Fast microstructure and phase analyses of nanopowders using combined analysis of transmission electron microscopy scattering patterns

The usefulness of combined analysis for fast determination of crystallite sizes and shapes of nanoparticle aggregates using electron diffraction patterns is demonstrated.

Anisotropic resistivity tensor of melt-cast Bi2212 superconductors from QTA

Bi2Sr2CaCu2O8+δ HTSC superconductor is characterized by a very strong normal-state resistivity anisotropy, with ρc/ρab typically above 10E4. The aim of this study is to use Quantitative Texture Analysis from x-ray diffraction measurements to estimate the orientation effect on the anisotropic macroscopic resistivity in melt-cast bulk Bi2Sr2CaCu2O8+δ superconductors. Our approach uses the geometric mean [1] of the single crystal resistivity tensor weighted by the Orientation Distribution Function (ODF) to quantitatively estimate the macroscopic resistivity tensor of the samples. The ODF is obtained from x-ray Combined Analysis [2], using the E-WIMV algorithm of the MAUD software. The GMA applies to the rank-two resistivity tensor of the orthorhombic space group considered tetragonal due to the small difference of a- and b-axes of the phase, with only two independent tensor components. We relate a relatively good agreement between measured and calculated macroscopic anisotropic resistivity ratios. Even with ρc/ρab between 10E4 and 10E5 for Bi2212 at room temperature in single crystals [3], we experiment macroscopic ratio in our bulk samples of around only 2. This small ratio is explained by the weak planar- or fiber-like (Figure) texture achieved in the melt-cast samples, characterized by maxima of orientation distributions not larger than 10 mrd. Calculated resistivities, based on homogeneous crystallites, perfect grain boundaries and no secondary phases, are 10 times larger than the observed ones. This suggests that the observed minor phases positively affect conductive pathways between grains. Calculated and measured anisotropic resistive ratios are coherent with one another, and Combined Analysis gives good predictions of these former.

Freshwater pearls as near single crystals for vaterite structure resolution

There has been lots of controversies about vaterite structure in the past decades. Extra peaks occurring out of the hexagonal structure and best described by Kamhi [1] still resist any indexing. Lower space group symmetries, superspace groups, microtwinning, and first-principle calculations [2], all failed in taking account of these minor peaks, surprisingly always present in all synthetic and biogenic vaterite formations. Recently, secondary interspersed domains observed in high-resolution TEM images indicated their incoherence and rather incompatible character with the vaterite matrix [3]. One of the major difficulty in resolving the vaterite structure lies in the absence of single crystals. Powder diffraction patterns are always composed of hexagonal and extra, but small, peaks, and temptation to index the pattern as a single phase is large, particularly since x-ray fluorescence invariably probes for CaCO3. We used Hyriopsis cumingii freshwater mussel pearls to help proving that vaterite is definitely crystallizing within the original hexagonal space group. Some of these pearls suffer defective growth toward vaterite. In such cases the hexagonal peaks clearly exhibit a strong texture while the extra peaks look more random. This is an invaluable evidence of the existence of clearly separated phases, though the minor phase (or phases) still resist indexing. The hexagonal structure refinement, thanks to the strong vaterite texture, is obtained with larger resolution than before.

Microstructure and crystallographic texture of Ranella Olearia Shell

Molluscs are soft-bodied animals, that why many of them invented complex strategies for protecting themselves [1]. One of these strategies consists in creating a rigid biomineral called mollusc shell. Mollusc shells are complex biocomposites of mineral and organic material (5% in volume) with high mechanical performances, compared to the geological mineral. Mollusc shells are mainly made of calcite and aragonite crystalline polymorphs of calcium carbonate. This organic part behaves as nanometer growth-control of the inorganic crystals. This has stimulated chemists and materials scientists to design materials with a microstructure similar to that of nacre. To achieve this, understanding the microstructure of the nacre at various scales is certainly the key [2]. In this work we made use of the Combined Analysis [3] to determine the structure and preferred orientations of constituting aragonite crystallites of the Ranella olerea shell (fig. 1a) layers using scanning electron microscopy and X-rays diffraction. SEM analyses (fig. 1b) show the presence of an inner layer composed of Radial Lamellar, an intermediate comarginal crossed lamellar layer and an outer crossed lamellar layer. The refinement of X-ray diffraction diagrams gives the textures of the three layers, their respective aragonite unit-cell distortions, and the macroscopic elastic tensor of their mineral parts. The textures of the three layers were found to be of high level, especially for the inner layer. Both intermediate and outer layers exhibit regular texture patterns for crossed lamellae with a split of the c-axis component around the normal to the shell. An anisotropic unit-cell distortion is quantified for the three layers which is attributed to the combined effects of inter- and intra-crystalline macromolecules. The simulation of the macroscopic elastic tensor shows that the strong orientations present in the successive layers give an optimisation in terms of rigidity and shear resistance.

Implementing graphic outputs for the Material Properties Open Database (MPOD)

The tensor nature of single- and polycrystalline materials' physical properties highlights both the diversity of possible technological applications and the difficulties of assimilation for those new to the subject. The Material Properties Open Database (MPOD) [1] is a useful tool that provides access to a wide spectrum of properties tensors for an extensive selection of materials. In the present contribution an extension of the MPOD system is reported. The introduced innovation is the output, in the form of a graphical representation, of registered second, third and fourth rank tensors. The objective, as an educational project, is to provide the crystallographic community a friendly means to help the intuitive understanding of crystalline anisotropy. The given graphical output is the so-called longitudinal surface representation [2]. The accompanying figure shows an example of the MPOD graphical output. Shown surfaces represent the compliance tensor and its inverse (Young's modulus) longitudinal surfaces for a silver single-crystal. MPOD's new version may be accessed by the original website http://www.materialproperties.org/ and also by its Mexican mirror http://mpod.cimav.edu.mx. The MPOD websites continue their development. The international MPOD group systematically adds new published data. Modeling and representing textured polycrystals' properties is on target [3].

Chemical information presentation in the Crystallography Open Database

Crystallography Open Database (COD, http://www.crystallography.net/) is the largest to date curated open-access collection of small to medium sized unit cell crystal structures [1,2]. Over 11 years of development, COD has accumulated over 1/4 million structures from the peer reviewed press and personal communications. COD has an automated data submission Web site, performs routine automatic quality checks on all incoming structures and is now recommended as a database for crystallographic deposition by several scientific journals. To facilitate automatic use and discoverability of COD data, and to increase usefulness of our database for chemists, two steps were undertaken. COD was now supplemented with software and data from the CrystalEye data aggregator. The new software permits extracting chemical data and presenting them as structural formula, unique moieties, and chemically significant fragments. We have also implemented search of crystal structures by the structural chemical formulae of the target compounds. The search is first of all performed among 70 000 hand-curated chemical structure descriptors, and can be extended to automatically generated descriptors. To facilitate data curation, a new software platform for data review is being developed. All COD structures will be evaluated using statistical distributions of observed geometrical and chemical properties (bond lengths, angles, dihedrals, planarities). The most statistically unusual structures will be forwarded to a COD reviewer Internet forum, where qualified reviewers will be asked whether they find provided evidence for a particular structure convincing or not. In this way, a set of human review indicators (convincing/unconvincing) will be available along with the match against the bulk of data (usual structure/unusual). Such indicators would be especially useful for deciding which COD records require special attention and which subsets of COD should be selected for reliable scientific inferences.

Launching the Theoretical Crystallography Open Database

As computational chemistry methods enjoy unprecedented growth, computer power increases and price/performance ratio drops, a large number of crystal structures can today be refined and their properties computed using modern theoretical approaches, such as DFT, post-HF, QM/MM, MCMM methods. Availability of several open source codes for computational and quantum chemistry and open-access crystallographic databases enables large scale computations of material structures and properties. We thus increasingly feel that an open collection of theoretically computed chemical structures would be a valuable resource for the scientific community. To address this need, we have launched a Theoretical Crystallography Open Database (TCOD, http://www.crystallography.net/tcod/). The TCOD sets a goal to collect a comprehensive set of computed crystal structures that would be made available under an Open Data license and invites all scientists to deposit their published results or pre-publication data. Accompanied with a large set of experimental structures in the COD database [1], the TCOD opens immediate possibilities for experimental and theoretical data cross-validation. To ensure high quality of deposited data, TCOD offers ontologies in a form of CIF [2] dictionaries that describe parameters of computed chemical and crystal structures, and an automated pipeline that checks each submitted structure against a set of community-specified criteria for convergence, computation quality and reproducibility. The scope of TCOD and validation tools make TCOD a high-quality, comprehensive theoretical structure database, useful in a broad range of disciplines. First-principle calculations are also of huge interest to simulate physical properties, either prospectively or for materials that do not grow as sufficiently large crystals. The property results can now be tested against the Material Properties Open Database [3] (http://www.materialproperties.org/) to ameliorate the used models.