Nanocrystalline n-AlO(OH) (Disperal P2 ® of Sasol) was investigated by means of the atomic pair distribution function (PDF). The PDF is derived from powder diffraction data, an ideally resolved PDF is obtained from a synchrotron source which provides a large maximal scattering vector length Q max > 300 nm -1 . Here, however, we were able to reveal atomic structure details of the ~ 4 nm particles from in-house diffraction data (Q max = 130 nm -1 ): PDF least squares model refinements show that n-AlO(OH) is of the layered Boehmite structure type (oC16, Cmcm). But the structure is uniformly distorted in domains of ~ 2 nm size within the nano particles. The hydrogen bonds between the layers are widened up significantly by +13 pm, accounting for the higher reactivity when compared to microcrystalline Boehmite. Our results from only one "nanocrystallographic" experiment are consistent with a trend which was found via the Rietveld technique on a series of AlO(OH) of different crystallite size (Bokhimi et al., 2001 [2]). In addition, the PDF contains information on structural distortion as a function of (coherence) domain size within the nano particles.
A new approach to solve quasicrystalline atomic structures in 3-dimensional (3D) real space is presented: The atomic pair distribution function (PDF) of face centred icosahedral Ho 9 Mg 26 Zn 65 [a(6D) = 2×5.18(3)Å] was obtained from in-house X-ray diffraction data (MoKα 1 ). Starting with rational approximant models, derived from 1/1and 2/1-Al-Mg-Zn, its local and medium range structure was refined (r < 27Å; R = 12.9%) using the PDF data. 85% of all atoms show Frank-Kasper (FK) type coordinations. Basic structural unit is the 3-shell, 104-atom Bergman cluster (d ≈ 15Å) comprising a void at its center. The clusters are interconnected sharing common edges and hexagonal faces of the 3rd shells. T he remaining space is filled by some glue atoms (9% of all atoms), yielding an almost tetrahedrally close packed structure. All Ho atoms are surrounded by 16 neighbours (FK-polyhedron "P"). Most of them (89%) are situated in the 2nd shell (pentagon dodecahedron), the other act as glue atoms. As a result and as can be expected for real matter, local atomic coordinations in quasicrystals are similar compared to common crystalline intermetallic compounds. From our results, the long range quasiperiodic structure of icosahedral Mg-Zn-RE (RE = Y and some rare earths) is anticipated to be a canonical cell tiling (CCT , after Henely) decorated with Bergman clusters.
The crystal structure of the nanocrystalline alpha phase of Pigment Yellow 213 (P.Y. 213) was solved by a combination of single-crystal electron diffraction and X-ray powder diffraction, despite the poor crystallinity of the material. The molecules form an efficient dense packing, which explains the observed insolubility and weather fastness of the pigment. The pair-distribution function (PDF) of the alpha phase is consistent with the determined crystal structure. The beta phase of P.Y. 213 shows even lower crystal quality, so extracting any structural information directly from the diffraction data is not possible. PDF analysis indicates the beta phase to have a columnar structure with a similar local structure as the alpha phase and a domain size in column direction of approximately 4 nm.
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