State-of-the-art oxides and sulfides with high Li-ion conductivity and good electrochemical stability are among the most promising candidates for solid-state electrolytes in secondary batteries. Yet emerging halides offer promising alternatives because of their intrinsic low Li + migration energy barriers, high electrochemical oxidative stability, and beneficial mechanical properties. Mechanochemical synthesis has enabled the characterization of LiAlX 4 compounds to be extended and the iodide, LiAlI 4 , to be synthesized for the first time (monoclinic P 2 1 / c , Z = 4; a = 8.0846(1) Å; b = 7.4369(1) Å; c = 14.8890(2) Å; β = 93.0457(8)°). Of the tetrahaloaluminates, LiAlBr 4 exhibited the highest ionic conductivity at room temperature (0.033 mS cm –1 ), while LiAlCl 4 showed a conductivity of 0.17 mS cm –1 at 333 K, coupled with the highest thermal and oxidative stability. Modeling of the diffusion pathways suggests that the Li-ion transport mechanism in each tetrahaloaluminate is closely related and mediated by both halide polarizability and concerted complex anion motions.
The need for a new approach to describing nanoparticle nucleation and growth different from the classical models is highlighted. In and ex situ total scattering experiments combined with additional characterization techniques are used to unravel the chemistry dictating ZnWO4 formation.
Gaining insight into crystal structure is essential for understanding thermoelectric transport mechanisms and predicting thermoelectric properties. The main challenge in studying thermoelectric mechanisms is often imprecise or wrong models of the crystal structure. This work examines the structure modifications observed in MgAgSb thermoelectric materials by multi-temperature high-resolution synchrotron radiation powder X-ray diffraction (SR-PXRD). Rietveld refinement reveals large atomic displacement parameters (ADPs) of the Ag1 atoms at the 4a position indicating possible atomic disorder, which may contribute to the low thermal conductivity observed in α-MgAgSb. The temperature dependence of anisotropic structural parameters indicates a tendency of increasing structural symmetry in α-MgAgSb with increasing temperature, largely contributing to the temperature evolution of the thermoelectric properties. Two MgAgSb polymorphs (β-MgAgSb and γ-MgAgSb) coexist at 700 K, and only the γ-MgAgSb crystalline phase is found at high temperatures (800-1000 K). The content of γ-MgAgSb phase decreases with temperature due to the increase of liquid impurities, and the sample is only 43.8% crystalline at 1000 K. At 800 K, the high resolution powder data are fitted equally well using type I (with Mg, Ag, and Sb on the 4b, 4c, and 4a sites, respectively) and type II (with Mg, Ag, and Sb on the 4a, 4b, and 4c sites, respectively) half-Heusler crystal structure models. Nonetheless, Maximum Entropy Method (MEM) analysis carried out on the extracted factors shows that the type II structure gives a more physically sound MEM electron density. The disorder in γ-MgAgSb consists of mixed sites of Mg and Ag as well as vacancies, and the strong disorder of the cation sublattice contributes to the low thermal conductivity.
The chemistry of ZnAl2O4 nanocrystal nucleation and growth is examined by X-ray scattering methods, and the results challenge the conventional understanding of its preparation by hydrothermal methods. The common assumption that a specific metal to hydroxide ion (M/OH) ratio is necessary to achieve a phase-pure product is shown to be inadequate. Pair distribution function analysis is used to identify distinct precursor structures, providing an understanding of why particular impurity phases are observed under certain M/OH ratios as heating is applied. In situ X-ray diffraction studies then probe the ZnAl2O4 growth in real time, from which optimal synthesis conditions and the influence of impurities is established. It is found that the heating rate plays a dominant role in impurity formation and dissolution. This observation is explored in three different hydrothermal synthesis methods (microwave, autoclave, and supercritical flow) having different intrinsic heating rates, and methodologies to prepare phase-pure ZnAl2O4 were successfully developed in each case. Ultimately, the atomic scale X-ray scattering information provides concrete guidance to tune the crystallite size, band gap, morphology, and defects of ZnAl2O4 nanocrystals in hydrothermal synthesis establishing a bottom up nonempirical approach to synthesis design.
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