On the amorphous layer in bone mineral and biomimetic apatite: a combined small- and wide-angle X-ray scattering analysis

The occurrence of an amorphous calcium phosphate layer covering the crystalline apatite core has been suggested to be an intrinsic feature of both bone mineral and synthetic biomimetic analogs. However, an exahustive quantitative picture of the amorphous-crystalline relationship in these materials is still missing. Here, we present a multiple scale modelling that combines small-angle X-ray scattering (SAXS) and synchrotron wide-angle X-ray total scattering (WAXTS) analyses to investigate the amorphous-crystalline spatial interplay in bone sample and biomimetic carbonated nano-apatites. SAXS analysis indicates the presence of a single morphology consisting of tiny nanoplates (NPLs) and provides a measure of their thickness (falling in the 3-5 nm range). WAXTS analysis was performed by developing atomistic models of apatite NPLs incorporating lattice strain, mostly attributed to the carbonate content, and calculating the X-ray patterns using the Debye Scattering Equation. Upon model optimization, the size and strain parameters of the crystalline platelets were derived and the amorphous component, co-existing with the crystalline one, separated and quantified (in the 23-33 wt% range). Notably, the thickness of the apatite core was found to exhibit nearly null (bone) or minor (< 0.5 nm, biomimetic samples) deviations from that of the entire NPLs, suggesting that the amorphous material remains predominantly distributed along the lateral sides of the NPLs, in a core-crown-like arrangement. The lattice strain analysis indicates a significant stiffness along the c axis, which is comparable in bone and synthetic samples, and larger deformations in the other directions. Statement of significance Current models of bone mineral and biomimetic nanoapatites suggest the occurrence of an amorphous layer covering the apatitic crystalline nanoplates in a core-shell arrangement. By combining X-ray scattering techniques in the small and wide angle regions, we propose a joint atomic-to-nanometre scale modelling to investigate the amorphous-crystalline interplay within the nanoplates. Estimates are extracted for the thickness of the entire nanoplates and the crystalline core, together with the quantification of the amorphous fraction and apatite lattice strain. Based on the thickness matching, the location of the amorphous material mostly along the edges of the nanoplates is inferred, with a vanishing or very thin layer in the thickness direction, suggesting a core-crown-like arrangement, with possible implications on the mineral surface reactivity.