Fig. 2: Coordination environments and Ca-pair distributions in ACC.
From: Geometrically frustrated interactions drive structural complexity in amorphous calcium carbonate
a, Histogram of Ca2+ coordination environments, decomposed into contributions from carbonate and water oxygen donors (Oc and Ow, respectively). b, Histogram of CO32− coordination environments, now decomposed into contributions from Ca2+ and water hydrogen donors (Hw). c, Representative Ca2+ coordination sphere for the modal coordination environment marked by a star in a. d, Representative CO32− coordination sphere for the modal coordination environment marked by a star in b. Note that pairs of Ca2+ ions within the same CO32− coordination sphere either share a common oxygen donor (for example, red arrow) or are connected by Ca–O–C–O–Ca pathways (for example, blue arrow). Ca, C, O and H atoms are shown in beige, grey, aquamarine and white, respectively. e, Ca-pair correlation functions gCa(r) extracted from HRMC, RMC and LJG configurations, compared against the normalized Fourier transform of the experimental X-ray total scattering function (which includes contributions from all atom pairs, for example, the Ca–O peak at 2.4 Å marked with an asterisk). The two principal peaks common to all functions, indicated by red and blue shading, can be assigned to the two types of Ca2+-ion pair illustrated in d.
