Geometrically frustrated interactions drive structural complexity in amorphous calcium carbonate

TC Nicholas and AE Stones and A Patel and FM Michel and RJ Reeder and DGAL Aarts and VL Deringer and AL Goodwin, NATURE CHEMISTRY (2023).

DOI: 10.1038/s41557-023-01339-2

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 CaMIDLINE HORIZONTAL ELLIPSISCa 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. Although amorphous calcium carbonate represents an important biomineralization precursor, its structure has been difficult to understand. Now, amorphous calcium carbonate's structure is shown to arise from the different bridging modes available to the calcium ions. This effective multi-well potential that drives calcium arrangements creates a geometric incompatibility between preferred Ca-Ca distances and frustrates crystallization.

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