RESUMO
We study the new details of electronic and thermoelectric properties of polycrystalline layered oxychalcogenide systems of (BiO)Cu Ch ( Ch = Se, Te) prepared by using a solid-state reaction. The systems were characterized by using photoemission (PE) spectroscopy and four-probe temperature-dependent electrical resistivity ρ( T). PE spectra are explained by calculating the electronic properties using the generalized-gradient approximation method. PE spectra and ρ( T) show that (BiO)CuSe system is a semiconductor, while (BiO)CuTe system exhibits the metallic behavior that induces the high thermoelectric performance. The calculation of electronic properties of (BiO)Cu Ch ( Ch = S, Se, Te) confirms that the metallic behavior of (BiO)CuTe system is mainly induced by Te 5p states at Fermi energy level, while the indirect bandgaps of 0.68 and 0.40 eV are obtained for (BiO)CuS and (BiO)CuSe systems, respectively. It is also shown that the local symmetry distortion at Cu site strongly stimulates Cu 3d-t2g to be partially hybridized with Ch p orbitals. This study presents the essential properties of the inorganic systems for novel functional device applications.
RESUMO
We study the structural, electronic, and magnetic properties of the antiferromagnetic-layered oxyarsenide (LaO)MnAs system from the first-principle calculation. The increasing Hubbard energy (U) in the Mn 3d orbital induces the increasing local-symmetry distortions (LSDs) in MnAs4 and OLa4 tetrahedra. The LSD in MnAs4 tetrahedra is possibly promoted by the second-order Jahn-Teller effect in the Mn 3d orbital. Furthermore, the increasing U also escalates the bandgap (E g) and the magnetic moment of Mn (µMn). The value of U = 1 eV is the most appropriate by considering the structural properties. This value leads to E g and µMn of 0.834 eV and 4.31 µB, respectively. The calculated µMn is lower than the theoretical value for the high-spin state of Mn 3d (5 µB) due to the hybridization between Mn 3d and As 4p states. However, d xy states are localized and show the weakest hybridization with valence As 4p states. The Mott-insulating behavior in the system is characterized by the E g transition between the valence and conduction d zx /d zy states. This work shows new physical insights for advanced functional device applications, such as spintronics.
RESUMO
We study the structural, electronic, and thermoelectric properties of p-type layered oxychalcogenides (La0.5Bi0.5O)CuCh (Ch = S, Se, Te) from first principles. Ch substitution from S to Te enhances the local-symmetry distortions (LSDs) in CuCh4 and OLa2Bi2 tetrahedra, where the LSD in OLa2Bi2 is more pronounced. The LSD in CuCh4 tetrahedra comes from the possible pseudo-Jahn-Teller effect, indicated by the degeneracy-lifted t2g and eg states of Cu 3d10 orbital. The Ch substitution decreases bandgap from 0.529, 0.256 (Γ â 0.4Δ), to 0.094 eV (Z â 0.4Δ), for Ch = S, Se, Te, respectively, implying the increasing carrier concentration and electrical conductivity. The split-off energy at Z and Γ points are also increased by the substitution. The valence band shows deep O 2p states in the electron-confining [LaBiO2]2+ layers, which is essential for thermoelectricity. (La0.5Bi0.5O)CuTe provides the largest thermoelectric power from the Seebeck coefficient and the carriers concentration, which mainly come from Te 5p x /p y , Cu 3d zx , and Cu 3d zy states. The valence band shows the partial hybridization of t2g and Chp states, implied by the presence of nonbonding valence t2g states. This study provides new insights, which predict experimental results and are essential for novel functional device applications.