The crystalline properties of silica biomorphs vary within and between morphologies

Abstract

Silica-witherite biomorphs are a class of emergent materials, i.e. composite microstructures made of nanometric barium carbonate surrounded by amorphous silica. They form via co-precipitation of barium carbonate and siliceous species, and self-organize into a multitude of shapes with a distinct long-range order of the carbonate nanocrystals. However, the internal structural organization within and across different morphologies remains insufficiently resolved. Here, we use X-ray texture and diffraction tomography to create three-dimensional, spatially resolved maps of crystallographic orientation and structural parameters in silica-witherite biomorphs. At the sub-micron voxel level, all morphologies exhibit a crystallographic order consistent with a fiber texture around the c-axis. At larger length scales, however, the orientation field as well as crystallite size, crystallite shape anisotropy and the unit cell volume show systematic spatial variations. Leaf-like and helical morphologies contain defined directions along which structural parameters change systematically. Furthermore, we find recurring structural regimes with strong similarities between these morphologies. Conversely, coral-like morphologies are overall less textured and outside of the nucleation region we do not find clear structural regimes in the crystalline properties. These results provide a three-dimensional description of the internal organization of crystallites in silica-witherite biomorphs and establish a basis for systematically relating crystallographic organization to morphology.