Dental enamel, a hierarchical material composed primarily of hydroxylapatite nanowires, is susceptible to degradation by plaque biofilm-derived acids. The solubility of enamel strongly depends on the presence of Mg(2+), F(-), and CO3(2-). However, determining the distribution of these minor ions is challenging. We show—using atom probe tomography, x-ray absorption spectroscopy, and correlative techniques—that in unpigmented rodent enamel, Mg(2+) is predominantly present at grain boundaries as an intergranular phase of Mg-substituted amorphous calcium phosphate (Mg-ACP). In the pigmented enamel, a mixture of ferrihydrite and amorphous iron-calcium phosphate replaces the more soluble Mg-ACP, rendering it both harder and more resistant to acid attack. These results demonstrate the presence of enduring amorphous phases with a dramatic influence on the physical and chemical properties of the mature mineralized tissue.
Ternary Fe
x
Ni2–x
P (0 < x < 2) phases exhibit
a range of useful properties that can be augmented or tuned by confinement
to the nanoscale including hydrotreating catalytic activity for small x and near-room temperature ferromagnetism for high x. In this work, a solution-phase arrested-precipitation
method was developed for the synthesis of Fe
x
Ni2–x
P over all values of x (0 < x < 2). The synthesis involves
preparation of Ni–P amorphous particles, introduction of the
Fe precursor to form amorphous Fe–Ni–P particles, and
high-temperature conversion of Fe–Ni–P particles into
crystalline ternary phosphide nanocrystals. The ternary Fe
x
Ni2–x
P nanocrystals
crystallize in the hexagonal Fe2P-type structure, and the
morphology of the nanocrystals showed a distinct compositional dependence,
transitioning from about 11 nm diameter spheres to rods with aspect
ratios approaching 2 as the Fe fraction is increased (x ≥ 1.2). Lattice parameters do not follow Vegard’s
law, consistent with Mössbauer data showing preferential site
occupation by Fe of the tetrahedral over the square pyramidal sites
at low Fe concentrations, and the opposite effect for x > 0.8. Magnetic measurements of Fe
x
Ni2–x
P (x = 1.8,
1.4,
and 1.2) nanorods showed a strong compositional dependence of the
Curie temperature (T
C) that differs from
observations in bulk phases, with the highest T
C (265 K) obtained for x = 1.4.
A synthetic cycle for the CO(2)-to-CO conversion (with subsequent release of CO) based on iron(II), a redox-active pydridinediimine ligand (PDI), and an O-atom acceptor is reported. This conversion is a passive-type ligand-based reduction, where the electrons for the CO(2) conversion are supplied by the reduced PDI ligand and the ferrous state of the iron is conserved.
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