Folyadék és amorf szerkezetű anyagok vizsgálata diffrakcióval és számítógépes modellezéssel = Investigations of liquid and amorphous structures by diffraction and computer modelling methods

Eredeti pályázatunknak megfelelően elsősorban folyadékok szerkezetét vizsgáltuk (elsősorban röntgen)diffrakciós módszerrel és az azt követő Reverse Monte Carlo (RMC) modellezéssel. Az elnyert támogatás a világ legnagyobb teljesítményű szinktrotronforrásánál, a SPring-8-nél (Hyogo, Japán) végzett (nagyenergiás) röntgendiffrakciós kísérleteket tett lehetővé. Meghatároztuk nagynyomású, szobahőmérsékletű folyékony (szuperkritikus állapotban lévő) oxigén és nitrogén lokális szerkezetét; úgy találtuk, hogy a szomszédos molekulák párhuzamos beállása mellett az 'X'-alakú konformáció is gyakori. Azonosítottuk az ón-tetrajodid folyadékfázisát mint azt az anyagot, amelyben a legnagyobb arányban fordulnak elő a szabályos tetraéder alakú molekulák 'csúcs-lap' (azaz 'Apollo') típusú illeszkedése. Az XCl4 szabályos tetraéder alakú molekulákból álló folyadékokban (X: C, Si, Ge, Sn) a vártnál (valamint az eddigiekben javasolthoz képest) lényegesebb hosszabb, néhány nanométerre kiterjedő orientációs rendezettséget találtunk. | According to our original project plan, we've investigated primarily liquid structures, using (primarily X-ray) diffraction methods and subsequent Reverse Monte Carlo modeling. The support awarded made it possible to carry out (high energy) X-ray diffraction experiments at the world's most powerful synchrotron source, SPring-8 (Hyogo, Japan). We have determined the local structure of high-pressure, room temperature liquid (supercritical fluid) oxygen and nitrogen; it was found that apart from the parallel configuration, neighboring molecules frequently choose 'X'-shaped mutual orientations. We have identified liquid tin tetraiodoide as the material which contains the highest ratio of 'corner-to-face' (or so-called 'Apollo') type conformations of molecules with perfect tetrahedral shape. In liquids of XCl4 (perfect tetrahedral) molecules (X: C, Si, Ge, Sn) a rather long (much longer than expected and than had been suggested before), nanometer range orientational ordering of molecules

Local motifs in GeS2_2-Ga2_2S3_3 glasses

The structure of (GeS$_2$)$_{0.75}$(Ga$_2$S$_3$)$_{0.25}$ and (GeS$_2$)$_{0.83}$(Ga$_2$S$_3$)$_{0.17}$ glasses was investigated by Raman scattering, high energy X-ray diffraction and extended X-ray absorption fine structure (EXAFS) measurements at the Ga and Ge K-edges. The reverse Monte Carlo simulation technique (RMC) was used to obtain structural models compatible with diffraction and EXAFS datasets. It was found that the coordination number of Ga is close to four. While Ge atoms have only S neighbors, Ga binds to S as well as to Ga atoms showing a violation of chemical ordering in GeS$_2$-Ga$_2$S$_3$ glasses. Analysis of the corner- and edge-sharing between [GeS$_{4/2}$] units revealed that about 30% of germanium atoms participate in the edge-shared tetrahedra.Comment: 23 pages, 7 figures, accepted for publication in Journal of Alloys and Compound

Folyadékok és amorf anyagok szerkezetvizsgálata diffrakcióval és számítógépes modellezéssel = Structural investigations of liquids and amorphous solids by diffraction and computer modelling

Folyadékok és amorf anyagok atomi szintű szerkezetét határoztuk meg neutron- és röntgendiffrakciós, valamint röntgen-abszorpciós (EXAFS) kísérletekkel. A Reverse Monte Carlo (RMC) számítógépes modellezési eljárás segítségével olyan 3 dimenziós, több ezer atomot tartalmazó szerkezeteket generáltunk, amelyek teljes mértékben (azaz a kísérleti hibán belül) konzisztensek voltak a felsorolt kísérleti adatok mindegyikével. A vázolt procedúra-sorozat ('protokoll') több, egymástól néha meglehetősen eltérő folyadék és amorf anyag szerkezetének megértését tette lehetővé a korábbiakban elképzelhetetlen részletességgel és bizonyossággal. E rövid összefoglalóba csak néhány példa említése fér: tetraéder alakú molekulákból álló folyadékokban (pl. szén-tetraklorid) a molekulák orientációs korrelációit tisztáztuk; vizes elektrolitoldatokban rögzítettük a diffrakciós adatok által megengedett koordinációs számokat és a hidrátszférákat jellemző szög-korrelációkat; a DVD-technológia alapjául szolgáló germánium-antimon-tellúr amorf ötvözet szerkezetét elsőként sikerült részletekbe menően tisztázni. A támogatott kutatómunka több mint 40, referált nemzetközi folyóiratban megjelent közleményt eredményezett, amelyek összesített impakt faktora meghaladja a 90-et (azaz 1 impakt-faktor pont 'egységára' kb. 150 eFt volt). | The atomic level structure of liquids and amorphous materials have been determined by neutron and X-ray diffraction, as well as X-ray absorption (EXAFS) experiments. By means of the Reverse Monte Carlo (RMC) computer modelling teachnique, 3 dimensional structures (containing thousands of particles) have been generated that were fully consistent (within experimental errors) with all of the aformentioned experimental data. The series of procedures ('protocol') outlined above made it possible to understand the structure of various (sometimes, rather different) liquids and amorphous materials at an unprecedented level and certainty. Only a few representative examples can be mentioned in such a short summary: we were able to clarify correlations between molecular orientations in liquids containing tethedral molecules (such as carbon tatrachloride); for some aqueous elctrolyte solutions, we have determined the range of coordinations numbers and angular corrrelations that are allowed by diffraction data; the structure of germanium-antimony-tellurium amorphous alloy that forms tha basis of DVD technology could be determined to fine details. The reserach supported by the present OTKA grant resulted in more than 40 publications in referred international scientific journals, with a cumulative impact factor that exceeds 90 (that is, the 'unit price' of 1 impact factor point was about 150 kHUF)

Structural, electronic and kinetic properties of the phase-change material Ge2Sb2Te5 in the liquid state

Phase-change materials exhibit fast and reversible transitions between an amorphous and a crystalline state at high temperature. The two states display resistivity contrast, which is exploited in phase-change memory devices. The technologically most important family of phase-change materials consists of Ge-Sb-Te alloys. In this work, we investigate the structural, electronic and kinetic properties of liquid Ge2Sb2Te5 as a function of temperature by a combined experimental and computational approach. Understanding the properties of this phase is important to clarify the amorphization and crystallization processes. We show that the structural properties of the models obtained from ab initio and reverse Monte Carlo simulations are in good agreement with neutron and X-ray diffraction experiments. We extract the kinetic coefficients from the molecular dynamics trajectories and determine the activation energy for viscosity. The obtained value is shown to be fully compatible with our viscosity measurements

Atomic level structure of Ge-Sb-S glasses: chemical short range order and long Sb-S bonds

The structure of Ge$_{20}$Sb$_{10}$S$_{70}$, Ge$_{23}$Sb$_{12}$S$_{65}$ and Ge$_{26}$Sb$_{13}$S$_{61}$ glasses was investigated by neutron diffraction (ND), X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) measurements at the Ge and Sb K-edges as well as Raman scattering. For each composition, large scale structural models were obtained by fitting simultaneously diffraction and EXAFS data sets in the framework of the reverse Monte Carlo (RMC) simulation technique. Ge and S atoms have 4 and 2 nearest neighbors, respectively. The structure of these glasses can be described by the chemically ordered network model: Ge-S and Sb-S bonds are always preferred. These two bond types adequately describe the structure of the stoichiometric glass while S-S bonds can also be found in the S-rich composition. Raman scattering data show the presence of Ge-Ge, Ge-Sb and Sb-Sb bonds in the S-deficient glass but only Ge-Sb bonds are needed to fit diffraction and EXAFS datasets. A significant part of the Sb-S pairs has 0.3-0.4 {\AA} longer bond distance than the usually accepted covalent bond length (~2.45 {\AA}). From this observation it was inferred that a part of Sb atoms have more than 3 S neighbors.Comment: 23 pages, 6 figures, submitted to Journal of Alloys and Compound

Capital flows in Italy: hints about a possible analysis

SIGLEAvailable from Bibliothek des Instituts fuer Weltwirtschaft, ZBW, Duesternbrook Weg 120, D-24105 Kiel C 141124 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Short-Range Order in Ge-As-Te Glasses

The structure of Te-rich (75–80 at.% Te) and Te-poor (40 at.% Te) Ge–As–Te glasses has been investigated by diffraction and extended X-ray absorption fine structure (EXAFS) measurements. Large-scale structural models have been created by fitting simultaneously diffraction and EXAFS datasets by the reverse Monte Carlo simulation technique. It is found that As–As bonds improve the fit quality in the case of Te-rich glasses while no Ge–Ge bonding is necessary in these compositions. In the Te-poor glasses, Te–Te homopolar bonds are also observed while Ge binds preferentially to Te rather than to As. Ge–As and Ge–Te coordination numbers do not change significantly with increasing Ge content