Revealing crystalline domains in a mollusc shell single-crystalline prism

Abstract

Biomineralization integrates complex processes leading to an extraordinary diversity of calcareous biomineral crystalline architectures, in intriguing contrast with the consistent presence of a sub-micrometric granular structure. Hence, gaining access to the crystalline architecture at the mesoscale, that is, over a few granules, is key to building realistic biomineralization scenarios. Here we provide the nanoscale spatial arrangement of the crystalline structure within the 'single-crystalline' prisms of the prismatic layer of a Pinctada margaritifera shell, exploiting three-dimensional X-ray Bragg ptychography microscopy. We reveal the details of the mesocrystalline organization, evidencing a crystalline coherence extending over a few granules. We additionally prove the existence of larger iso-oriented crystalline domains, slightly misoriented with respect to each other, around one unique rotation axis, and whose shapes are correlated with iso-strain domains. The highlighted mesocrystalline properties support recent biomineralization models involving partial fusion of oriented nanoparticle assembly and/or liquid droplet precursors.

Numerical study of Bragg CDI on thick polycrystalline specimens

“…Hence, the support condition does not bring useful information to the phase problem, which can be solved only if additional a priori knowledge is available [14]. Bragg ptychography has been proposed to address the case of an extended crystal, but it still requires that the sample fulfills the longitudinal coherence length condition (implying a limit on thickness of typically about 1 μm) [15]. However, many areas of research encountered in materials science, engineering, and geoscience, require the investigations of thicker samples comprised of a myriad of micro-crystals producing super-imposed diffraction Bragg peaks.…”

Three-Dimensional Imaging of Phase Ordering in an Fe-Al Alloy by Bragg Ptychography

“…In this Letter, we present the visualization of APD structures in three dimensions together with the associated general lattice strain. This is achieved by Bragg ptychography [21][22][23][24][25], which has shown robustness in retrieving images of materials exhibiting strong phase shifts [26], allowing us to compare and combine 3D images obtained at two different reflections, a fundamental and a superlattice peak, but originating from the same crystal grain. The superlattice peak is sensitive to both antiphase domain boundaries and phase variations from lattice strain and tilts while the fundamental peak is only sensitive to strain and tilts.…”

“…For every ðx; yÞ position of the beam a high resolution θ scan was performed hence acquiring a pixel detector image of the scattering at every ðx; y; θÞ point. For both reflections this provides 3D data sets of approximately 40 GB to be analyzed with the inversion algorithms [25].…”

“…To accomplish the Bragg ptychography reconstructions a GPU-based algorithm was developed, introducing a 3D complex valued effective electron density to represent the diffracting sample structure [23][24][25]. The modulus is proportional to the electron density and the phase holds information about the lattice displacement field, e.g., strain, crystalline tilt, and potentially phase domain boundaries [for the (001) reflection only].…”

“…This is approximately what is expected as typical APD size of the sample after annealing [20] and it falls below the longitudinal coherence limit of this setup. This information was used for both data sets and introduced as a regularization term in the inversion process favoring a confinement of the solution within a finite thickness support [25]. The reconstructions employed a total of 4000 iterations using a combination of two different methods namely the ordered subset and conjugated gradient algorithms.…”