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PRINCIPAL PUBLICATION AND AUTHORS

Mesocrystalline structure and mechanical properties of biogenic calcite from sea urchin spine, H. Cölfen (a), H.-B. Bürgi (b,c), D. Chernyshov (d), M. Stekiel (e), A. Chumakova (f), A. Bosak (f), B. Wehinger (f), B. Winkler (g), Acta Cryst. B B78, 356-358 (2022); https:/doi.org/10.1107/S2052520622000634 (a) Chemical Department, University Konstanz (Germany) (b) Department of Chemistry, University of Zürich (Switzerland) (c) Department of Chemistry, Biochemistry, and Pharmacy (DCBP), Universität Bern (Switzerland) (d) Swiss-Norwegian Beam Lines, ESRF (e) Department of Physics, Technical University Munich (Germany) (f) ESRF (g) Institute of Geosciences, Goethe University Frankfurt (Germany)

REFERENCES

[1] A. Bosak et al., J. Phys. D: Appl. Phys. 48, 504003 (2015). [2] M. Stekiel et al., Phys. Rev. B 99, 054101 (2019). [3] J. Seto et al., Proc. Natl. Acad. Sci. USA, 109, 3699-3704 (2012).

In Figures 99a and b, diffuse scattering intensity maps show a number of distinct features for inorganically formed natural calcite, the atomistic ab-initio models and the sea urchin spine. The diffuse clouds around the Bragg nodes are more isotropic for biogenic than for mineral calcite. Figures 99c and d present inelastic scattering data for a few representative directions as intensity maps in momentum transfer (Q) energy (E) coordinates. The scattering intensity of inorganically formed natural calcite vanishes at zero energy transfer and its dynamical structure factor reliably corresponds to the results of the ab-initio calculations [1] using phonon eigenvalues and eigenvectors [2].

Intensity maps for the sea urchin spine show essentially identical phonon dispersions and do not contain any extra features except for a divergence of the intensity of the elastic line towards the Brillouin zone centre. Variations of Ca/Mg composition and associated displacements and fluctuations in shape and size of the structurally coherent volumes serve as the main components of the static disorder in biocalcite. Thus, a ~50% increase in ADPs can be attributed to static disorder but not to the difference in the vibrational spectra.

Additionally, a difference in diffuse scattering due to a mesoscale structure with {104} nanofaceting of biogenic calcite crystals was identified. Figure 99e, in turn, illustrates the shape function or power spectrum of the crystal shape of an equilateral crystal. Real spine domains are neither equilateral nor monodisperse, but the anisotropic envelope of their shape functions with diffuse tails protruding along 104 is conserved, while interference ripples are lost. Nanoscale faceting with {104} facets derived from the diffuse scattering perfectly corresponds to the electron micrographs taken for similar samples in previous studies [3].

To conclude, sea urchin spines have unique micromechanical properties that are likely related to their intricate mesoscopic structure and microscopic architecture. In this experiment, the inelastic scattering component was similar for single-crystal abiotic natural calcite and the sea urchin spine: the elastic stiffness coefficients were closely similar. Hence, the highly porous single crystal with a developed mesoscale structure extracted from the urchin s spines may serve as a prototype for novel lightweight construction materials of natural origin. Furthermore, the obtained information is critical for creating functional materials with a wide range of practical applications, from self-sharpening knives to high-strength drills and cutting tools. Future studies may confirm and extend the insight gained in this study by employing diffuse and small-angle scattering in various subclasses of Echinoidea both spines and individual calcite plates of the internal skeleton.

Fig. 99: a) DS intensity maps for inorganically formed natural calcite in comparison to ab-initio calculations (left column) and

(b) the sea urchin spine. Results of IXS intensity maps in (Q-E) coordinates, measured for abiotic (c) and biogenic (d) calcite.

e) Equilateral calcite rhombohedron with {104} faceting; power spectrum of such a crystal in isointensity surface representation, and isointensity surface representation of small-angle scattering

on sea urchin spine tails are pointing towards 104 spots. Therefore, the mesoscopic architecture of the crystal should have

caused the differences in the mechanical properties.