Synergistic Effects of Intrinsic Cation Disorder and Electron-Deficient Substitution on Ion and Electron Conductivity in La_1–<i>x</i>Sr_<i>x</i>Co_0.5Mn_0.5O_3−δ (<i>x</i> = 0, 0.5, and 0.75)

The effects of intrinsic cation disorder and electron-deficient substitution for La_1–<i>x</i>Sr_<i>x</i>Co_0.5Mn_0.5O_3−δ (LSCM, <i>x</i> = 0, 0.5, and 0.75) on oxygen vacancy formation, and their influence on the electrochemical properties, were revealed through a combination of computer simulation and experimental study. First-principles calculations were first performed and found that the tendency of the oxygen vacancy formation energy was Mn³⁺-O*-Mn⁴⁺ < Co²⁺-O*-Co³⁺ < Co²⁺-O*-Mn⁴⁺, meaning that antisite defects not only facilitate the formation of oxygen vacancy but introduce the mixed-valent transition-metal pairs for high electrical conductivity. Detailed partial density of states (PDOS) analysis for Mn on Co sites (Mn_Co) and Co on Mn sites (Co_Mn) indicate that Co²⁺ is prone to being Co³⁺ while Mn⁴⁺ is prone to being Mn³⁺ when they are on antisites, respectively. Also it was found that the holes introduced by Sr tend to enter the Co sublattice for <i>x</i> = 0.5 and then the O sublattice when <i>x</i> = 0.75, which further promotes oxygen vacancy formation, and these results are confirmed by both the calculated PDOS results and charge-density difference. On the basis of microscopic predictions, we intentionally synthesized a series of pure LSCM compounds and carried out comprehensive characterization. The crystal structures and their stability were characterized via powder X-ray Rietveld refinements and in situ high-temperature X-ray diffraction. X-ray photoelectron spectroscopy testified to the mixed oxidation states of Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺. The thermal expansion coefficients were found to match the Ce_0.8Sm_0.2O_2−δ electrolyte well. The electrical conductivities were about 41.4, 140.5, and 204.2 S cm^–1 at doping levels of <i>x</i> = 0, 0.5, and 0.75, and the corresponding impedances were 0.041, 0.027, and 0.022 Ω cm² at 850 °C, respectively. All of the measured results testify that Sr-doped LaCo_0.5Mn_0.5O₃ compounds are promising cathode materials for intermediate-temperature solid oxide fuel cells

Shape Control of Ternary Sulfide Nanocrystals

Synthesis of semiconductor nanocrystals with a definite shape is the foundation of their anisotropy properties investigation; however, it is more challenging in ternary metal sulfides than that of noble metal and binary sulfides. In this paper, we report a solvent polarity control strategy to prepare a family of ternary sulfide (Ag₃SbS₃) semiconductor nanocrystals with tunable polyhedral shapes. The crystal growth speed along different directions was confined by the capping effect of the polarity of solvents that was defined by reaction temperature. Crystal shape of Ag₃SbS₃ nanocrystals could be tailored as a sphere, hexagonal plate, and prism. A shape-controllable growth mechanism was analyzed based on the Bravais–Friedel–Donnay–Harker theory by taking crystal structure characteristics and the polarity of solvents into consideration. The semiconductor nanocrystals show a near value of the band gaps for different shaped samples and facet-dependent photocatalytic water-splitting activities, which may result from the discrimination of the terminal surface structure and binding energy of Sb and S for the three different shaped nanocrystals. Thus, we provide a new crystal shape tunable strategy for ternary sulfide nanocrystal synthesis, which is important for optimizing properties and applications of sulfide semiconductor nanocrystals