Nanodomains and local structure in ternary alkaline-earth hexaborides

Abstract

The local structures of ternary alkaline-earth hexaborides (MB 6 , M = Ca 0.5 Sr 0.5 , Ca 0.5 Ba 0.5 and Sr 0.5 Ba 0.5 ) have been analysed using X-ray pair distribution function (PDF) analysis, Raman spectroscopy and transmission electron microscopy (TEM). The results show significant local deviations from the average cubic structure within the boron sub-lattice and support the conclusion that rapid synthesis processes lead to the formation of coherent nanodomains over length scales of about 10 nm. Reverse Monte Carlo fitting of the PDFs allows for quantification of the displacement disorder within the boron sublattice as a function of sample composition. Detailed Raman spectroscopy studies and high-resolution TEM support the models derived from X-ray scattering. The average magnitude of the static displacement disorder varies by sample composition and positively correlates with the cation radius ratios across the three compositions. The new models form a foundation for future computational and experimental studies aimed at understanding and predicting properties of hexaborides.

One-Step Solvothermal Preparation of Mn-Doped VO2(B) Nanosheets for High-Performance Supercapacitors

“…5), behavior which has been attributed to the presence of local distortions in an otherwise bulk material. [48][49][50] Most notably, the c lattice parameter, refined from considering only the long-range structure (30-70 Å), increases from 6.421 Å in the undoped sample to 6.424 Å in the doped sample. Similarly, when considering only the short-range structure (1.35-15 Å), the c lattice parameter increases from 6.435 Å in the undoped sample to 6.443 Å in the doped sample.…”

Phase Stability Analysis of Ternary Alkaline-Earth Hexaborides: Insights from DFT Calculations

“…Thus, the histogram incorporates 11 atomic configurations and represents an ensemble average over many different local environments, including locally Ca-rich or Ba-rich configurations. Koch and co-workers 67 used reverse Monte Carlo fitting of X-ray pair distribution function data to extract the various B−B bond lengths from a large supercell, and found that the B−B distances range from 1.43 to 1.97 Å, with a clear larger peak near 1.8 Å and a smaller long tail in the distribution toward shorter bond lengths. The result from our computational work (Figure 5) mimics the form of the distribution found in the experimental result, though the bond distance range is smaller (1.65 to 1.78 Å), and the peak in the distribution is smaller by 0.25 Å.…”

“…Figure shows that it is possible to derive a histogram of B–B bond lengths from the various DFT-derived configurations. In order to compare to the experimental work of Koch et al, we show an average over the three types of ordered states for each of the three Ca/Ba ratios and also include the end members. Thus, the histogram incorporates 11 atomic configurations and represents an ensemble average over many different local environments, including locally Ca-rich or Ba-rich configurations.…”

Morphology Control of Tantalum Carbide Nanoparticles through Dopant Additions

“…Strain effects have also been explored as parameters for morphological modification of copper and nickel particles, and doping has been applied for obtaining nanorods of magnesium . In addition, modeling − and experimental techniques − have been applied for exploring the behavior of borides of cubic and other morphologies. Moreover, experimental efforts have investigated shape change in cubic carbides as a function of carbon stoichiometry, which influences the relative growth rate of the dominant {111} and {100} facets. − …”