Structure and triclustering in Ba-Al-O glass

Glass-forming ability in the (BaO) x(Al 2O 3) 1-x system (0≤x≤1) was investigated by using the containerless aerodynamic levitation and laser-heating method. The main glass-forming region was found to occur for 0.40(2) ≤x≤ 0.48(2), where there is insufficient oxygen to form an ideal network of corner-sharing AlO 4 tetrahedra in which the oxygen atoms are twofold coordinated, with another narrow glass-forming region at x = 0.62(2) around the eutectic composition. The glass corresponding to x = 0.4 was chosen for further investigation by using both neutron and x-ray diffraction, and a detailed atomistic model was built by applying a combination of molecular dynamics and reverse Monte Carlo methods. The results show a network structure based predominantly on corner-sharing tetrahedral AlO 4 motifs in which triclusters (OAl 3 units formed by three tetrahedral Al atoms sharing a common vertex) play an integral part, with as many as 21% of the oxygen atoms involved in these configurations. The barium ions bind to an average of 7.4 O atoms, most of which are twofold-coordinated bridging oxygen atoms. The larger size of barium compared to calcium narrows the range of glass-forming compositions in alkaline-earth aluminates such that the main glass-forming range corresponds to a regime in which an oxygen-deficient Al-O network is stabilized by the formation of triclusters

Neutron diffraction as a probe of liquid and glass structures under extreme conditions

Scanning electron micrograph taken of palladium/platinum coated structures with an FEI Quanta 600 FEG environmental scanning electron microscope

Structural transformations on vitrification in the fragile glass-forming system CaAl₂O₄

International audienceThe structure of the fragile glass-forming material CaAl 2 O 4 was measured by applying the method of neutron diffraction with Ca isotope substitution to the laser-heated aerodynamically levitated liquid at 1973(30) K and to the glass at 300(1) K. The results, interpreted with the aid of molecular dynamics simulations, reveal key structural modifications on multiple length scales. Specifically, there is a reorganization on quenching that leads to an almost complete breakdown of the AlO 5 polyhedra and threefold coordinated oxygen atoms present in the liquid, and to their replacement by a predominantly corner-sharing network of AlO 4 tetrahedra in the glass. This process is accompanied by the formation of branched chains of edge and face-sharing Ca-centered polyhedra that give cationic ordering on an intermediate length scale, where the measured coordination number for O around Ca is 6.0(2) for the liquid and 6.4(2) for the glass. Calcium aluminates ðCaOÞ x ðAl 2 O 3 Þ 1Àx (0 x 1) have been extensively studied on account of their geological , technological, and scientific importance [1-20]. For example, they are a significant component of the Earth's mantle so that the liquid structure is of interest for understanding magma-related processes [21], they are an integral component of aluminous cement [22], the glasses have a favorable infrared transmission window that extends up to a wavelength $6 m [23] giving them optical applications [24,25], and the rare-earth-metal-doped materials exhibit persistent luminescence [26]. From a glass physics perspective, calcium aluminates are very fragile glass for-mers [1,4] and, in contrast to strong network glass formers such as SiO 2 , large structural alterations should accompany the rapid change in viscosity and other dynamical properties as the glass transition temperature T g is approached [27]. Experimental information on the extent of structural transformation is therefore essential to understanding the processes occurring around T g and the material properties to which they are linked. An experimental exploration of liquid aluminates is, however, challenging because of the high temperatures involved. The containerless method of aerodynamic levitation offers a way forward, and by minimizing heterogeneous nucleation, it extends the narrow glass-forming region centered at x ¼ 0:65 in the calcium aluminate system to include the equimolar composition CaAl 2 O 4 [16] which has a fragility index of m ¼ 116 [1,28]. At this composition , the O:Al ratio is 2:1 such that it is just feasible to form an ideal network of fully connected corner-sharing AlO 4 tetrahedra where the oxygen atoms are twofold coordinated, as in the crystalline phase which has a tridymite-like structure where the tetrahedra form a fully polymerized network of six-membered rings [29]. This has motivated a range of experimental and computer simulation studies on the liquid and glass structure [2-20]. It has, however, proved difficult to measure unambiguously the Al and Ca coordination environments. For example, in the liquid state 27 Al nuclear magnetic resonance (NMR) experiments observe the fast exchange limit such that individual Al coordination environments cannot be identified [2-5], and in diffraction experiments , the nearest-neighbor Ca-O and other pair correlations are strongly overlapped [16-19]. The powerful method of neutron diffraction with isotope substitution has been used to probe directly the coordination environment of Ca in ðCaOÞ 48 ðSiO 2 Þ 49 ðAl 2 O 3 Þ 3 glass [30,31], but the method is usually limited to large samples [32]. In this Letter we show, however, that the neutron diffraction with isotope substitution method can be used to measure the detailed atomic structure of a single aerodynamically levitated laser-heated drop of liquid CaAl 2 O 4 at 1973(30) K. The structure of the glass at 300(1) K is also investigated. The results, interpreted with the aid of molecular dynamics (MD) simulations, characterize the nature of the structural transformations that occur on vitrification on both the local and intermediate atomic length scales. The total structure factor measured by neutron diffraction is given by FðQÞ ¼ P P c c b b ½S ðQÞ À 1

High-Pressure Transformation of SiO2 Glass from a Tetrahedral to an Octahedral Network:A Joint Approach Using Neutron Diffraction and Molecular Dynamics

International audienceA combination of in situ high-pressure neutron diffraction at pressures up to 17.5(5) GPa and moleculardynamics simulations employing a many-body interatomic potential model is used to investigate thestructure of cold-compressed silica glass. The simulations give a good account of the neutron diffractionresults and of existing x-ray diffraction results at pressures up to ∼60 GPa. On the basis of the moleculardynamics results, an atomistic model for densification is proposed in which rings are “zipped” by a pairingof five- and/or sixfold coordinated Si sites. The model gives an accurate description for the dependence ofthe mean primitive ring size hni on the mean Si-O coordination number, thereby linking a parameter that issensitive to ordering on multiple length scales to a readily measurable parameter that describes the localcoordination environment

Mechanisms of network collapse in GeO(2) glass:high-pressure neutron diffraction with isotope substitution as arbitrator of competing models

International audienceThe structure of the network forming glass GeO2 is investigated by making the first application of the method of in situ neutron diffraction with isotope substitution at pressures increasing from ambient to 8 GPa. Of the various models, the experimental results are in quantitative agreement only with molecular dynamics simulations made using interaction potentials that include dipole-polarization effects. When the reduced density = 0 & 1:16, where 0 is the value at ambient pressure, network collapse proceeds via an interplay between the predominance of distorted square pyramidal GeO5 units versus octahedral GeO6 units as they replace tetrahedral GeO4 units. This replacement necessitates the formation of threefold coordinated oxygen atoms and leads to an increase with density in the number of small rings, where a preference is shown for sixfold rings when = 0 D 1 and fourfold rings when = 0 D 1:64

Neutron diffraction as a probe of liquid and glass structures under extreme conditions

Neutrons provide a unique tool for probing the structure of liquid and glassy materials, and deliver information that cannot be obtained from other experimental techniques. Advances in neutron diffraction instrumentation and measurement protocols now make it possible to measure the structure of these disordered materials under extremes of high temperatures or high pressures. Here, we consider the use of aerodynamic levitation with laser heating to explore the structure of glass-forming oxide melts at high temperatures, and the use of a Paris Edinburgh press to investigate the mechanisms of density-driven network collapse for glassy materials in the gigapascal (GPa) pressure regime