Publications

2024

1. Nat. Chem.

   Geometrically frustrated interactions drive structural complexity in amorphous calcium carbonate

   T. C. Nicholas, A. E. Stones, A. Patel, and 5 more authors

   Nat. Chem., 2024

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   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.

   @article{Nicholas2024,
     title = {Geometrically frustrated interactions drive structural complexity in amorphous calcium carbonate},
     author = {Nicholas, T. C. and Stones, A. E. and Patel, A. and Michel, F. Marc and Reeder, Richard J. and Aarts, Dirk G. A. L. and Deringer, Volker L. and Goodwin, Andrew L.},
     journal = {Nat. Chem.},
     volume = {16},
     pages = {36--41},
     year = {2024},
     doi = {10.1038/s41557-023-01339-2},
     publisher = {Nature Publishing Group},
     dimensions = {true},
   }

2021

1. J. Phys. Chem. B

   Intermediate Range Order in Metal–Ammonia Solutions: Pure and Na-Doped Ca-NH3

   Thomas C. Nicholas, Thomas F. Headen, Jonathan. C. Wasse, and 3 more authors

   J. Phys. Chem. B, 2021

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   The local and intermediate range ordering in Ca–NH₃ solutions in their metallic phase is determined through H/D isotopically differenced neutron diffraction in combination with empirical potential structure refinements. For both low and high relative Ca concentrations, the Ca ions are found to be octahedrally coordinated by the NH₃ solvent, and these hexammine units are spatially correlated out to lengthscales of ∼7.4−10.3 Å depending on the concentration, leading to pronounced ordering in the bulk liquid. We further demonstrate that this liquid order can be progressively disrupted by the substitution of Ca for Na, whereby a distortion of the average ion primary solvation occurs and the intermediate range ion−ion correlations are disrupted.

   @article{Nicholas2021,
     author = {Nicholas, Thomas C. and Headen, Thomas F. and Wasse, Jonathan. C. and Howard, Christopher. A. and Skipper, Neal. T. and Seel, Andrew G.},
     title = {Intermediate Range Order in Metal–Ammonia Solutions: Pure and Na-Doped Ca-NH3},
     journal = {J. Phys. Chem. B},
     volume = {125},
     pages = {7456--7461},
     year = {2021},
     doi = {10.1021/acs.jpcb.1c03843},
     dimensions = {true},
   }

2020

1. Chem. Sci.

   Understanding the geometric diversity of inorganic and hybrid frameworks through structural coarse-graining

   Thomas C. Nicholas, Andrew L. Goodwin, and Volker L. Deringer

   Chem. Sci., 2020

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   Much of our understanding of complex structures is based on simplification: for example, metal–organic frameworks are often discussed in the context of “nodes” and “linkers”, allowing for a qualitative comparison with simpler inorganic structures. Here we show how such an understanding can be obtained in a systematic and quantitative framework, combining atom-density based similarity (kernel) functions and unsupervised machine learning with the long-standing idea of “coarse-graining” atomic structure. We demonstrate how the latter enables a comparison of vastly different chemical systems, and we use it to create a unified, two-dimensional structure map of experimentally known tetrahedral AB2 networks – including clathrate hydrates, zeolitic imidazolate frameworks (ZIFs), and diverse inorganic phases. The structural relationships that emerge can then be linked to microscopic properties of interest, which we exemplify for structural heterogeneity and tetrahedral density.

   @article{Nicholas2020,
     author = {Nicholas, Thomas C. and Goodwin, Andrew L. and Deringer, Volker L.},
     title = {Understanding the geometric diversity of inorganic and hybrid frameworks through structural coarse-graining},
     journal = {Chem. Sci.},
     year = {2020},
     volume = {11},
     pages = {12580--12587},
     publisher = {The Royal Society of Chemistry},
     doi = {10.1039/D0SC03287E},
     dimensions = {true},
   }