[en] The fine structures of the electron energy-loss spectra (EELS) for the B-K edge have been examined in NbB₂ and superconducting Nb_0.75Mg_0.25B₂. The experimental results are analyzed based on the calculations of density functional theory (DFT) using the Wien2k code. The results of the EELS spectra and the angular decomposition of the density of states (DOS) reveal that both the B p_z and B p_x+p_y states in NbB₂ have large weights at the Fermi energy due to intersheet covalent bonding with notable hybridization between the Nb 4d and B 2p states. This kind of hybridization also results in different core-hole behaviors for the B-K edge in two orthogonal crystallographic orientations. The best fit between experimental and theoretical data is achieved with consideration of the core-hole effect of the B 1 s states, in particular for the q-perpendicular c spectra. Analysis of the electronic structure of the Nb_1-xMg_xB₂ superconductors suggests that confinement of the intersheet covalent bonding is likely to be favorable for the improvement of superconductivity in this kind of materials

[en] Electrolyte ceramic of lithium lanthanum titanate [Li_0.5La_0.5TiO₃ (LLTO)] was prepared by microwave sintering method. The lattice structure, microstructure and transport properties were studied and compared with those of the LLTO ceramics prepared via conventional sintering method. Though the sintering methods both led to pure perovskite LLTO phase, they produced different crystallites. The LLTO prepared via microwave technique presented cubic perovskite-type crystallite, while the LLTO via conventional method offered tetragonal crystallite. In addition, the microwave sintering resulted in the formation of better-grained LLTO ceramics but lower conductivity than the conventional one did. This was explained by considering the different microstructures and conduction mechanisms of lithium ions in the two cases.

[en] A new preparation route for LiTi₂O₄ superconductor was presented. The upper critical field and low temperature thermal expansion of LiTi₂O₄ were investigated. The upper critical field H _C2(0) determined by the measurements of resistance and magnetization versus temperature are 8.9 ± 0.5 T and 9.3 ± 0.3 T, respectively. The linear coefficient of thermal expansion in the temperature range of 90-298 K determined by low temperature X-ray diffraction is 7.97(±0.13) x 10^-5 K^-1