Early stages of solid state reactions: insights from micro-XRD and XAS

The mechanism of a solid state reaction in its early stages can be explored by investigating the time evolution of a model reactive system made of a thin layer of one reagent deposited onto a single crystal slab of the other reagent. Insights can be retrieved by comparing results at both local and long length scales obtained with films of different thicknesses and deposited onto different crystal orientations. In particular, reaction between ZnO and Al2O3has been chosen, as the spinel-forming reactions have been and still remain a model experimental system for investigating solid state reactions and because in the ZnO/Al2O3phase diagram, spinel is the only stable compound. The reaction initial steps have been investigated by using synchrotron X-ray diffraction, atomic force microscopy and X-ray absorption spectroscopy at the Zn-K edge starting from zincite films deposited onto (110)-, (012)-, (001)-oriented corundum single crystals [1,2]. The reaction eventually yields ZnAl2O4spinel but via a complex mechanism involving side and intermediate non-equilibrium compounds that do not appear in the equilibrium phase diagram of the pseudo-binary system. Spinel, when occurs, is polycrystalline at the end but initially forms with a few preferred orientations. Intermediate phases form before and in parallel with the growth of the spinel. Their number, composition, structure and kinetic role strongly depend on substrate orientation and film thickness. A more detailed understanding of the reactivity can be inferred by comparing EXAFS results to those of grazing incidence diffraction experiments of the films deposited on the (001) face of Al2O3and heat-treated at 10000C for different lengths of time. Information on the structure of the intermediate phases is given and results are discussed by comparing different films thickness to clarify the role of interfacial free energy and crystallographic orientation

Non-normal identification for price discovery in high-frequency financial markets

The possibility to measure the relative contribution of agents and exchanges to the price formation process in high-frequency financial markets acquired increasingly importance in the financial econometric literature. In this paper I propose to adopt fully data-driven approaches to identify structural vector error correction models (SVECM) typically used for price discovery. Exploiting the non-Normal distributions of the variables under consideration, I propose two novel variants of the widespread Information Share (IS) measure which are able to identify the leaders and the followers in the price formation process. The approaches will be illustrated both from a semiparametric and parametric standpoints, solving the identification problem with no need of increasing the computational complexity which usually arises when working at incredibly short time scales. Finally, an empirical application on IBM intraday data will be provided

Uncovering the network structure of non-centrally cleared derivative markets: Evidences from regulatory data

The network structure of non-centrally cleared derivative markets, uncovered via the European Market Infrastructure Regulation (EMIR), is investigated with a focus on the Covid-19 market turmoil period. Initial and variation margin networks are reconstructed to analyze channels of potential losses and liquidity dynamics. Despite the absence of central clearing, the derivative network is found to be ultrasmall and a filtering tool is proposed to identify channels in the network characterized by the highest exposures. I find these exposures to be mainly toward institutions outside the euro-area (EA), emphasizing the need for cooperation across different jurisdictions. Anomalous behavior in terms of diverging first and second moments on the degree and strength distributions are detected, signaling the presence of large exposures generating extreme liquidity outflows. A reference table of parameters' estimates based on real data is provided for different network sizes, with no break of confidentiality, making possible to simulate in a realistic way the liquidity dynamic in global derivative markets even when the access to supervisory data is not granted

Mixing and ordering behaviour in manganocolumbite-ferrocolumbite solid solution: a single-crystal X-ray diffraction study

The structural changes upon cation substitution in natural AB2O6 columbites have been studied by means of single-crystal X-ray diffraction. Most of the structural variations across the MnNb2O6- FeNb2O6 solid solution in completely ordered samples can be simply understood in terms of ionic radii. The substitution of Fe for the larger Mn cation causes a linear decrease of all unit-cell parameters. Going from manganocolumbite to ferrocolumbite the site A is reduced in volume and becomes less distorted. The oxygen cage around the cation assumes a more regular arrangement since the mismatch between A and B chains decreases. At the same time, the divalent cation moves toward the barycenter of the polyhedron. The B site, which is not involved in the Fe-Mn cation substitution, maintains its geometry unchanged. Ordering of divalent cations at A sites and pentavalent cations at B sites causes linear variations of a and c cell parameters. A non-linear behavior is shown by the b cell parameter that shows a minimum at order parameter Qm ∼ 0.7. A discontinuity at this Qm value is also shown by other structural parameters. Cation ordering also causes volume variations of the two octahedral sites as a consequence of the different ionic radii of the various species. Octahedral bond-length distortion parameters show that the B site is in general more distorted than the A site; distortion of the B site increases with ordering due to higher cation-cation repulsion along the B octahedral chain and to the second-order Jahn-Teller (SOJT) effect. Octahedral chains respond to modiÞ cations of the polyhedra by folding along the common edge

Low-alkali metal content in beta-vanadium mixed bronzes: the crystal structures of β-Kx(V,Mo)6O15 (x = 0.23 and 0.32) by single-crystal X-ray diffraction

The vanadium–molybdenum mixed oxide bronzes of composition K0.23(V5.35Mo0.65)O15 and K0.32(V5.48Mo0.52)O15 have a monoclinic structure with s.g. C2/m, Z ¼ 2, and unit-cell dimensions a ¼ 15.436(2), b ¼ 3.6527(5), c ¼ 10.150(1)A ˚ , b ¼ 108.604(3)1 and a ¼ 15.452(2), b ¼ 3.6502(5), c ¼ 10.142(1)A ˚ , b ¼ 109.168(3)1, respectively, as determined by single-crystal X-ray diffraction. These compounds show the b-NaxV6O15 tunnel structure, which is isostructural with bannermanite, natural sodium–potassium vanadate. Structure refinements from diffracted intensities collected in the 2–381y range converged to final R ¼ 5.58% and 7.48% for the two crystals, respectively. The V atoms are distributed on three different crystallographic sites. Partial substitution of V with Mo occurs in only one of these positions. Oxygen atoms involved in vanadyl groups point toward the tunnels. The K ions in the tunnels are coordinated by seven oxygen atoms. The alkali metal content in these crystals is much lower than the solubility limit found for the analogous Na containing compound

08_1641Tarantino.indd

ABSTRACT The structural changes upon cation substitution in natural AB 2 O 6 columbites have been studied by means of single-crystal X-ray diffraction. Most of the structural variations across the MnNb 2 O 6 -FeNb 2 O 6 solid solution in completely ordered samples can be simply understood in terms of ionic radii. The substitution of Fe for the larger Mn cation causes a linear decrease of all unit-cell parameters. Going from manganocolumbite to ferrocolumbite the site A is reduced in volume and becomes less distorted. The oxygen cage around the cation assumes a more regular arrangement since the mismatch between A and B chains decreases. At the same time, the divalent cation moves toward the barycenter of the polyhedron. The B site, which is not involved in the Fe-Mn cation substitution, maintains its geometry unchanged. Ordering of divalent cations at A sites and pentavalent cations at B sites causes linear variations of a and c cell parameters. A non-linear behavior is shown by the b cell parameter that shows a minimum at order parameter Q m ∼ 0.7. A discontinuity at this Q m value is also shown by other structural parameters. Cation ordering also causes volume variations of the two octahedral sites as a consequence of the different ionic radii of the various species. Octahedral bond-length distortion parameters show that the B site is in general more distorted than the A site; distortion of the B site increases with ordering due to higher cation-cation repulsion along the B octahedral chain and to the second-order Jahn-Teller (SOJT) effect. Octahedral chains respond to modiÞ cations of the polyhedra by folding along the common edge