RESUMO
The current work investigates a novel three-dimensional boron nitride called bulk B4N4 and its corresponding two-dimensional monolayer B4N4 based on the first-principles of density functional theory. The phonon spectra prove that bulk B4N4 and monolayer B4N4 are dynamically stable. The molecular dynamics simulations verify that bulk B4N4 and monolayer B4N4 have excellent thermal stability of withstanding temperature up to 1000 K. The calculated elastic constants state that bulk B4N4 and monolayer B4N4 are mechanically stable, and bulk B4N4 has strong anisotropy. The theoretically obtained electronic structures reveal that bulk B4N4 is an indirect band-gap semiconductor with a band gap of 5.4 eV, while monolayer B4N4 has a direct band gap of 6.1 eV. The valence band maximum is mainly contributed from B-2p and N-2p orbits, and the conduction band minimum mainly derives from B-2p orbits. The electron transitions from occupied N-2p states to empty B-2p states play important roles in the dielectric functions of bulk B4N4 and monolayer B4N4. The newly proposed monolayer B4N4 is a potential candidate for designing optoelectronic devices such as transparent electrodes due to its high transmissivity.
RESUMO
The crystal structures, mechanical properties, lattice dynamics, electronic structures and optical properties of Sr2CoNbO6 and Ba2CoNbO6 have been studied by the first principles of density functional theory. The theoretically obtained crystal parameters of Sr2CoNbO6 and Ba2CoNbO6 are consistent with their experimental ones. Both Sr2CoNbO6 and Ba2CoNbO6 belong to the [Formula: see text] space group at the low-temperature limit and have very weak elastic anisotropy. The former is slightly brittle while the latter is more brittle. Their electronic structures are similar to each other, and Co-3d and O-2pâ orbitals constitute the top valence bands while Co-3d orbitals form the bottom conduction bands. Sr2CoNbO6 and Ba2CoNbO6 are indirect band gap semiconductors, and their band gaps are respectively 2.916 and 3.050 eV. The close band gaps are mainly dominated by the similar [Formula: see text] octahedrons in their crystal structures. The electron transitions from O-2pâ orbitals in the valence bands to Co-3d orbitals in the conduction bands play important roles in the optical properties of Sr2CoNbO6 and Ba2CoNbO6. Due to the same point group, Sr2CoNbO6 and Ba2CoNbO6 have the same five active lattice vibration modes of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] and one static lattice vibration mode of [Formula: see text], and the typical displacement patterns are also analyzed in detail.
RESUMO
The electronic, optical, and lattice dynamical properties of tetracalcium trialuminate (Ca4Al6O13) with a special sodalite cage structure were calculated based on the density functional theory. Theoretical results show that Ca4Al6O13 is ductile and weakly anisotropic. The calculated Young's modulus and Poisson ratio are 34.18 GPa and 0.32, respectively. Ca4Al6O13 is an indirect-gap semiconductor with a band gap of 5.41 eV. The top of the valence band derives from O 2p states, and the bottom of conduction band consists of Ca 3d states. Transitions from O 2p, 2s states to empty Ca 4s, 3d and Al 3s, 3p states constitute the major peaks of the imaginary part of the dielectric function. Ca4Al6O13 is a good UV absorber for photoelectric devices due to the high absorption coefficient and low reflectivity. The lattice vibration analysis reveals that O atoms contribute to the high-frequency portions of the phonon spectra, while Ca and Al atoms make important contributions to the middle- and low-frequency portions. At the center of the first Brillouin zone, lattice vibrations include the Raman active modes (E, A1), infrared active mode (T2), and silentmodes (T1, A2). Typical atomic displacement patterns were also investigated to understand the vibration modes more intuitively.
