Change In Coordination Number Research Articles

Water makes glass elastically stiffer under high-pressure

Because of its potentially broad industrial applications, a new synthesis of elastically stiffer and stronger glass has been a long standing interest in material science. Various chemical composition and synthesis condition have so far been extensively tested to meet this requirement. Since hydration of matter, in general, significantly reduces its stiffness, it has long been believed that an anhydrous condition has to be strictly complied in synthesis processes. Here we report elastic wave velocities of hydrous SiO2 glass determined in-situ up to ultrahigh-pressures of ~180 gigapascals, revealing that the elastic wave velocities of hydrous glass unexpectedly show the rapid increase with pressure and eventually become greater than those of anhydrous glass above ~15 gigapascals. Furthermore, anomalous change in the velocity gradient at ~100 gigapascals, probably caused by the change in Si-O coordination number from 6 to 6+, was also found at ~40 gigapascals lower pressure condition than that previously reported in anhydrous silica glass, implying that water is a highly effective impurity to make SiO2 glass much denser. This experimental discovery strongly indicates that hydration combined with pressurization is highly effective to synthesize elastically stiffer glass materials, which offers a new insight into the fabrication of industrially useful novel materials.

Structural Compromise between High Hardness and Crack Resistance in Aluminoborate Glasses.

Alkali aluminoborate glasses have recently been shown to exhibit a high threshold for indentation cracking compared to other bulk oxide glasses. However, to enable the use of these materials in engineering applications, there is a need to improve their hardness by tuning the chemical composition. In this study, we substitute alkaline earth for alkali network-modifying species at fixed aluminoborate base glass composition and correlate it with changes in the structure, mechanical properties, and densification behavior. We find that the increase in field strength (i.e., the charge-to-size ratio) achieved by substituting alkaline earth oxide from BaO to MgO manifests itself in a monotonic increase in several properties, such as atomic packing density, glass-transition temperature, densification ability, indentation hardness, and crack resistance. Although the use of alkaline earth oxides as modifier enables higher hardness values (increasing from 2.0 GPa for Cs to 5.8 GPa for Mg), their crack resistance is generally lower than that of the corresponding alkali aluminoborate glasses. We discuss the origin of this compromise between hardness and crack resistance in terms of the ability of the glass networks to undergo structural transformations and self-adapt under stress. We show that the extent of volume densification scales linearly with the number of pressure-induced coordination number changes of B and Al.

GLASS FORMATION IN TERNARY SYSTEM La2O3–As2S3–Pr6O11

Boundaries of glass formation in ternary system La2O3-As2S3-Pr6O11 were studied using methods of physical-chemical analysis (DTA, DTG, X-ray analysis, MSA). It was established that in glass formation region the range of concentration covers from As2S3-La2O3 15 mol%, but from As2S3-Pr6O11 it is 13 mol%. To our opinion the decrease in glass formation region of Pr6O11 comparing to La2O3 is related to crystalline structure of oxide, i.e. to the change in coordination number of lanthanides. Because properties of compounds of lanthanide are more sensitive to the change in coordination numbers relative to radii of ion. In the system the region of transparent glass formation is limited with the region of non-transparent one. The reason of non-transparency of a glass is the formation of primary centers of crystallization, which was confirmed by the results of X-ray analysis and SEM. Raman spectra of transparent glass containing(La2O3)0.07(As2S3)0.90(Pr6O11)0.03 were studied. Intensity of bands which characterize the bonds of As–S, La–O, Pr–O in Raman spectrum and shift of energy of waves in the big party are related to the strengthening of bonds and probably, appearance of new bonds in glasses. When heating glasses of the composition (La2O3)0.07(As2S3)0.90(Pr6O11)0.03 in inert atmosphere (He) at 224,61 °С it softens which is followed by endoeffects. Observed exoeffect at 315,80 °С satisfies crystallization of glass, but at 380,80 °С thermal decomposition begins. Samples in the region of glass formation are stable at 300 K to air, water and organic solvents. When heated, glasses are dissolved in chromic mixture. They decompose in mineral acids and alkalies.Forcitation:Bakhtiyarly I.B., Fatullayeva G.M., Kerimli O.Sh. Glass formation in ternary system La2O3–As2S3–Pr6O11. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N 4-5. P. 43-48

The existence form and synergistic effect of P in improving the structural stability and electrochemical performance of Li2Mn0.5Fe0.5SiO4/C cathode materials

Nano-Li2Mn0.5Fe0.5SiO4/C cathode material is synthesized by a hydrothermal route and phosphorus substitution is applied to improve structural stability and electrochemical properties. At low substitution content, P element completely enters into the lattice, forms [PO4] tetrahedrons and partially replaces [SiO4] tetrahedrons, which is confirmed by X-ray diffraction and X-ray photoelectron spectroscope measurements. Phosphorus substitution helps to suppress the change of coordination number of Mn and stabilize the material structure to some extent, obtaining better electrochemical performance in the early cycle. With the increase of P content, parts of P element exist in Li3PO4 which distributes uniformly and co-exists with active substance. Electrochemical tests prove that existing Li3PO4 has positive impacts on cycle and rate performance, and the lithium ion diffusion coefficient increases by about 14 times than pristine sample. Under the synergistic effects of phosphate substation and proper Li3PO4, Li2Mn0.5Fe0.5SiO4/C shows enhanced electrochemical performances.

The Random Change of Coordination Number in the Blume-Capel Model

The spin-1 Blume-Capel model is investigated under the random change of coordination numbers on the Bethe lattice (BL). Various coordination numbers are taken as couples and varied randomly on the shells of the BL. The randomization is realized in the standard-random approach and the formulation is obtained in terms of exact recursion relations. The phase diagrams are obtained by calculating the places of the second- and first-order phase transitions and tricritical points.

Homoleptic U(iii) and U(iv) amidate complexes.

The syntheses of the first homoleptic U(iii) and U(iv) amidate complexes are described. These can be interconverted by chemical reduction/oxidation, showing an unusual change in coordination number from four in the U(iii) complex to eight in the U(iv) complex in the solid state structures.

Layering and phase transition of liquid aluminum confined by different substrates

Molecular dynamics simulation has been performed to study the layering in the nano-confined liquid aluminum, a magic phenomenon characterized by oscillatory density profile across the molecular thin films. Carbon and copper walls are served as the confinement substrates, which can bring about discrepancy in the layering behavior, coordination number and diffusion coefficient. We highlight the abnormal changes of potential energy and volume when the new layer forms, accompanying the distinctive changes of the average coordination number and diffusion coefficient, which is a typical character of phase transition. The role of pressure and temperature on the layering is also reported. Specifically, the pressure-confinement and temperature-confinement diagrams are plotted to further analyze the layering transition.

Phase stability and mechanical properties of Mo1-xNx with 0 ≤ x ≤ 1

First-principle density-functional calculations coupled with the USPEX evolutionary phase-search algorithm are employed to calculate the convex hull of the Mo-N binary system. Eight molybdenum nitride compound phases are found to be thermodynamically stable: tetragonal β-Mo3N, hexagonal δ-Mo3N2, cubic γ-Mo11N8, orthorhombic ε-Mo4N3, cubic γ-Mo14N11, monoclinic σ-MoN and σ-Mo2N3, and hexagonal δ-MoN2. The convex hull is a straight line for 0 ≤ x ≤ 0.44 such that bcc Mo and the five listed compound phases with x ≤ 0.44 are predicted to co-exist in thermodynamic equilibrium. Comparing the convex hulls of cubic and hexagonal Mo1-xNx indicates that cubic structures are preferred for molybdenum rich (x < 0.3) compounds, and hexagonal phases are favored for nitrogen rich (x > 0.5) compositions, while similar formation enthalpies for cubic and hexagonal phases at intermediate x = 0.3–0.5 imply that kinetic factors play a crucial role in the phase formation. The volume per atom Vo of the thermodynamically stable Mo1-xNx phases decreases from 13.17 to 9.56 Å3 as x increases from 0.25 to 0.67, with plateaus at Vo = 11.59 Å3 for hexagonal and cubic phases and Vo = 10.95 Å3 for orthorhombic and monoclinic phases. The plateaus are attributed to the changes in the average coordination numbers of molybdenum and nitrogen atoms, which increase from 2 to 6 and decrease from 6 to 4, respectively, indicating an increasing covalent bonding character with increasing x. The change in bonding character and the associated phase change from hexagonal to cubic/orthorhombic to monoclinic cause steep increases in the isotropic elastic modulus E = 387–487 GPa, the shear modulus G = 150–196 GPa, and the hardness H = 14–24 GPa in the relatively narrow composition range x = 0.4–0.5. This also causes a drop in Poisson's ratio from 0.29 to 0.24 and an increase in Pugh's ratio from 0.49 to 0.64, indicating a ductile-to-brittle transition between x = 0.44 and 0.5.

In-Situ Structural Characterization of CH3NH3PbI3 Nano-Crystal Using Advance Synchrotron-Radiation Techniques

Although CH3NH3PbI3 is a promising light harvesting material1, 2, its structural fluctuations and instability impede our understanding and its technological applications. It is now demonstrated for the first time that advanced synchrotron techniques: in-situ extended X-ray absorption fine structure, X-ray absorption near edge structure and temporally coherent X-ray diffraction and coherent X-ray scattering were used to study in-situ microstructure and structural evolution of CH3NH3PbI3 nano-crystals during thermal heating. The measurements were performed from room temperature up to 100 0C. Microstructure evolution, oxidation state, coordination number, structural distortion and phase heterogeneity changes were examined. Except the microstructure, there was no change in oxidation state and coordination number with increase in temperature.

Nanosecond Dynamics at Protein Metal Sites: An Application of Perturbed Angular Correlation (PAC) of γ-Rays Spectroscopy.

Metalloproteins are essential to numerous reactions in nature, and constitute approximately one-third of all known proteins. Molecular dynamics of proteins has been elucidated with great success both by experimental and theoretical methods, revealing atomic level details of function involving the organic constituents on a broad spectrum of time scales. However, the characterization of dynamics at biomolecular metal sites on nanosecond time scales is scarce in the literature. The aqua ions of many biologically relevant metal ions exhibit exchange of water molecules on the nanosecond time scale or faster, often defining their reactivity in aqueous solution, and this is presumably also a relevant time scale for the making and breaking of coordination bonds between metal ions and ligands at protein metal sites. Ligand exchange dynamics is critical for a variety of elementary steps of reactions in metallobiochemistry, for example, association and dissociation of metal bound water, association of substrate and dissociation of product in the catalytic cycle of metalloenzymes, at regulatory metal sites which require binding and dissociation of metal ions, as well as in the transport of metal ions across cell membranes or between proteins involved in metal ion homeostasis. In Perturbed Angular Correlation of γ-rays (PAC) spectroscopy, the correlation in time and space of two γ-rays emitted successively in a nuclear decay is recorded, reflecting the hyperfine interactions of the PAC probe nucleus with the surroundings. This allows for characterization of molecular and electronic structure as well as nanosecond dynamics at the PAC probe binding site. Herein, selected examples describing the application of PAC spectroscopy in probing the dynamics at protein metal sites are presented, including (1) exchange of Cd2+ bound water in de novo designed synthetic proteins, and the effect of remote mutations on metal site dynamics; (2) dynamics at the β-lactamase active site, where the metal ion appears to jump between the two adjacent sites; (3) structural relaxation in small blue copper proteins upon 111Ag+ to 111Cd2+ transformation in radioactive nuclear decay; (4) metal ion transfer between two HAH1 proteins with change in coordination number; and (5) metal ion sensor proteins with two coexisting metal site structures. With this Account, we hope to make our modest contribution to the field and perhaps spur additional interest in dynamics at protein metal sites, which we consider to be severely underexplored. Relatively little is known about detailed atomic motions at metal sites, for example, how ligand exchange processes affect protein function, and how the amino acid composition of the protein may control this facet of metal site characteristics. We also aim to provide the reader with a qualitative impression of the possibilities offered by PAC spectroscopy in bioinorganic chemistry, especially when elucidating dynamics at protein metal sites, and finally present data that may serve as benchmarks on a relevant time scale for development and tests of theoretical molecular dynamics methods applied to biomolecular metal sites.

Synthesis, spectral, crystal structures, Hirshfeld surface analysis and DFT studies on bis(N,N-di(4-methoxybenzyl)dithiocarbamato-S,S′)zinc(II) and its imine adducts: new precursor for the preparation of hexagonal ZnS

ABSTRACT[Zn(4-mbzdtc)2] (1), [Zn(4-mbzdtc)2(py)] (2), [Zn2(4-mbzdtc)4(4,4′-bipy)] (3), and [Zn(4-mbzdtc)2(2,2′-bipy)] (4), (where, 4-mbzdtc = N,N-di(4-methoxybenzyl)dithiocarbamate) were prepared and characterized by elemental analysis, infrared, 1H and 13C NMR spectroscopy and the structures of 1 and 3 were elucidated by X-ray crystallography. The monomeric structure of 1 shows a distorted tetrahedral geometry for zinc(II). The coordination geometry around zinc in dinuclear complex 3 is an intermediate between square pyramidal and trigonal bipyramidal. The downfield shift of NCS2 carbon signal for imine adducts of 2–4 compared to those found in parent complex 1 is attributed to the change in coordination number and steric effect exerted by the nitrogen donor ligand. Hirshfeld surface analysis was used to visualize intermolecular interactions in the crystal structure of 1 and 3. The geometry optimization of 1 is performed using DFT with the B3LYP-LANL2DZ basis set. Solvothermal decomposition of 1 yielded only hexagonal zinc sulfide nanoparticles. Zinc sulfide nanoparticles were characterized using powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectra and diffuse reflectance spectra. DRS study of ZnS nanoparticles shows the quantum confinement effect with a band gap of 3.74 eV.

The local structure of As[sbnd]Se[sbnd]S chalcogenide glasses studied by neutron diffraction and Raman scattering

The local structures of amorphous As40Se60, As40Se30S30, As33.3Se33.3S33.4 chalcogenide glass semiconductors have been studied by neutron diffraction and Raman scattering methods. The neutron diffraction data-sets were modeled by Reverse Monte Carlo (RMC) simulation technique. Several first and second neighbour distances, coordination numbers and bond-angle distributions have been calculated. It is established that the first neighbour atomic distances are overlapping at three characteristic distances, namely the SS and SeS are centered at 2.2(5)Å, while the AsS and SeSe are centered at 2.35Å, the AsAs and AsSe are centered at 2.4(5)Å. The average coordination numbers in As40Se60 (ZAs=3.03; ZSe=2.02) and As40Se30S30 (ZAs=3; ZSe=2.07; ZS=2) compositions were determined and found to be consistent with, “8-N” rule. The slight deviations from this rule is discovered in case of As33.3Se33.3S33.4 (ZAs=3.07; ZSe=2.09; ZS=1.95) composition. The main role in the formation of medium range order belongs to SeSe bonds. It has been identified that the replacement of Se with S atoms causes a slight change in coordination numbers. The similarity of θSeAsSe and θAsSeAs bond angle distributions suggest that S atoms have a similar role in the structure formation as Se atoms. The RMC models highlighted a glassy network built-up from AsSe3 trigonal bipyramids, in all binary and ternary samples.

Rigidity Theory-Based Approximation of Vibrational Entropy Changes upon Binding to Biomolecules.

We introduce a computationally efficient approximation of vibrational entropy changes (ΔSvib) upon binding to biomolecules based on rigidity theory. From constraint network representations of the binding partners, ΔSvib is estimated from changes in the number of low frequency ("spongy") modes with respect to changes in the networks' coordination number. Compared to ΔSvib computed by normal-mode analysis (NMA), our approach yields significant and good to fair correlations for data sets of protein-protein and protein-ligand complexes. Our approach could be a valuable alternative to NMA-based ΔSvib computation in end-point (free) energy methods.

Graphene-like monolayer low-buckled honeycomb germanium film

Molecular dynamics simulations have been performed to study the cooling process of two-dimensional liquid germanium under nanoslit confinement. The results clearly indicates that the liquid germanium undergoes an obvious liquid-solid phase transition to a monolayer honeycomb film with the decrease of temperature, accompanying the rapid change in potential energy, atomic volume, coordination number and lateral radial distribution function. During the solidification process, some hexagonal atomic islands first randomly emerge in the disordered liquid film and then grow up to stable crystal grains which keep growing and finally connect together to form a honeycomb polycrystalline film. It is worth noting that the honeycomb germanium film is low-buckled, quite different from the planar graphene.

DFT Analysis of Spin Crossover in Mn(III) Complexes: Is a Two-Electron S = 2 to S = 0 Spin Transition Feasible?

Six-coordinate, rigorously octahedral d4 Mn(III) spin crossover (SCO) complexes are limited by symmetry to an S = 1 (intermediate spin, IS) to S = 2 (high spin, HS) transition. In order to realize the potential S = 0 to S = 2 transition, a lower symmetry and/or change in coordination number is needed, which we explore here computationally. First, a number of complexes are analyzed to develop a reliable and relatively fast DFT protocol for reproducing known Mn(III) spin state energetics. The hybrid meta-GGA functional TPSSh with a modest split valence plus polarization basis set and an empirical dispersion correction is found to predict correctly the ground spin state of Mn(III) complexes, including true low-spin (LS) S = 0 systems, with a range of donor sets including the hexadentate [N4O2] Schiff base ligands. The electronic structure design criteria necessary for realizing a ΔS = 2 SCO transition are described, and a number of model complexes are screened for potential SCO behavior. Five-coordinate trigonal-bipyramidal symmetry fails to yield any suitable systems. Seven-coordinate, approximately pentagonal bipyramidal symmetry is more favorable, and when a known pentadentate macrocyclic donor is combined with π-acceptor axial ligands, a novel Mn(III) complex, [Mn(PABODP)(PF3)2]3+ (PABODP = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), is predicted to have the right spin state energetics for an S = 0 to S = 2 transition. Successful synthesis of such a complex could provide the first example of a ΔS = 2 SCO transition for d4 Mn(III). However, the combination of a rigid macrocycle and a high coordination number dilutes the stereochemical activity of the d electrons, leading to relatively small structural changes between HS and LS systems. It may therefore remain a challenge to realize strong cooperative effects in Mn(III) systems.

Non-Dissipative Structural Evolutions in Granular Materials

The structure of the contact network in granular assemblies can evolve due to either dissipative mechanisms such as sliding at contact points, or non-dissipative mechanisms through the phenomenon of contact gain and loss. Being associated with negligible deformations, non-dissipative mechanisms is actually active even in the small strain range of ~ 10−3 , especially in the case of densely packed assemblies. Hence, from a constitutive modelling point of view, it is crucial to be able to estimate such non-dissipative evolutions since both elastic and plastic properties of granular assemblies highly depend on contact network characteristics. The current study proposes an analytical scheme that allows us to estimate the non-dissipative contact gain/loss regime in terms of directional changes in the average contact force. The probability distribution of contact forces is used to compute the number of lost contact for each direction. Similarly, the number of newly formed contacts is estimated by considering the probability distribution of the gap between neighbouring particles. Based on the directional contact gain/loss computed, the changes in coordination number and fabric anisotropy can be found which, together with statistical treatments of Love-Weber stress expression, form a complete system of equations describing the evolution of other controlling microvariables. Finally, the results of the calculations have been compared with DEM simulations which verify the accuracy of the proposed scheme.

X-ray absorption spectroscopies of Mg-Al-Ni hydrotalcite like compound for explaining the generation of surface acid sites

Hydrotalcite-like compound containing metal cations such as Mg2+, Al3+ and Ni2+ was characterized using Ni K-edge EXAFS and in situ Ni K-edge XANES techniques for clarifying its bonding environment around Ni2+ sites and structure changes during calcination from room temperature to 550 °C, respectively. At the fixed molar ratio of Mg/Ni/Al of 2/1/1, the results obtained from EXAFS analysis showed a slight blue shift before and after the calcination at 550 °C and a reduction in white line peak; the best fits of the two samples revealed tiny change in coordination number about 7 for Ni-O path but considerable difference for Ni-Mg(Al) path from about 4.5 to 9.5, confirming a modification from brucite like to mixed oxide structure. On the other hand, bond distances of the Ni-O and Ni-Mg paths nearly fixed at about 2.06 A to 3.0 A reflected stability of the cationic bond order on each plane, but partial collapse and decomposition of the interlayer formed by water molecules and anion CO 3 2− after the calcination. Linear combination fit extracted from the in situ Ni K-edge XANES also confirmed the changes along with the calcination such as slow and fast decreases of brucite fraction at 150 °C and 330 °C, respectively, in corresponding to the mixed oxide fraction increases. The achieved bonding structures were also applied to explain acid-base occurrence of the hydrotalcite-like material, especially the acid sites generated by different static charges along with the bonds. The explanation was illustrated by NH3-TPD method.

Polymorphic germanium films forming in slit nanopore

The solidification of two-dimensional liquid germanium confined to slit nanopores with different sizes has been studied using molecular dynamics simulations. The results clearly show that the system undergoes an obvious liquid-solid phase transition to polymorphic ordered germanium films with the decrease of temperature, accompanied by dramatic change in potential energy and coordination number. Moreover, it is found that the slit size is of vital importance to the transition point and the structure of the germanium films. The results of radial distribution functions, density distribution functions and angular distribution functions suggest the obtained germanium films have various novel structures different from the graphene.

Toward Quantitatively Accurate Calculation of the Redox-Associated Acid–Base and Ligand Binding Equilibria of Aquacobalamin

Redox processes in complex transition metal-containing species are often intimately associated with changes in ligand protonation states and metal coordination number. A major challenge is therefore to develop consistent computational approaches for computing pH-dependent redox and ligand dissociation properties of organometallic species. Reduction of the Co center in the vitamin B12 derivative aquacobalamin can be accompanied by ligand dissociation, protonation, or both, making these properties difficult to compute accurately. We examine this challenge here by using density functional theory and continuum solvation to compute Co-ligand binding equilibrium constants (Kon/off), pKas, and reduction potentials for models of aquacobalamin in aqueous solution. We consider two models for cobalamin ligand coordination: the first follows the hexa, penta, tetra coordination scheme for Co(III), Co(II), and Co(I) species, respectively, and the second model features saturation of each vacant axial coordination site on Co(II) and Co(I) species with a single, explicit water molecule to maintain six directly interacting ligands or water molecules in each oxidation state. Comparing these two coordination schemes in combination with five dispersion-corrected density functionals, we find that the accuracy of the computed properties is largely independent of the scheme used, but including only a continuum representation of the solvent yields marginally better results than saturating the first solvation shell around Co throughout. PBE performs best, displaying balanced accuracy and superior performance overall, with RMS errors of 80 mV for seven reduction potentials, 2.0 log units for five pKas and 2.3 log units for two log Kon/off values for the aquacobalamin system. Furthermore, we find that the BP86 functional commonly used in corrinoid studies suffers from erratic behavior and inaccurate descriptions of Co-axial ligand binding, leading to substantial errors in predicted pKas and Kon/off values. These findings demonstrate the effectiveness of the present approach for computing electrochemical and thermodynamic properties of a complex transition metal-containing cofactor.

Ultrahigh-pressure acoustic wave velocities of SiO2-Al2O3 glasses up to 200 GPa

Extensive experimental studies on the structure and density of silicate glasses as laboratory analogs of natural silicate melts have attempted to address the nature of dense silicate melts that may be present at the base of the mantle. Previous ultrahigh-pressure experiments, however, have been performed on simple systems such as SiO2 or MgSiO3, and experiments in more complex system have been conducted under relatively low-pressure conditions below 60 GPa. The effect of other metal cations on structural changes that occur in dense silicate glasses under ultrahigh pressures has been poorly understood. Here, we used a Brillouin scattering spectroscopic method up to pressures of 196.9 GPa to conduct in situ high-pressure acoustic wave velocity measurements of SiO2-Al2O3 glasses in order to understand the effect of Al2O3 on pressure-induced structural changes in the glasses as analogs of aluminosilicate melts. From 10 to 40 GPa, the transverse acoustic wave velocity (V S ) of Al2O3-rich glass (SiO2 + 20.5 mol% Al2O3) was greater than that of Al2O3-poor glass (SiO2 + 3.9 mol% Al2O3). This result suggests that SiO2-Al2O3 glasses with higher proportions of Al ions with large oxygen coordination numbers (5 and 6) become elastically stiffer up to 40 GPa, depending on the Al2O3 content, but then soften above 40 GPa. At pressures from 40 to ~100 GPa, the increase in V S with increasing pressure became less steep than below 40 GPa. Above ~100 GPa, there were abrupt increases in the P-V S gradients (dV S /dP) at 130 GPa in Al2O3-poor glass and at 116 GPa in Al2O3-rich glass. These changes resemble previous experimental results on SiO2 glass and MgSiO3 glass. Given that changes of dV S /dP have commonly been related to changes in the Si-O coordination states in the glasses, our results, therefore, may indicate a drastic structural transformation in SiO2-Al2O3 glasses above 116 GPa, possibly associated with an average Si-O coordination number change to higher than 6. Compared to previous acoustic wave velocity data on SiO2 and MgSiO3 glasses, Al2O3 appears to promote a lowering of the pressure at which the abrupt increase of dV S /dP is observed. This suggests that the Al2O3 in silicate melts may help to stabilize those melts gravitationally in the lower mantle.