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In this theoretical study we investigate the electronic, optical, and photocatalytic properties of monolayer titanium dioxide (TiO2), nickel dioxide (NiO2), and germanium dioxide (GeO2), as well as the effects of strain and stacking on these monolayers. The first principle calculations were performed using density functional theory (DFT) to determine the band structure, density of states, and optical properties of the monolayers. The photocatalytic activity of the monolayers was evaluated based on the water-splitting reaction. The results showed that the electronic and optical properties of the monolayers were significantly affected by strain and stacking. Specifically, the bandgap of TiO2, NiO2, and GeO2 decreased under tensile strain, while it increased under compressive strain. Furthermore, the stacking order of the monolayers affected their electronic and optical properties. The photocatalytic activity of the monolayers was found to be highest for TiO2. This study provides a valuable insight into the properties of monolayer oxides TiO2 and GeO2 and their van der Waal heterostructure GeO2–TiO2 as a potential candidate for photocatalytic water splitting. [ABSTRACT FROM AUTHOR]

Effects of nickel substitution on the physical properties of perovskite SrFe1−xNixO3 (x = 0, 0.25, 0.50, 0.75 and 1) are investigated by density functional theory (DFT) using local density approximation (LDA), generalized gradient approximation (GGA) and GGA supplemented with a localization correction (GGA + U). It is found that the GGA + U results are the much consistent with experiments. The lattice constants decreases with the increase of Ni substitution and are found to be stable for all concentrations. The electron charge densities reveal that the bond between Sr-O is ionic; the Fe/Ni-O bond is covalent and the Ni-Fe bond is metallic. Electronic band structure comparisons and electrical properties reveal the metallic nature of samples. DFT as well as post-DFT calculations confirm that the magnetic interactions are anti-ferromagnetic for SrFeO3, while they are ferromagnetic for all Ni concentrations. The mechanism is elucidated by exchange interactions with magnetic transitions above room temperature. Due to the metallic nature and good electrical conductivity of these materials, it is expected that they could be used as electrode materials in fuel cells. [ABSTRACT FROM AUTHOR]

Quaternary perovskites CaA3V4O12 (A = Mn, Fe, Co, Ni and Cu) have been studied theoretically using a generalized gradient approximation along with Hubbard potential (GGA + U) in the domain of density functional theory (DFT). A decrease in the lattice constants of these compounds is observed when going from CaMn3V4O12 to CaCu3V4O12 due to the increase in the number of electrons in the trending of the metals. Electron charge densities in different crystallographic planes show that the bonds between Ca–O, A/V–O and A–V are ionic, covalent and metallic, respectively. The electronic band structures show the metallic behavior of these compounds except that CaMn3V4O12 and CaFe3V4O12 are half-metals. The elastic moduli of these compounds indicate the hardness and increases in a sequence going from CaMn3V4O12 to CaCu3V4O12, and also reveal their ductile nature. The optimized energies in different magnetic phases and the post-DFT calculations confirm that CaMn3V4O12 and CaCo3V4O12 are anti-ferromagnetic, and CaFe3V4O12 and CaNi3V4O12 are ferromagnetic, whereas CaCu3V4O12 is a paramagnetic material. Based on the above properties, it is expected that these compounds are potential candidates for storage devices. [ABSTRACT FROM AUTHOR]

Ternary palladates CdPdO and TlPdO have been studied theoretically using the generalized gradient approximation (GGA), modified Becke-Johnson, and spin-orbit coupling (GGA-SOC) exchange-correlation functionals in the density functional theory (DFT) framework. From the calculated ground-state properties, it is found that SOC effects are dominant in these palladates. Mechanical properties reveal that both compounds are ductile in nature. The electronic band structures show that CdPdO is metallic, whereas TlPdO is an indirect-bandgap semiconductor with energy gap of 1.1 eV. The optical properties show that TlPdO is a good dielectric material. The dense electronic states, narrow-gap semiconductor nature, and Seebeck coefficient of TlPdO suggest that it could be used as a good thermoelectric material. The magnetic susceptibility calculated by post-DFT treatment confirmed the paramagnetic behavior of these compounds. [ABSTRACT FROM AUTHOR]