Structure resolution by electron diffraction tomography of the complex layered iron-rich Fe-2234-type Sr5Fe6O15.4

Na2VO(HPO4)2: an original phase solved by continuous 3D electron diffraction and powder X-ray diffraction

The new phase Na2VO(HPO4)2 was synthesized by sodium/proton ion exchange between NaI and VO(H2PO4)2 in hexanol. The exchange of two protons by two sodium ions causes a structural reorganization leading to a new original phase. The crystal structure was solved by continuous 3D Electron Diffraction, consisting of recording a video in diffraction mode during the continuous sample holder rotation in order to acquire a complete dataset in a shortest time in order to avoid the deterioration of this electron beam sensitive material. The individual Electron Diffraction patterns were extracted from the video, processed by conventional electron diffraction crystallography programs (PETS, JANA2006) and the resulting structural model calculated by the charge flipping algorithm was refined from powder X-ray diffraction data. This material crystallizes in an orthorhombic unit cell in the Iba2 (45) space group, with the cell parameters a = 13.86852(19), b = 13.7985(2), c = 7.47677(9). Electrochemical studies show that up to 0.66 Na f.u.-1 could be removed from Na2VO(HPO4)2.

3D Electron Diffraction: The Nanocrystallography Revolution

Crystallography of nanocrystalline materials has witnessed a true revolution in the past 10 years, thanks to the introduction of protocols for 3D acquisition and analysis of electron diffraction data. This method provides single-crystal data of structure solution and refinement quality, allowing the atomic structure determination of those materials that remained hitherto unknown because of their limited crystallinity. Several experimental protocols exist, which share the common idea of sampling a sequence of diffraction patterns while the crystal is tilted around a noncrystallographic axis, namely, the goniometer axis of the transmission electron microscope sample stage. This Outlook reviews most important 3D electron diffraction applications for different kinds of samples and problematics, related with both materials and life sciences. Structure refinement including dynamical scattering is also briefly discussed.

Structure analysis of materials at the order–disorder borderline using three-dimensional electron diffraction

Diffuse scattering, observed as intensity distribution between the Bragg peaks, is associated with deviations from the average crystal structure, generally referred to as disorder. In many cases crystal defects are seen as unwanted perturbations of the periodic structure and therefore they are often ignored. Yet, when it comes to the structure analysis of nano-volumes, what electron crystallography is designed for, the significance of defects increases. Twinning and polytypic sequences are other perturbations from ideal crystal structure that are also commonly observed in nanocrystals. Here we present an overview of defect types and review some of the most prominent studies published on the analysis of defective nanocrystalline structures by means of three-dimensional electron diffraction.

Ba19Cr12O45: A High Pressure Chromate with an Original Structure Solved by Electron Diffraction Tomography and Powder X-ray Diffraction

We report on the discovery of a Ba-based chromate obtained by high pressure-high temperature treatment of the low pressure orthorhombic Ba₂CrO₄ phase. By combining transmission electron microscopy and powder X-ray diffraction measurements, we have determined its crystallographic structure. This new Cr-oxide has a cubic lattice with a = 13.3106(6) Å built from a three-dimensional network of two Cr sites, Cr1 and Cr2, both in octahedral environments, with face sharing between Cr1 and Cr2 octahedra and corner-sharing between two Cr1 octahedra. The resulting chemical composition Ba₁₉Cr₁₂O₄₅ and bond valence sum analysis suggest a possible charge disproportion between Cr⁴⁺ in the Cr1 site and Cr⁵⁺ in the Cr2 site. Finally analysis of magnetization measurements indicates antiferromagnetic correlations between Cr cations and also points toward a probable charge disproportion between Cr sites.