the conduction band of Zr₃V₃O are dominated by contributions of the V2 3d-like states, with slightly smaller contributions of the V1 3d-like states as well. Significant contributions of the Zr 4d-like states throughout the whole valence-band region and near the bottom of the conduction band are also characteristic of the electronic structure of Zr₃V₃O. The XPS valence-band spectra and the XES Zr Lβ_2,15, V L_α and O K_α bands have been derived and compared on a common energy scale for Zr₃V₃O and Zr₃V₃O_0.6 oxides. This comparison of the experimental spectra was found to be in excellent agreement with the results of the FP-LAPW calculations. In addition, the XPS Zr 3d, V 2p and O 1s core-level binding energies have been measured for Zr₃V₃O and Zr₃V₃O_0.6 oxides.     

Atomic and Electronic Structure of 3C-SiC(111)-( $$2\sqrt 3 \times 2\sqrt 3 $$ 2 3 × 2 3 )-R30° Surface

Physics of the Solid State - The theoretical studies and ab initio calculations have been carried out for the first time for atomic and electronic structure of four variants of 3C-SiC(111)-(...     

Electronic structure of non-centrosymmetric AgCd2GaS4 and AgCd2GaSe4 single crystals

X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion irradiated (0 0 1) surfaces of AgCd₂GaS₄ and AgCd₂GaSe₄ single crystals grown, respectively, by the Bridgman method and the method of direct crystallization have been measured in the present work. The X-ray photoelectron spectroscopy (XPS) results reveal high chemical stability of (0 0 1) surfaces of AgCd₂GaS₄ and AgCd₂GaSe₄ single crystals. Electronic structure of AgCd₂GaS₄ has been calculated employing the full potential linearized augmented plane wave method. For the AgCd₂GaS₄ compound, the X-ray emission bands representing the energy distribution of the valence Ag d-, Cd d-, Ga p- and S p-like states were recorded and compared on a common energy scale with the XPS valence-band spectrum. The theoretical and experimental data regarding the occupation of the valence band of AgCd₂GaS₄ were found to be in excellent agreement to each other. Second harmonic generation (SHG) efficiency of AgCd₂GaS₄ by using the 320 ns CO laser at 5.5 μm has been recorded within the temperature range 80–300 K. Substantial increase of the photoinduced SHG which in turn is substantially dependent on the temperature has been detected for the AgCd₂GaS₄ compound.     

Atomic and electron structure of reconstructed (111) surface in ZnSe and CdSe crystals

The atomic and electron structure of four variants of polar (111)-(2 × 2) surfaces in ZnSe and CdSe terminated by a cation, namely, the ideal, relaxed, reconstructed, and relaxed after reconstruction surfaces, are calculated for the first time from the first principles. The surface is simulated by a film with a thickness of 12 atomic layers and a vacuum gap of ~16 Å in the layered superlattice approximation. Four fictitious hydrogen atoms with a charge of 0.5 electrons each are added for closing dangling Se bonds on the opposite side of the film. Ab initio calculations are performed using the QUANTUM ESPRESSO software based on the density functional theory. It is shown that relaxation results in splitting of atomic layers. We calculate and analyze the band structures and total and layer-wise densities of electron states for four variants of the surface.