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

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

Bonding trends within ternary Isocoordinate chalcogenide glasses GeAsSe

A structural study is presented of ab-initio molecular dynamics simulations of Ge-As-Se calcogenide glasses performed at the same mean coordination number but differing stoichiometry ranging between Se rich and Se poor glasses. Starting configurations are generated via Reverse Monte Carlo (RMC) simulations of Extended X-ray Absorption Fine Structure (EXAFS) measurements of experimental samples. Structural analysis is presented illustrating the bonding trends found with changing stoichiometry.Comment: 26 pages, 9 figures. Submitted to Physical Review

Chemical Short-Range Order in Selenide and Telluride Glasses

International audienc

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

The boson peak and the first sharp diffraction peak in (As2S3)x(GeS2)1–x glasses

The parameters of the boson peak (BP) and the first sharp diffraction peak (FSDP) in (As2S3)x(GeS2)1x glasses measured using high-resolution Raman spectroscopy and high-energy synchrotron X-ray diffraction measurements are examined as a function of x. It has been found that there is no correlation between the positions of BP and FSDP. The BP position shows a nonlinear composition behavior with a maximum at about x = 0.4, whereas the FSDP position changes virtually linearly with x. The intensities of both BP and FSDP show nonlinear composition dependences with the slope changes at x = 0.4, although there is no direct proportionality. Analysis of the partial structure factors for the glasses with x = 0.2, 0.4 and 0.6 obtained in another study has shown that the cation-cation atomic pairs of Ge–Ge, Ge–As and As–As make the largest contribution to FSDP, where the Ge–Ge and Ge–As pairs are dominant

Short range order in Ge-Ga-Se glasses

International audienc