At the Gates: The Tantalum-Rich Phase Hf3Ta2O11 and its Commensurately Modulated Structure

Generic mixtures in the system (Zr,Hf)O2–(Nb,Ta)2O5 are employed as tunable gate materials for field-effect transistors. Whereas production processes and target compositions are well-defined, resulting crystal structures are vastly unexplored. In this study, we summarize the sparse reported findings and present the new phase Hf3Ta2O11 as synthesized via a sol–gel route. Its commensurately modulated structure represents the hitherto unknown, metal(V)-richest member of the family (Zr,Hf)x(Nb,Ta)2O2x+5. Based on electron, neutron, and X-ray diffraction, the crystal structure is described within modern superspace [Hf1.2Ta0.8O4.4, Z = 2, a = 4.7834(13), b = 5.1782(17), c = 5.064(3) Å, q = 1/5c*, orthorhombic, superspace group Xmcm(00γ)s00] and supercell formalisms [Hf3Ta2O11, Z = 4, a = 4.7834(13), b = 5.1782(17), c = 25.320(13) Å, orthorhombic, space group Pbnm]. Transmission electron microscopy shows the microscopic structure from film-like aggregates down to atomic resolution. Cation ordering within the different available coordination environments is possible, but no significant hint at it is found within the limits of standard diffraction techniques. Hf3Ta2O11 is an unpredicted compound in the above-mentioned oxide systems, in which stability ranges have been disputably fuzzy and established only by syntheses via solid-state routes so far.DFG, SPP 1613, Regenerativ erzeugte Brennstoffe durch lichtgetriebene Wasserspaltung: Aufklärung der Elementarprozesse und Umsetzungsperspektiven auf technologische Konzept

Transformation of ACC into aragonite and the origin of the nanogranular structure of nacre

Currently a basic tenet in biomineralization is that biominerals grow by accretion of amorphous particles, which are later transformed into the corresponding mineral phase. The globular nanostructure of most biominerals is taken as evidence of this. Nevertheless, little is known as to how the amorphousto-crystalline transformation takes place. To gain insight into this process, we have made a highresolution study (by means of transmission electron microscopy and other associated techniques) of immature tablets of nacre of the gastropod Phorcus turbinatus, where the proportion of amorphous calcium carbonate is high. Tablets displayed a characteristic nanoglobular structure, with the nanoglobules consisting of an aragonite core surrounded by amorphous calcium carbonate together with organic macromolecules. The changes in composition from the amorphous to the crystalline phase indicate that there was a higher content of organic molecules within the former phase. Within single tablets, the crystalline cores were largely co-oriented. According to their outlines, the internal transformation front of the tablets took on a complex digitiform shape, with the individual fingers constituting the crystalline cores of nanogranules. We propose that the final nanogranular structure observed is produced during the transformation of ACC into aragonite

Large Scale Cross-Correlations in Internet Traffic

The Internet is a complex network of interconnected routers and the existence of collective behavior such as congestion suggests that the correlations between different connections play a crucial role. It is thus critical to measure and quantify these correlations. We use methods of random matrix theory (RMT) to analyze the cross-correlation matrix C of information flow changes of 650 connections between 26 routers of the French scientific network `Renater'. We find that C has the universal properties of the Gaussian orthogonal ensemble of random matrices: The distribution of eigenvalues--up to a rescaling which exhibits a typical correlation time of the order 10 minutes--and the spacing distribution follow the predictions of RMT. There are some deviations for large eigenvalues which contain network-specific information and which identify genuine correlations between connections. The study of the most correlated connections reveals the existence of `active centers' which are exchanging information with a large number of routers thereby inducing correlations between the corresponding connections. These strong correlations could be a reason for the observed self-similarity in the WWW traffic.Comment: 7 pages, 6 figures, final versio

Crystallographic orientation inhomogeneity and crystal splitting in biogenic calcite

The calcitic prismatic units forming the outer shell of the bivalve Pinctada margaritifera have been analysed using scanning electron microscopy–electron back-scatter diffraction, transmission electron microscopy and atomic force microscopy. In the initial stages of growth, the individual prismatic unit sare single crystals. Their crystalline orientation is not consistent but rather changes gradually during growth. The gradients in crystallographic orientation occurmainly in a direction parallel to the long axis of the prism, i.e. perpendicular to the shell surface anddo not showpreferential tilting along anyof the calcite lattice axes. At a certain growth stage, gradients begin to spread and diverge, implying that the prismatic units split into several crystalline domains. In this way, a branched crystal, inwhich the ends of the branches are independent crystalline domains, is formed. At the nanometre scale, the material is composed of slightly misoriented domains, which are separated by planes approximately perpendicular to the c-axis. Orientational gradients and splitting processes are described in biocrystals for the first time and are undoubtedly related to the high content of intracrystalline organic molecules, although the way in which these act to induce the observed crystalline patterns is amatter of future research

In-situ redox cycling behaviour of Ni-BaZr0.85Y0.15O3-delta cermet anodes for Protonic Ceramic Fuel Cells

The current work investigates the redox behaviour of peak performing Ni-BaZr0.83Y0.15O3-delta (Ni-BZY) cermet anodes for protonic ceramic fuel cells (PCFCs) by electrochemical impedance measurements, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Peak performing PCFC cermet anodes are documented to require much lower porosity levels than those needed in oxide-ion conducting counterparts. The polarisation behaviour of these optimised PCFC anodes is shown to be drastically impaired by redox cycling, with depletions in performance that correspond to around 80% of the original resistance values noted after the first redox cycle. The ohmic resistance (Rohmic) is also shown to be increased due to delamination at the electrode/electrolyte interface, as confirmed by postmortem microstructural analysis. In-situ measurements by environmental scanning electron microscopy (ESEM) reveal that degradation proceeds due to volume expansion of the nickel phase during the re-oxidation stage of redox cycling. The present study reveals degradation to be very fast for peak performing Ni-BZY cermets of low porosity. Hence, methods to improve redox stability can be considered to be essential before such anodes can be implemented in practical devices. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

New Crystallographic Relationships in Biogenic Aragonite: The Crossed-Lamellar Microstructures of Mollusks

Crossed-lamellar microstructures are the most common shell-forming biomaterials in mollusks. Because of their complex hierarchical 3D arrangement and small crystallite size, previous crystallographic studies are scarce and have centered on particular species with no comprehensive analysis available. To evaluate the crystallographic diversity of the crossed-lamellar microstructures, we have studied a large set of bivalve and gastropod species with crossed-lamellar layers using X-ray diffraction and electron backscatter diffraction. From the number, distribution, and relationships of maxima, we have classified pole figures into nine different recurring crystallographic patterns. According to their crystallographic equivalences, these patterns can be grouped into five groups. A first division is established according to whether there is one or two main orientations for the <i>c</i>-axis of aragonite. In the latter case, each orientation corresponds to one of the two sets of alternating first-order lamellae. The two main orientations of the <i>c</i>-axis diverge by rotation within the plane of the first-order lamellae around either a common <i>a</i>- or <i>b</i>-axis. We also show how some patterns may derive from others. Patterns with two <i>c</i>-axis orientations represent crystal relationships until now completely unknown in biogenic and abiogenic aragonite and are most likely produced by particular proteomic pools

Interplay of magnetic properties and doping in epitaxial films of h-REFeO3 multiferroic oxides

Multiferroic materials demonstrating coexistence of magnetic and ferroelectric orders are promising candidates for magnetoelectric devices. While understanding the underlying mechanism of interplaying of ferroic properties is important, tailoring their properties to make them potential candidates for magnetoelectric devices is challenging. Here, the antiferromagnetic Neel ordering temperature above 200 K is realized in successfully stabilized epitaxial films of (Lu,Sc)FeO3 multiferroic oxide. The first-principles calculations show the shrinkage of in-plane lattice constants of the unit cells of the films on different substrates which corroborates well the enhancement of the Neel ordering temperature (TN). The profound effect of lattice strain/stress at the interface due to differences of in-plane lattice constants on out of plane magnetic properties and on spin reorientation temperature in the antiferromagnetic region is further elucidated in the epitaxial films with and without buffer layer of Mn-doped LuFeO3. Writing and reading ferroelectric domains reveal the ferroelectric response of the films at room temperature. Detailed electron microscopy shows the presence of lattice defects in atomic scale. First-principles calculations show that orbital rehybridization of rare-earth ions and oxygen is one of the main driving force of ferroelectricity along c-axis in thin films of hexagonal ferrites.This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement. A.B. acknowledges FCT fellowship SFRH/BPD/115625/2016. The authors acknowledge the access to electron microscopy facilities of the ScopeM, ETH. A.B. appreciates the Inorganic Materials Science group, MESA+ Institute, University of Twente, Netherlands for the use of PFM and XRD for some samples within ECIU mobility research grant