Strain Effects on the Electronic and Thermoelectric Properties of n(PbTe)-m(Bi2Te3) System Compounds

Owing to their low lattice thermal conductivity, many compounds of the n(PbTe)-m(Bi2Te3) homologous series have been reported in the literature with thermoelectric (TE) properties that still need improvement. For this purpose, in this work, we have implemented the band engineering approach by applying biaxial tensile and compressive strains using the density functional theory (DFT) on various compounds of this series, namely Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5. All the fully relaxed Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5 compounds are narrow band-gap semiconductors. When applying strains, a semiconductor-to-metal transition occurs for all the compounds. Within the range of open-gap, the electrical conductivity decreases as the compressive strain increases. We also found that compressive strains cause larger Seebeck coefficients than tensile ones, with the maximum Seebeck coefficient being located at −2%, −6%, −3% and 0% strain for p-type Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5, respectively. The use of the quantum theory of atoms in molecules (QTAIM) as a complementary tool has shown that the van der Waals interactions located between the structure slabs evolve with strains as well as the topological properties of Bi2Te3 and PbBi2Te4. This study shows that the TE performance of the n(PbTe)-m(Bi2Te3) compounds is modified under strains.

Electrophysical characteristics of nanodimensional cobalte-cuprate-manganite LaNa2CoCuMnO6 and nickelite-cuprate-manganite LaNa2NiCuMnO6

The temperature dependences of the electric capacity, dielectric constant and electrical resistance of cobaltecuprate-manganite of lanthanum and sodium of LaNa2CoCuMnO6 and nickelite-cuprate-manganite of lanthanum and sodium of LaNa2NiCuMnO6 were investigated on the LCR-800 serial device (manufactured by Taiwan) at the operating frequencies of 1 kHz, 5 kHz, and 10 kHz in interval of 293–483 K through 10 K continuously in dry air. It was determined that LaNa2CoCuMnO6 in interval of 293–483 K shows the semiconductor conductivity. A band gap ( Е) is 0.54eV. The compound has the high values of the dielectric constant, which are equal 2.17106 (1 kHz), 2.31105 (5 kHz), 8.22104 (10 kHz) at 293 K and 8.49108 (5 kHz), 7.87107 (10 kHz) at 483 K. LaNa2NiCuMnO6 in interval of 293–483 K demonstrates the semiconductor conductivity ( Е = 0.48 eV), at 433–443 K — the metallic conductivity and at 453–483 K — the semiconductor conductivity ( Е = 2.33 eV).The values of the dielectric constant are 4.97103 (1 kHz), 9.2102 (5 kHz), 5.1101 (10 kHz) at 293 K and 1.02106 (1 kHz), 1.98105 (5 kHz) and 9.76104 (10 kHz) at 483 K. The compounds can be classified as the narrow-band gap semiconductors and they are of interest for the semiconductor and microcapacitor technologies.

Enhanced visible-active photocatalytic behaviors observed in Mn-doped BiFeO3

A series of BiFeO3 and BiFe[Formula: see text]Mn[Formula: see text]O3 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) were synthesized by a hydrothermal method. The samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy (EDS) and UV–Vis diffuse reflectance spectroscopy, and their photocatalytic activity was studied by photocatalytic degradation of methylene blue in aqueous solution under visible light irradiation. The band gap of BiFeO3 was significantly decreased from 2.26 eV to 1.90 eV with the doping of Mn. Furthermore, the 6% Mn-doped BiFeO3 photocatalyst exhibited the best activity with a degradation rate of 94% after irradiation for 100 min. The enhanced photocatalytic activity with Mn doping could be attributed to the enhanced optical absorption, increment of surface reactive sites and reduction of electron–hole recombination. Our results may be conducive to design more efficient photocatalysts responsive to visible light among narrow band gap semiconductors.