Energetics of the Charge-Coupled Substitution Si4+ Na++ T3+ in the-Glasses NaTO2–SiO2 (T = Al, Fe, Ga, B)

Heats of solution in molten 2PbO·B2O3 at 973 K are reported for glasses xNaT3+O2–(1 –x)SiO2 for T = Fe, Ga. These measurements, combined with previous data for T = Al, B, give a relative measure of the enthalpy of the charge-coupled substitution Si4+→ Na++ T3+. The heats of solution become more endothermic with increasing x for x→ 0.5 and exhibit a maximum near x= 0.5. This indicates an exothermic enthalpy for the substitution and an overall stabilization of the glasses. The degree to which the glasses are stabilized decreases in the order Al > Ga > Fe > B. On the basis of molecular orbital calculations, X-ray scattering, and Raman spectroscopy, it is argued that this trend is primarily due to a decrease in the range of energetically favorable T–O–T bond angles as Al, Ga, Fe, and B are substituted for Si

Structure and Stability of the Iodide Elpasolite, Cs2AgBiI6

Iodide elpasolites (or double perovskites, A2B'B"I6, B' = M+, B" = M3+) are predicted to be promising alternatives to lead-based perovskite semiconductors for photovoltaic and optoelectronic applications, but no iodide elpasolite has ever been definitively prepared or structurally characterized. Iodide elpasolites are widely predicted to be unstable due to favorable decomposition to the competing A3B2I9 (B = M3+) phase. Here, we report the results of synchrotron XRD and X-ray total scattering measurements on putative Cs2AgBiI6 nanocrystals made via anion exchange from parent Cs2AgBiBr6 nanocrystals. Rietveld refinement of XRD and PDF data shows that these nanocrystals indeed exhibit a tetragonal (I4-m) elpasolite structure, making them the first example of a structurally characterized iodide elpasolite. A series of experiments probing structural relaxation and the effects of surface ligation or grain size all point to the critical role of surface free energy in stabilizing the iodide elpasolite phase in these nanocrystals.Comment: 7 figures, 1 scheme, plus supporting information fil

Surface Aggregation of Urinary Proteins and Aspartic Acid-Rich Peptides on the Faces of Calcium Oxalate Monohydrate Investigated by In Situ Force Microscopy

The growth of calcium oxalate monohydrate in the presence of Tamm-Horsfall protein (THP), osteopontin, and the 27-residue synthetic peptides (DDDS)6DDD and (DDDG)6DDD (D = aspartic acid, S = serine, and G = glycine) was investigated via in situ atomic force microscopy. The results show that these four growth modulators create extensive deposits on the crystal faces. Depending on the modulator and crystal face, these deposits can occur as discrete aggregates, filamentary structures, or uniform coatings. These proteinaceous films can lead to either the inhibition of or an increase in the step speeds (with respect to the impurity-free system), depending on a range of factors that include peptide or protein concentration, supersaturation, and ionic strength. While THP and the linear peptides act, respectively, to exclusively increase and inhibit growth on the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\bar{1}01} \right)$$\end{document} face, both exhibit dual functionality on the (010) face, inhibiting growth at low supersaturation or high modulator concentration and accelerating growth at high supersaturation or low modulator concentration. Based on analyses of growth morphologies and dependencies of step speeds on supersaturation and protein or peptide concentration, we propose a picture of growth modulation that accounts for the observations in terms of the strength of binding to the surfaces and steps and the interplay of electrostatic and solvent-induced forces at the crystal surface

Controlled synthesis of highly-branched plasmonic gold nanoparticles through peptoid engineering

In nature, specific biomolecules interacting with mineral precursors are responsible for the precise production of nanostructured inorganic materials that exhibit complex morphologies and superior performance. Despite advances in developing biomimetic approaches, the design rules for creating sequence-defined molecules that lead to the synthesis of inorganic nanomaterials with predictable complex morphologies are unknown. Herein we report the design of sequence-defined peptoids for controlled synthesis of highly branched plasmonic gold particles. By engineering peptoid sequences and investigating the resulting particle formation mechanisms, we develop a rule of thumb for designing peptoids that predictively enabled the morphological evolution from spherical to coral-shaped nanoparticles. Through a combination of hyperspectral UV-Vis extinction microscopy and three-photon photoemission electron microscopy, we demonstrate that the individual coral-shaped gold nanoparticles exhibit a plasmonic enhancement as high as 105-fold. This research significantly advances our ultimate vision of predictive bio-inspired materials synthesis using sequence-defined synthetic molecules that mimic proteins and peptides

Energy-Economical Heuristically Based Control of Compass Gait Walking on Stochastically Varying Terrain

Investigation uses simulation to explore the inherent tradeoffs ofcontrolling high-speed and highly robust walking robots while minimizing energy consumption. Using a novel controller which optimizes robustness, energy economy, and speed of a simulated robot on rough terrain, the user can adjust their priorities between these three outcome measures and systematically generate a performance curveassessing the tradeoffs associated with these metrics

Formation of stable uranium(VI) colloidal nanoparticles in conditions relevant to radioactive waste disposal

The favored pathway for disposal of higher activity radioactive wastes is via deep geological disposal. Many geological disposal facility designs include cement in their engineering design. Over the long term, interaction of groundwater with the cement and waste will form a plume of a hyperalkaline leachate (pH 10-13), and the behavior of radionuclides needs to be constrained under these extreme conditions to minimize the environmental hazard from the wastes. For uranium, a key component of many radioactive wastes, thermodynamic modeling predicts that, at high pH, U(VI) solubility will be very low (nM or lower) and controlled by equilibrium with solid phase alkali and alkaline-earth uranates. However, the formation of U(VI) colloids could potentially enhance the mobility of U(VI) under these conditions, and characterizing the potential for formation and medium-term stability of U(VI) colloids is important in underpinning our understanding of U behavior in waste disposal. Reflecting this, we applied conventional geochemical and microscopy techniques combined with synchrotron based in situ and ex situ X-ray techniques (small-angle X-ray scattering and X-ray adsorption spectroscopy (XAS)) to characterize colloidal U(VI) nanoparticles in a synthetic cement leachate (pH > 13) containing 4.2-252 μM U(VI). The results show that in cement leachates with 42 μM U(VI), colloids formed within hours and remained stable for several years. The colloids consisted of 1.5-1.8 nm nanoparticles with a proportion forming 20-60 nm aggregates. Using XAS and electron microscopy, we were able to determine that the colloidal nanoparticles had a clarkeite (sodium-uranate)-type crystallographic structure. The presented results have clear and hitherto unrecognized implications for the mobility of U(VI) in cementitious environments, in particular those associated with the geological disposal of nuclear waste