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Sanchez-Munoz L.,CSIC - Institute of Ceramics and Glass | Sanz J.,Instituto Ciencia Of Materiales Of Madrid Csic | Sobrados I.,Instituto Ciencia Of Materiales Of Madrid Csic | Gan Z.,Florida State University
American Mineralogist

The structures of K-rich feldspar, (K>Na)AlSi3O8, are currently described as "ideal" crystals with periodic average structures from Bragg diffraction maxima obtained by reciprocal-space techniques. Polymorphism is explained by variable substitutional disorder of framework Si and Al cations in tetrahedral T sites, and positional disorder of cavity alkali cations in a single M site. Here, high-resolution magic angle spinning multinuclear magnetic resonance spectroscopy, leading to 29Si, 27Al, and 23Na spectra at 9.4 as well as 27Al, 39K, and 23Na spectra at 19.6 T, has been used to investigate the "real" structures along the order-disorder series of K-feldspar crystals. The "ideal" and "real" structure coincides only in the perfectly long-range ordered triclinic end-member of the low-microcline structure. Long-range disordered structures (either with monoclinic or triclinic symmetry by X-ray diffraction) show non-random disorder at the medium-range scale, triclinic-like distortions with four sets of T sites for framework atoms, two sets of M sites for alkali atoms, and Al-O-(K,Na) multi-site correlations by NMR spectroscopy. The K-feldspar structures can be described by a medium-range structure using the number of Al atoms per four-membered rings of tetrahedra, with "...-2-0-2-0-..." chains for microcline and orthoclase where the Al-occupancies t1O > t2m > t2O ≈ t1m, and with "...-1-1-1-1-..." chains for valencianite and sanidine, in which t1O > t2m ≈ t2O ≈ t1m. Framework cations respect Loewenstein's rule (Al-O-Al avoidance), as well as some additional constraints of charge dispersion involving deficiency of Si atoms in Q4 (4Si,0Al), (1Si,3Al), and (0Si,4Al) environments, constraints which are particularly strong in valencianite. These "real" structure features cannot be described by "ideal" structures owing to the lack of resolving power of the reciprocal-space techniques. Source

Merino P.,CSIC - National Institute of Aerospace Technology | Svec M.,ASCR Institute of Physics Prague | Martinez J.I.,Instituto Ciencia Of Materiales Of Madrid Csic | Jelinek P.,ASCR Institute of Physics Prague | And 8 more authors.
Nature Communications

Polycyclic aromatic hydrocarbons as well as other organic molecules appear among the most abundant observed species in interstellar space and are key molecules to understanding the prebiotic roots of life. However, their existence and abundance in space remain a puzzle. Here we present a new top-down route to form polycyclic aromatic hydrocarbons in large quantities in space. We show that aromatic species can be efficiently formed on the graphitized surface of the abundant silicon carbide stardust on exposure to atomic hydrogen under pressure and temperature conditions analogous to those of the interstellar medium. To this aim, we mimic the circumstellar environment using ultra-high vacuum chambers and investigate the SiC surface by in situ advanced characterization techniques combined with first-principles molecular dynamics calculations. These results suggest that top-down routes are crucial to astrochemistry to explain the abundance of organic species and to uncover the origin of unidentified infrared emission features from advanced observations. © 2014 Macmillan Publishers Limited. All rights reserved. Source

Arbi K.,Instituto Ciencia Of Materiales Of Madrid Csic | Arbi K.,CSIC - Eduardo Torroja Institute for Construction Science | Sobrados I.,Instituto Ciencia Of Materiales Of Madrid Csic | Hoelzel M.,TU Munich | And 3 more authors.
Materials Research Society Symposium Proceedings

Lithium mobility in LiM 2(PO 4) 3 compounds, with M= Ge, Ti, Sn, Zr and Hf, has been investigated by 7Li Nuclear Magnetic Resonance (NMR) spectroscopy in the temperature range 100-500 K. From the analysis of 7Li NMR quadrupole interactions (C Q and η parameters), Li sites occupancy and exchange processes between structural sites have been studied. Below 250K, Li ions are preferentially located at M 1 sites in rhombohedral phases, but occupy M 12 sites in triclinic ones. At increasing temperatures, Li mobility has been deduced from spin-spin (T 2 -1) and spin-lattice relaxation (T 1 -1) rates. In this analysis, the presence of two relaxation mechanisms in T 1 -1 plots has been associated with departures of conductivity from the Arrhenius behavior. At high temperatures, residence times at M 1 and M 12 sites become similar and conductivity significantly increase. This superionic state can be achieved by enlarged order-disorder transformations in rhombohedral phases, or by sharp first order transitions in triclinic ones. Results described in the LiTi 2(PO 4) 3 sample have been compared with those obtained in rhombohedral Li 1+xTi 2-xAl x(PO 4) 3 and LiTi 2-xZr x(PO 4) 3 series showing respectively higher and lower conductivities. In the case of Li 1.2Ti 1.8Al 0.2(PO 4) 3, displaying the highest reported conductivity, NMR results are discussed in relation with those obtained by Neutron Diffraction (ND) and Impedance Spectroscopy (IS). Diffusion coefficients determined by NMR Pulse Field Gradient (PFG) technique are similar to those deduced from Impedance Spectroscopy and NMR relaxation data. © 2011 Materials Research Society. Source

Otero-Irurueta G.,University of Aveiro | Hernandez-Rodriguez I.,Instituto Ciencia Of Materiales Of Madrid Csic | Martinez J.I.,Instituto Ciencia Of Materiales Of Madrid Csic | Palacios-Rivera R.,CSIC - National Institute of Aerospace Technology | And 8 more authors.
Chemical Communications

Direct sublimation of a Cu4Cl4 metal-organic cluster on Cu(110) under ultra-high vacuum allows the formation of ultra-large well-organized metal-organic supramolecular wires. Our results show that the large monomers assemble with each other by π-π interactions connecting dipyrimidine units and are stabilized by the surface. This journal is © The Royal Society of Chemistry 2015. Source

Otero G.,Instituto Ciencia Of Materiales Of Madrid Csic | Gonzalez C.,Instituto Ciencia Of Materiales Of Madrid Csic | Pinardi A.L.,Instituto Ciencia Of Materiales Of Madrid Csic | Merino P.,CSIC - National Institute of Aerospace Technology | And 10 more authors.
Physical Review Letters

We have studied large areas of (√3×√3)R30° graphene commensurate with a Pt(111) substrate. A combination of experimental techniques with ab initio density functional theory indicates that this structure is related to a reconstruction at the Pt surface, consisting of an ordered vacancy network formed in the outermost Pt layer and a graphene layer covalently bound to the Pt substrate. The formation of this reconstruction is enhanced if low temperatures and polycyclic aromatic hydrocarbons are used as molecular precursors for epitaxial growth of the graphene layers. © 2010 The American Physical Society. Source

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