Medium-range structural properties of vitreous germania obtained through first-principles analysis of vibrational spectra

Accurate and Transferable Machine Learning Potential for Molecular Dynamics Simulation of Sodium Silicate Glasses

An accurate and transferable machine learning (ML) potential for the simulation of binary sodium silicate glasses over a wide range of compositions (from 0 to 50% Na2O) was developed. The potential energy surface is approximated by the sum of atomic energy contributions mapped by a neural network algorithm from the local geometry comprising information on atomic distances and angles with neighboring atoms using the DeePMD code [Wang, H. Comput. Phys. Commun. 2018, 228, 178-184]. Our model was trained on a large data set of total energies and atomic forces computed at the density functional theory level on structures extracted from classical molecular dynamics (MD) simulations performed at several temperatures from 300 to 3000 K. This allows for the generation of a robust and transferable ML potential applicable over the full compositional range of glass formability at different temperatures that outperforms the empirical potentials available in the literature in reproducing structures and properties such as bond angle distribution, total distribution functions, and vibrational density of state. The generality of the approach enables the future training of a potential with other or more elements allowing for simulations of structures, properties, and behavior of ternary and multicomponent oxide glasses with nearly ab initio accuracy at a fraction of the computational cost.

Threefold coordinated germanium in a GeO2 melt

The local structure around germanium is a fundamental issue in material science and geochemistry. In the prevailing viewpoint, germanium in GeO2 melt is coordinated by at least four oxygen atoms. However, the viewpoint has been debated for decades due to several unexplained bands present in the GeO2 melt Raman spectra. Using in situ Raman spectroscopy and density functional theory (DFT) computation, we have found a [GeOØ2]n (Ø = bridging oxygen) chain structure in a GeO2 melt. In this structure, the germanium atom is coordinated by three oxygen atoms and interacts weakly with two neighbouring non-bridging oxygen atoms. The bonding nature of the chain has been analyzed on the basis of the computational electronic structure. The results may settle down the longstanding debate on the GeO2 melt structure and modify our view on germanate chemistry.

Enhanced medium-range order in vapor-deposited germania glasses at elevated temperatures

Glasses are nonequilibrium solids with properties highly dependent on their method of preparation. In vapor-deposited molecular glasses, structural organization could be readily tuned with deposition rate and substrate temperature. Here, we show that the atomic arrangement of strong network-forming GeO₂ glass is modified at medium range (<2 nm) through vapor deposition at elevated temperatures. Raman spectral signatures distinctively show that the population of six-membered GeO₄ rings increases at elevated substrate temperatures. Deposition near the glass transition temperature is more efficient than postgrowth annealing in modifying atomic structure at medium range. The enhanced medium-range organization correlates with reduction of the room temperature internal friction. Identifying the microscopic origin of room temperature internal friction in amorphous oxides is paramount to design the next-generation interference coatings for mirrors of the end test masses of gravitational wave interferometers, in which the room temperature internal friction is a main source of noise limiting their sensitivity.

Networks under pressure: the development of in situ high-pressure neutron diffraction for glassy and liquid materials

The pressure-driven collapse in the structure of network-forming materials will be considered in the gigapascal (GPa) regime, where the development of in situ high-pressure neutron diffraction has enabled this technique to obtain new structural information. The improvements to the neutron diffraction methodology are discussed, and the complementary nature of the results is illustrated by considering the pressure-driven structural transformations for several key network-forming materials that have also been investigated by using other experimental techniques such as x-ray diffraction, inelastic x-ray scattering, x-ray absorption spectroscopy and Raman spectroscopy. A starting point is provided by the pressure-driven network collapse of the prototypical network-forming oxide glasses B2O3, SiO2 and GeO2. Here, the combined results help to show that the coordination number of network-forming structural motifs in a wide range of glassy and liquid oxide materials can be rationalised in terms of the oxygen-packing fraction over an extensive pressure and temperature range. The pressure-driven network collapse of the prototypical chalcogenide glass GeSe2 is also considered where, as for the case of glassy GeO2, site-specific structural information is now available from the method of in situ high-pressure neutron diffraction with isotope substitution. The application of in situ high-pressure neutron diffraction to other structurally disordered network-forming materials is also summarised. In all of this work a key theme concerns the rich diversity in the mechanisms of network collapse, which drive the changes in physico-chemical properties of these materials. A more complete picture of the mechanisms is provided by molecular dynamics simulations using theoretical schemes that give a good account of the experimental results.

Voronoi dipole moments for the simulation of bulk phase vibrational spectra

Voronoi tessellation of the electron density in ab initio molecular dynamics simulations is used to calculate vibrational spectra.

Mechanisms of network collapse in GeO2 glass: high-pressure neutron diffraction with isotope substitution as arbitrator of competing models

The structure of the network forming glass GeO(2) 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 ρ/ρ(0) > or approximately equal to 1.16, where ρ(0) is the value at ambient pressure, network collapse proceeds via an interplay between the predominance of distorted square pyramidal GeO(5) units versus octahedral GeO(6) units as they replace tetrahedral GeO(4) 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 ρ/ρ(0) = 1 and fourfold rings when ρ/ρ(0) = 1.64.

Vibrational properties of vitreous GeSe2 with the Becke-Lee-Yang-Parr density functional

The vibrational properties of vitreous GeSe(2) are studied within the Becke-Lee-Yang-Parr (BLYP) generalized gradient approximation to density functional theory. For this purpose, we consider two models. The first one is derived from a chemically ordered network, previously generated through classical molecular dynamics. The second one is generated through first-principles molecular dynamics simulations based on the BLYP functional and shows a larger variety of structural motifs. We describe the structural and electronic properties of our models, including bond lengths, first-neighbor coordinations, and the electronic density of states. The vibrational properties are addressed through the vibrational density of states, the infrared spectrum, and the Raman spectrum. For the chemically ordered model, the vibrational properties obtained with the BLYP functional are of comparable quality with respect to previous results obtained with the Perdew-Wang (PW) functional. However, a global consideration of our results indicates that the BLYP functional yields an overall improved description for the vibrational properties of vitreous GeSe(2). The BLYP dielectric constants are closer to experimental values. Furthermore, we found that the Raman spectrum of the model generated through first-principles molecular dynamics within the BLYP scheme is largely improved with respect to an analogous model generated within the PW scheme.

Overlayer surface-enhanced Raman spectroscopy for studying the electrodeposition and interfacial chemistry of ultrathin ge on a nanostructured support

Ultrathin films of germanium (Ge) have been electrodeposited onto surface-enhanced raman spectroscopy (SERS)-active, polycrystalline gold (Au) nanoparticle film electrodes from aqueous solutions containing dissolved GeO2. An overlayer SERS strategy was employed to use the SERS-activity of the underlying Au electrode to enhance the Raman signatures separately for the Ge phonon mode and vibrational modes of surface groups. Electrochemical and spectroscopic data are presented that demonstrate monolayer-level detection of the electrodeposited material and the preparation of crystalline Ge films exhibiting quantum-confinement effects. Potential-dependent Raman spectra are shown that identify electrodeposition conditions where Ge films can be deposited with either long- or short-range crystalline order. Raman spectra collected with electrodeposited Ge films immersed in solutions containing CN-(aq) did not indicate a significant presence of pinholes that exposed the underlying Au(s) substrate. Raman spectra were also collected that identified a potential-dependence for Ge hydride formation at the interface of these films. Separate spectra were collected for the oxidative dissolution of Ge in solution and the complete dry oxidation of Ge to GeOx in air. These data sets cumulatively represent the first demonstration of the overlayer SERS strategy to follow surface chemical processes at crystalline, nanostructured, Ge materials in situ and in real time.

High-pressure behaviour of GeO2: a simulation study

In this work we study the high-pressure behaviour of liquid and glassy GeO(2) by means of molecular dynamics simulations. The interaction potential, which includes dipole polarization effects, was parametrized using first-principles calculations. Our simulations reproduce the most recent experimental structural data very well. The character of the pressure-induced structural transition in the glassy system has been a matter of controversy. We show that our simulations and the experimental data are consistent with a smooth transition from a tetrahedral to an octahedral network with a significant number of pentacoordinated germanium ions appearing over an extended pressure range. Finally, the study of high-pressure, liquid germania confirms that this material presents an anomalous behaviour of the diffusivity as observed in analogous systems such as silica and water. The importance of pentacoordinated germanium ions for such behaviour is stressed.

First-principles investigation of the relation between structural and NMR parameters in vitreous GeO2

NMR parameters of (73)Ge and (17)O in vitreous GeO(2) and quartz GeO(2), including the isotropic shifts, the quadrupole coupling constants C(Q), and the electric-field-gradient asymmetry parameters η, are determined through density functional calculations. Clear correlations are established between (73)Ge shifts and the mean of the four neighboring Ge-O-Ge bond angles, and between C(Q) and η parameters of (17)O and the local Ge-O-Ge angle. Available experimental data for C(Q) and the corresponding established correlation are used to extract the value of 135° for the average Ge-O-Ge angle in vitreous GeO(2). The features of the Ge-O-Ge bond angle distribution of vitreous GeO(2) derived in this work are consistent with those inferred from other experimental probes.

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

A new EXAFS investigation of local structural changes in amorphous and crystalline GeO(2) at high pressure

Structural transformations at high pressure in amorphous and quartz-like crystalline GeO(2) have been investigated by using a Paris-Edinburgh press coupled to EXAFS spectroscopy. From both the germanium absorption edge position and the Ge-O distance evolution, new detailed information has been obtained about the pressure behavior of the short range order. Crystalline GeO(2) undergoes a transformation from four- to six-fold coordination at about 8.5 GPa, but at least the whole 6-12 GPa pressure range should be considered as the transition region. On the other hand, amorphous GeO(2) is characterized by a much more gradual structural change and the full octahedral state is not reached at 13 GPa as commonly believed. Furthermore, no support to the recently claimed fully pentahedral intermediate state can be given. EXAFS signals of glassy GeO(2) beyond the first Ge-O shell qualitatively confirm the continuous breakdown of the intermediate range order up to 10 GPa.

Vibrational spectra of vitreous SiO2 and vitreous GeO2 from first principles

Using a density-functional approach, we calculate the principal vibrational spectra of vitreous SiO₂ and vitreous GeO₂ and discuss their analogies and differences. For both glasses, we generate model structures consisting of a random network of corner-sharing tetrahedra and differing only by their packing density. The comparison between calculated and measured neutron structure factors supports the validity of our model structures. Our investigation then extends to the vibrational properties, including the inelastic-neutron, infrared, and Raman spectra. For these spectra, good agreement with experiment is also found. Our results support the picture that silica and germania are constituted by a continuous random network of corner-sharing tetrahedra. In particular, the good agreement with experiment for the Raman spectra supports the average intertetrahedral angles of 148° and 135° found in our models of vitreous SiO₂ and vitreous GeO₂, respectively. The concentration of small ring structures in these glasses is also discussed.

Structure of glassy GeO2

The full set of partial structure factors for glassy germania, or GeO₂, were accurately measured by using the method of isotopic substitution in neutron diffraction in order to elucidate the nature of the pair correlations for this archetypal strong glass former. The results show that the basic tetrahedral Ge(O_1/2)₄ building blocks share corners with a mean inter-tetrahedral Ge-Ô-Ge bond angle of 132(2)°. The topological and chemical ordering in the resultant network displays two characteristic length scales at distances greater than the nearest neighbour. One of these describes the intermediate range order, and manifests itself by the appearance of a first sharp diffraction peak in the measured diffraction patterns at a scattering vector k_FSDP≈1.53 Å^-1, while the other describes so-called extended range order, and is associated with the principal peak at k_PP = 2.66(1) Å^-1. We find that there is an interplay between the relative importance of the ordering on these length scales for tetrahedral network forming glasses that is dominated by the extended range ordering with increasing glass fragility. The measured partial structure factors for glassy GeO₂ are used to reproduce the total structure factor measured by using high energy x-ray diffraction and the experimental results are also compared to those obtained by using classical and first principles molecular dynamics simulations.

Glass fragility and atomic ordering on the intermediate and extended range

The relation between the fragility of glass-forming systems, a parameter which describes many of their key physical characteristics, and atomic scale structure is investigated by using neutron diffraction to measure the topological and chemical ordering for germania, or GeO(2), which is an archetypal strong glass former. We find that the ordering for this and other tetrahedral network-forming glasses at distances greater than the nearest neighbor can be rationalized in terms of an interplay between the relative importance of two length scales. One of these is associated with an intermediate range, the other with an extended range and, with increasing glass fragility, it is the extended range ordering which dominates.