In biomineralization, amorphous precursors enable formation of complicated crystal shapes and incorporation of organic and inorganic impurities. In some cases the concentrations of the inorganic impurity atoms greatly exceed the thermodynamical (equilibrium) solubility limits. To shed additional light on this, crystallization experiments with Sr-rich amorphous calcium carbonate (ACC) are carried out. The Sr-concentration in ACC, Sr/(Ca+Sr) 0.3, is more than 60 times larger than its solubility limit in calcite. Crystallization of ACC is accomplished using two different pathways: i) in a high humidity environment at room temperature, and ii) upon heating at 300°C. In both cases, X-ray diffraction measurements revealed the formation of particular crystal phases with an ability to take up substantial amounts of Sr. Crystals formed via the humidity route comprised spatially separated regions of almost pure rhombohedral calcite and orthorhombic aragonite/strontianite solid solution with a Sr/(Ca+Sr) ratio equal nearly 0.5. Crystallization of ACC upon heating resulted in the appearance of a new calcite-like phase with reduced space symmetry, R3m, instead of R3c for regular calcite. Symmetry reduction in this Sr-rich R3m-phase is due to the rotational disorder of flat carbonate groups about the c-axis, which is responsible for the enhanced lattice capacity of Sr incorporation.

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In this study, a bio-inspired hybrid material is investigated by in situ X-ray scattering experiments in combination with mechanical tensile testing. The material is composed of nanometer-sized spherical magnesium fluoride particles which are linked via material-specific peptide poly(ethylene glycol)-PEG conjugates to a semi-crystalline poly(ethylene oxide) PEO matrix. Mechanically relevant changes in crystal size and orientation in the PEO matrix are followed by wide angle X-ray scattering during the application of tensile stress. The amorphous phase of PEO is stabilized by the surfaceengineered MgF 2 nanoparticles, leading to increased Young's modulus and tensile strength. Furthermore, small angle X-ray scattering experiments allowed the identification of a layer on the MgF 2 particle surfaces, which increases in thickness with the conjugate amount and leads to suppression of the agglomeration of MgF 2 nanoparticles. In conclusion, the use of selected peptide-PEG conjugates tailored to link MgF 2 particles to a PEO matrix successfully mimics the biological principle of interface polymers and suggests new directions for material fabrication for bio-applications.