Structural analysis of Ni₃Si₂ by EDT and dynamical refinement

Transition metal silicides are known for properties such as low resistivity, high melting point, low cost and low toxicity, which are of great interest for applications in current silicon nanotechnology such as nano-complementary metal-oxide semiconductor (CMOS) devices, photovoltaics and ohmic contacts. In all these technologies the materials are used on nanoscale. To gain better insight into their properties, it is necessary to be able to determine the structure of the nanoparticles of these materials. Electron diffraction tomography combined with the precession electron diffraction (PED) are ideal techniques for structural analysis of nanocrystals. In this work Ni3Si2 nanowires with diameter of 25 nm were analyzed by EDT both with and without PED. The structure was refined using the kinematical and dynamical diffraction theory. The results show that the best results can be obtained of EDT and PED.

Accurate structure refinement from 3D electron diffraction data

Structure determination from electron diffraction data has seen an enormous progress over the past few years. At present, complex structures with hundreds of atoms in the unit cell can be solved from electron diffraction using the concept of electron diffraction tomography (EDT), possibly combined with precession electron diffraction (PED) [1]. Unfortunately, the initial model is typically optimized using the kinematical approximation to calculate model diffracted intensities. This approximation is quite inaccurate for electron diffraction and leads to high figures of merit and inaccurate results with unrealistically low standard uncertainties. The obvious remedy to the problem is the use of dynamical diffraction theory to calculate the model intensities in structure refinement. This technique has been known and used before, but it has not become very popular, because good fits could be obtained only for sufficiently perfect and sufficiently thin crystals. It has been shown recently on several zone-axis patterns [2] that the quality of the refinement can be improved by using precession electron diffraction. In the present contribution we demonstrate that the same approach can be successfully used to refine crystal structures against non-oriented patterns acquired by EDT combined with PED (PEDT in short). Because the PEDT technique provides three-dimensional diffraction information, it can be used for a complete structure refinement. Several test examples demonstrate that the dynamical structure refinement yields better figures of merit and more accurate results than the refinement using kinematical approximation.

Use of Precession Electron and X Powder for solution and refinement of materials

This communication will present the case study of ALa5O5(VO4)2 (A= Li, Na, K, Rb), example of the use of a combination of Precession Electron and X-ray Powder Data for the solution and the refinement of new materials. Indeed, an original structural type has been evidenced in the system (A, La, V, O) with A=Li, Na, K, Rb. Attempts to solve the structure ab initio on X-ray powder data were unsuccessful (more particularly because the powder was a mixture of the title compound and of unreacted precursors). The structure was finally solved by charge flipping using Precession Electron Data (3D tomography) on a nanocrystal, enabling a posteriori the good formulation of a pure powder. This powder was then classically refined by Rietveld method showing the correctness of the electron-solved structure. It crystallizes in a monoclinic unit cell with space group C2/m and a=20.2282(14) b=5.8639(4) c=12.6060(9) Å and β=117.64(1)⁰. The ALa5O5(VO4)2 structure is built of (OLa4) tetrahedral units creating Crenel-like 2D ribbons. These ribbons, surrounded by four isolated VO4 tetrahedra, are creating channels parallel to b axis in which A+ ions are located.

Electron Diffraction vs aperiodic crystal: from indexation to structure solution

Can we solve aperiodic structures using intensities from electron diffraction? Yes! How? No mystery about it: the data analysis and the tools used for structure solution are essentially the same as the ones used in X-ray crystallography. The trick actually lies in new approaches used in electron crystallography. In analogy to X-ray diffraction, the so-called Electron Diffraction Tomography (EDT) [1] corresponds to a phi-scan data collection on a single crystal. There lies one major advantage of this technique: a powder sample is easily converted to infinitely large number of single crystals for electron diffraction. In case of aperiodic crystals this makes the difference over X-ray or neutron powder diffraction where, often, the lack of peaks clearly assignable to satellite reflections prevents any indexation and further analysis of the structure [2]. EDT allows for an accurate estimation of the modulation vector and a good guess of the super space group. These informations can be advantageously used as an input for X-ray or neutron powder diffraction. Not limited to indexation, EDT combined with Precession Electron Diffraction (PED) [3], offers a unique tool for solving modulated structures when crystals suitable for X-ray diffraction are missing. By limiting the paths for multiple scattering, PED makes the diffracted intensities closer to kinematical approximation so that they can be used efficiently for structure solution. Regarding aperiodic crystals, the superspace electron density map, generated as an output of the charge flipping algorithm used in Superflip, can be interpreted to obtain a structural model. This will be illustrated on a series of layered materials closely related to the Aurivillius phases belonging to the pseudo-binary Bi5Nb3O15-ABi2Nb2O9 (A=Pb, Sr, Ca, Ba). Limitations and possible combination with powder diffraction patterns will be discussed.

Semiconductors and Aperiodic Structures in Organocyanide-Based Materials

Organocyanides such as TCNQ (tetracyanoquinodimethane) and DCNQI (dicyanoquinodiimine) are excellent electron acceptors and have been extensively studied in electrically conducting/switching and magnetic materials. Supramolecular interactions such as π-stacking and hydrogen bonding in addition to coordination bonds with metal ions play an important role in the self-assembly of these functional inorganic materials. A novel semiconductor Cd2(TCNQ)3.5(H2O)2 with non-integer valences of TCNQ was synthesized and is the first example that exhibits four bridging modes of TCNQ in one structure (Figure 1). Despite the rather large stacking distance of 3.687(1) Å between the two μ3-TCNQ species, which constitute a "broken link" in the electron conducting pathway, the semiconductor exhibits a room temperature conductivity of 5.8×10-3 S·cm-1. The hydrogen bonding interactions between the coordinated water molecules and nitrogen atoms of the mu2- and mu3-TCNQ help to stabilize these coordinatively unsaturated TCNQ species. Another application of the chemistry of organocyanides in the context of this research is the study of reactions of the meta-dicyanamidobenzene dianion (DCYD2-) which was predicted by Ruiz and coworkers to facilitate ferromagnetic interactions between certain paramagnetic metal ions. The DCYD2- anion self-assembles with Mn(II) building blocks to afford a rare example of inorganic quadruple helices with an incommensurate modulated structure (Figure 2a). A supercell in the high symmetry space group P4/nnc is obtained with a c parameter five times that of the basic structure and a further 15-fold expansion in the space group of P1 along the c axis reveals that ~2% of Mn positions are disordered. The packing of these helical chains through π-stacking of pyridyl groups leads to 1D nanochannels with an estimated empty volume of 38.6% of the super cell volume (31,587Å3). However, in the presence of water, hydrogen bonding between the water molecules and the cyano-N atoms dominates and only zig-zag chains are formed (Figure 2b).