VTechWorks staff will be away for the Thanksgiving holiday beginning at noon on Wednesday, November 27, through Friday, November 29. We will resume normal operations on Monday, December 2. Thank you for your patience.

Browsing by Author "Goodwin, Andrew L."

Now showing 1 - 2 of 2
  • Thumbnail Image
    Geometrically frustrated interactions drive structural complexity in amorphous calcium carbonate
    Nicholas, Thomas C.; Stones, Adam Edward; Patel, Adam; Michel, F. Marc; Reeder, Richard J.; Aarts, Dirk G. A. L.; Deringer, Volker L.; Goodwin, Andrew L. (Nature Portfolio, 2023-09-25)
    Amorphous calcium carbonate is an important precursor for biomineralization in marine organisms. Key outstanding problems include understanding the structure of amorphous calcium carbonate and rationalizing its metastability as an amorphous phase. Here we report high-quality atomistic models of amorphous calcium carbonate generated using state-of-the-art interatomic potentials to help guide fits to X-ray total scattering data. Exploiting a recently developed inversion approach, we extract from these models the effective Ca⋯Ca interaction potential governing the structure. This potential contains minima at two competing distances, corresponding to the two different ways that carbonate ions bridge Ca2+-ion pairs. We reveal an unexpected mapping to the Lennard-Jones–Gauss model normally studied in the context of computational soft matter. The empirical model parameters for amorphous calcium carbonate take values known to promote structural complexity. We thus show that both the complex structure and its resilience to crystallization are actually encoded in the geometrically frustrated effective interactions between Ca2+ ions. [Figure not available: see fulltext.]
  • Thumbnail Image
    Nanocomposite structure of two-line ferrihydrite powder from total scattering
    Funnell, Nicholas P.; Fulford, Maxwell F.; Inoue, Sayako; Kletetschka, Karel; Michel, F. Marc; Goodwin, Andrew L. (2020-02-21)
    Ferrihydrite is one of the most important iron-containing minerals on Earth. Yet determination of its atomic-scale structure has been frustrated by its intrinsically poor crystallinity. The key difficulty is that physically-different models can appear consistent with the same experimental data. Using X-ray total scattering and a nancomposite reverse Monte Carlo approach, we evaluate the two principal contending models-one a multi-phase system without tetrahedral iron(III), and the other a single phase with tetrahedral iron(III). Our methodology is unique in considering explicitly the complex nanocomposite structure the material adopts: namely, crystalline domains embedded in a poorly-ordered matrix. The multi-phase model requires unphysical structural rearrangements to fit the data, whereas the single-phase model accounts for the data straightforwardly. Hence the latter provides the more accurate description of the short- and intermediate-range order of ferrihydrite. We discuss how this approach might allow experiment-driven (in)validation of complex models for important nanostructured phases beyond ferrihydrite. Although a geologically important mineral, the atomic-scale structure of ferrihydrite remains unresolved. Here the authors combine X-ray total scattering and reverse Monto Carlo to evaluate the two principal contending models, explicitly considering the material's complex nanocomposite structure.