Structural and electronic properties of liquid and amorphous SiO2: An ab initio molecular dynamics study

First-principles investigation of positively charged and neutral oxygen vacancies in amorphous silica

The structural parameters, electron localization functions, electron paramagnetic resonance (EPR) parameters, formation energies, and thermodynamic transition levels of various oxygen vacancy defects in amorphous silica are comprehensively and integrally investigated by using density functional theory. The trends of changes in the oxygen vacancy defect structure and electron localization induced by the increase in distance between defective silicon atoms are clearly identified. It is shown that the dimer configuration may be the potential structure of the Eδ' center. For the back-projected unpuckered configuration and the puckered configuration, whose EPR parameters are more consistent with the experimental values of the Eγ' center, the unpaired electron localized on the sp3 hybridized silicon atom is a common feature. Due to the three-coordinated oxygen atom in the forward-oriented configuration, the EPR parameters are closest to those of the Eα' center. Transformations of oxygen vacancy defects under different charge states are studied by sequentially adding and removing electrons. The thermodynamic transition level analysis reveals that the dimer and forward configurations may behave as deep traps for electron accumulation. The back-projected puckered fourfold-coordinated and fivefold-coordinated configurations are comparatively stable and may be able to function as shallow traps for electron transport. The neutral double unpuckered, neutral back-projected puckered fourfold-coordinated, and neutral back-projected unpuckered configurations are more likely to lose electrons during hole trapping. As the bias voltage is repeatedly changed, the defect density of the puckered configuration may reduce, while that of the dimer and unpuckered configuration may take an opposite trend.

Developments in Atomistic and Nano Structure Evolution Mechanisms of Molten Slag Using Atomistic Simulation Methods

Molten slag has different properties depending on its composition. The relationship between its composition, structure, and properties has been the focus of attention in industrial manufacturing processes. This review describes the atomistic scale mechanisms by which oxides of different compositions affect the properties and structure of slag, and depicts the current state of research in the atomic simulation of molten slag. At present, the research on the macroscopic properties of molten slag mainly focuses on viscosity, free-running temperature, melting point, and desulphurization capacity. Regulating the composition has become the most direct and effective way to control slag properties. Analysis of the microevolution mechanism is the fundamental way to grasp the macroscopic properties. The microstructural evolution mechanism, especially at the atomic and nanoscale of molten slag, is reviewed from three aspects: basic oxides, acidic oxides, and amphoteric oxides. The evolution of macroscopic properties is analyzed in depth through the evolution of the atomic structure. Resolution of the macroscopic properties of molten slag by the atomic structure plays a crucial role in the development of fundamental theories of physicochemistry.

Energy Landscapes for Lithium Incorporation and Diffusion in Multidomain Silicon Suboxide Anode Materials

In-depth understanding of the lithium interaction characteristics within multidomain silicon suboxide is indispensable for optimizing the electrochemical performance of silicon suboxide anode materials for lithium-ion batteries. In this study, we investigate the domain-dependent thermodynamic and kinetic properties of lithium atoms within systematically designed multidomain silicon suboxide models composed of Si, SiO2, and Si/SiO2 interface by performing a series of computational simulations combined with a unique tomography-like sampling scheme. We find that the Si/SiO2 interfacial region exhibits preferential thermodynamics and kinetics for lithiation and can serve as a critical lithium transport channel during charge-discharge cycles, while the SiO2 domain is likely to be excluded from lithiation due to its high resistance to lithium diffusion. Consequently, a significant fraction of lithium is expected to be trapped at the Si/SiO2 interface during the discharge process, which ultimately contributes to a low initial Coulombic efficiency. This theoretical understanding suggests that the formation of continuously connected lithium-transportable Si/SiO2 interfacial channels surrounding the Si domains, along with a well-structured shallow SiO2 framework through the use of appropriate synthesis methods, is essential for maximizing the electrochemical performance of silicon suboxide anode materials.

Machine learning molecular dynamics reveals the structural origin of the first sharp diffraction peak in high-density silica glasses

The first sharp diffraction peak (FSDP) in the total structure factor has long been regarded as a characteristic feature of medium-range order (MRO) in amorphous materials with a polyhedron network, and its underlying structural origin is a subject of ongoing debate. In this study, we utilized machine learning molecular dynamics (MLMD) simulations to explore the origin of FSDP in two typical high-density silica glasses: silica glass under pressure and permanently densified glass. Our MLMD simulations accurately reproduce the structural properties of high-density silica glasses observed in experiments, including changes in the FSDP intensity depending on the compression temperature. By analyzing the simulated silica glass structures, we uncover the structural origin responsible for the changes in the MRO at high density in terms of the periodicity between the ring centers and the shape of the rings. The reduction or enhancement of MRO in the high-density silica glasses can be attributed to how the rings deform under compression.

Ab initio molecular dynamics study on the disordered Li-Ga-Sn system

The eutectic Ga_91.6Sn_8.4 liquid metal could serve as the anode in Li-ion batteries to avoid dendrite growth issue and volume expansion, and maintain a good cycle life. However, the microstructure and the basic physical properties of the lithiated Ga_91.6Sn_8.4 are ignored in experiments and still unclear. In this work, we assume that a disordered structure is formed in the initial stage of lithiation of Ga_91.6Sn_8.4, and the structure, equilibrium density, thermal expansion coefficient, mixing enthalpy, self-diffusion coefficient and viscosity of the disordered Li-Ga-Sn system are investigated systematically by ab initio molecular dynamics. The radial distribution function, structure factor and bond angle distribution function are calculated to obtain local structure information. Our calculations show that the lithiation of Ga_91.6Sn_8.4 is exothermic, and for most cases, the diffusion coefficients for Li, Ga and Sn decrease with increasing Li content. Based on structural information and diffusion coefficients, we reveal that the lithiation of Ga_91.6Sn_8.4 will make the liquid Ga_91.6Sn_8.4 alloy form a solid-like structure. With the increase of Li content, it is more likely to form a solid-like structure. Furthermore, our simulations reveal that the chemical interaction of Li-Sn and Li-Ga is stronger than that of Ga-Sn, and Li is prone to combine with Sn firstly in the lithiation process of Ga_91.6Sn_8.4.

Electronic, mechanical and piezoelectric properties of glass-like complex Na2Si1−xGexO3 (x = 0.0, 0.25, 0.50, 0.75, 1.0)

Motivated by our previous work on pristine Na₂SiO₃, we proceeded with calculations on the structural, electronic, mechanical and piezoelectric properties of complex glass-like Na₂Si_1−xGe_xO₃ (x = 0.0, 0.25, 0.50, 0.75, 1.0) by using density functional theory (DFT). Interestingly, the optimized bond lengths and bond angles of Na₂SiO₃ and Na₂GeO₃ resemble each other with high similarity. On doping we report the negative formation energy and feasibility of transition of Na₂SiO₃ → Na₂GeO₃ while the structural symmetry is preserved. Analyzing the electronic profile, we have observed a reduced band gap on increasing x = Ge concentration at Si-sites. All the systems are indirect band gap (Z–Γ) semiconductors. The studied systems have shown mechanical stabilities by satisfying the Born criteria for mechanical stability. The calculated results have shown highly anisotropic behaviour and high melting temperature, which are a signature of glass materials. The piezoelectric tensor (both direct and converse) is computed. The results thus obtained predict that the systems under investigation are potential piezoelectric materials for energy harvesting.

Motivated by our previous work on pristine Na₂SiO₃, we proceeded with calculations on the structural, electronic, mechanical and piezoelectric properties of complex glass-like Na₂Si_1−xGe_xO₃ (x = 0.0, 0.25, 0.50, 0.75, 1.0) by using density functional theory (DFT).

Intrinsic Point Defects in Silica for Fiber Optics Applications

Due to its unique properties, amorphous silicon dioxide (a-SiO2) or silica is a key material in many technological fields, such as high-power laser systems, telecommunications, and fiber optics. In recent years, major efforts have been made in the development of highly transparent glasses, able to resist ionizing and non-ionizing radiation. However the widespread application of many silica-based technologies, particularly silica optical fibers, is still limited by the radiation-induced formation of point defects, which decrease their durability and transmission efficiency. Although this aspect has been widely investigated, the optical properties of certain defects and the correlation between their formation dynamics and the structure of the pristine glass remains an open issue. For this reason, it is of paramount importance to gain a deeper understanding of the structure-reactivity relationship in a-SiO2 for the prediction of the optical properties of a glass based on its manufacturing parameters, and the realization of more efficient devices. To this end, we here report on the state of the most important intrinsic point defects in pure silica, with a particular emphasis on their main spectroscopic features, their atomic structure, and the effects of their presence on the transmission properties of optical fibers.

Strong UV laser absorption source near 355 nm in fused silica and its origination

As a high-performance optical material, fused silica is widely applied in high-power laser and photoelectric systems. However, laser induced damage (LID) of fused silica severely limits the output power and performance of these systems. Due to the values in strong field physics and improving the load capacity and performance of high power systems at UV laser, LID at 355 nm of fused silica has attracted much attention. It has been found that, even be treated by advanced processing technologies, the actual damage threshold of fused silica at 355 nm is far below the intrinsic threshold. It means that there is an absorption source near 355 nm in fused silica. However, to date, the absorption source is still unknown. In this paper, a absorption source near 355 nm is found by first-principles calculations. We find that the absorption source near 355 nm is neutral oxygen-vacancy defect (NOV, ≡Si-Si≡) and this defect originates from the oxygen deficiency of fused silica. Our results indicate that NOV defect can be taken as a damage precursor for 355 nm UV laser, and this precursor can be obviously reduced by increasing the ratio of oxygen to silicon. Present work is valuable for exploring damage mechanisms and methods to improve the damage threshold of fused silica at UV laser.

Self-learning hybrid Monte Carlo method for isothermal-isobaric ensemble: Application to liquid silica

Self-learning hybrid Monte Carlo (SLHMC) is a first-principles simulation that allows for exact ensemble generation on potential energy surfaces based on density functional theory. The statistical sampling can be accelerated with the assistance of smart trial moves by machine learning potentials. In the first report [Nagai et al., Phys. Rev. B 102, 041124(R) (2020)], the SLHMC approach was introduced for the simplest case of canonical sampling. We herein extend this idea to isothermal-isobaric ensembles to enable general applications for soft materials and liquids with large volume fluctuation. As a demonstration, the isothermal-isobaric SLHMC method was used to study the vibrational structure of liquid silica at temperatures close to the melting point, whereby the slow diffusive motion is beyond the time scale of first-principles molecular dynamics. It was found that the static structure factor thus computed from first-principles agrees quite well with the high-energy x-ray data.

Structural and dynamic properties of aluminosilicate melts: a molecular dynamics study

In the present work, the structural and dynamic properties of aluminosilicates (Al₂O₃) _x -(SiO₂)_1-x (AS) as a function of the Al₂O₃ concentration x are studied by means of molecular dynamics simulations. Firstly, the parametrization of the Born-Mayer-Huggins type potential developed recently for the more general CaO-Al₂O₃-SiO₂ ternary system is assessed. Comparison of local structural properties, such as the x-ray structure factor, partial pair-correlation functions, distributions of coordination numbers and bond angles, as well as the dynamics through the viscosity and self-diffusion coefficients to experimental data and other molecular dynamics simulations found in the literature, shows that this potential is transferable to AS melts for all compositions and is more reliable than other empirical potentials used so far. The evolution of viscosity with temperature in stable liquid and undercooled regions is studied in the whole composition range and results show a progressive increase of the fragility with increasing Al₂O₃ content correlated to that of local structural entities like the triply bonded oxygen (TBO), AlO₅ and AlO₆.

Comparison of different machine learning models for the prediction of forces in copper and silicon dioxide

Recently, the machine learning (ML) force field has emerged as a powerful atomic simulation approach because of its high accuracy and low computational cost.

Diffusion and aggregation of oxygen vacancies in amorphous silica

Using density functional theory (DFT) calculations, we investigated oxygen vacancy diffusion and aggregation in relation to dielectric breakdown in amorphous silicon dioxide (a-SiO₂). Our calculations indicate the existence of favourable sites for the formation of vacancy dimers and trimers in the amorphous network with maximum binding energies of approximately 0.13 eV and 0.18 eV, respectively. However, an average energy barrier height for neutral vacancy diffusion is found to be about 4.6 eV, rendering this process unfeasible. At Fermi level positions above 6.4 eV with respect to the top of the valence band, oxygen vacancies can trap up to two extra electrons. Average barriers for the diffusion of negative and double negatively charged vacancies are found to be 2.7 eV and 2.0 eV, respectively. These barriers are higher than or comparable to thermal ionization energies of extra electrons from oxygen vacancies into the conduction band of a-SiO₂. In addition, we discuss the competing pathways for electron trapping in oxygen deficient a-SiO₂ caused by the existence of intrinsic electron traps and oxygen vacancies. These results provide new insights into the role of oxygen vacancies in degradation and dielectric breakdown in amorphous silicon oxides.

Modelling the local atomic structure of molybdenum in nuclear waste glasses with ab initio molecular dynamics simulations

The nature of chemical bonding of molybdenum in high level nuclear waste glasses has been elucidated by ab initio molecular dynamics simulations. Two compositions, (SiO₂)_57.5-(B₂O₃)₁₀-(Na₂O)₁₅-(CaO)₁₅-(MoO₃)_2.5 and (SiO₂)_57.3-(B₂O₃)₂₀-(Na₂O)_6.8-(Li₂O)_13.4-(MoO₃)_2.5, were considered in order to investigate the effect of ionic and covalent components on the glass structure and the formation of the crystallisation precursors (Na₂MoO₄ and CaMoO₄). The coordination environments of Mo cations and the corresponding bond lengths calculated from our model are in excellent agreement with experimental observations. The analysis of the first coordination shell reveals two different types of molybdenum host matrix bonds in the lithium sodium borosilicate glass. Based on the structural data and the bond valence model, we demonstrate that the Mo cation can be found in a redox state and the molybdate tetrahedron can be connected with the borosilicate network in a way that inhibits the formation of crystalline molybdates. These results significantly extend our understanding of bonding in Mo-containing nuclear waste glasses and demonstrate that tailoring the glass composition to specific heavy metal constituents can facilitate incorporation of heavy metals at high concentrations.

Volume dependence of the dielectric properties of amorphous SiO2

The dielectric properties of amorphous SiO₂ and other SiO₂ polymorphs are linked by simple volume dependence.

Substrate-induced variations of molecular packing, dynamics, and intermolecular electronic couplings in pentacene monolayers on the amorphous silica dielectric

Charge-carrier transport in thin-film organic field-effect transistors takes place within the first (few) molecular layer(s) of the active organic material in contact with the gate dielectric. Here, we use atomistic molecular dynamics simulations to evaluate how interactions with bare amorphous silica surfaces that vary in terms of surface potential influence the molecular packing and dynamics of a monolayer pentacene film. The results indicate that the long axis of the pentacene molecules has a non-negligible tilt angle away from the surface normal. Grazing-incidence X-ray diffraction patterns for these models are calculated, and we discuss notable differences in the shapes of the Bragg rods as a function of the molecular packing, also in relation to previously published experimental reports. Intermolecular electronic couplings (transfer integrals) evaluated for the monolayers show marked differences compared to bulk crystal calculations, a result that points to the importance of fully considering the molecular packing environment in charge-carrier mobility models for organic electronic materials.

Universal medium-range order of amorphous metal oxides

We propose that the structure of amorphous metal oxides can be regarded as a dual-dense-random-packing structure, which is a superposition of the dense random packing of metal atoms and that of oxygen atoms. Our ab initio molecular dynamics simulations show that the medium-range order of amorphous HfO2, ZrO2, TiO2, In2O3, Ga2O3, Al2O3, and Cu2O is characterized by the pentagonal-bipyramid arrangement of metal atoms and that of oxygen atoms, and prove the validity of our dual-random-sphere-packing model. In other words, we find that the pentagonal medium-range order is universal independent of type of metal oxide.

A Review of High-Energy X-Ray Diffraction from Glasses and Liquids

This paper summarizes the scientific trends associated with the rapid development of the technique of high-energy X-ray diffraction over the past decade pertaining to the field of liquids, glasses, and amorphous materials. The measurement of high-quality X-ray structure factors out to large momentum transfers leads to high-resolution pair distribution functions which can be directly compared to theory or combined with data from other experimental techniques. The advantages of combining highly penetrating radiation with low angle scattering are outlined together with the data analysis procedure and formalism. Also included are advances in high-energy synchrotron beamline instrumentation, sample environment equipment, and an overview of the role of simulation and modeling for interpreting data from disordered materials. Several examples of recent trends in glass and liquid research are described. Finally, directions for future research are considered within the context of past and current developments in the field.

Tailored nanoheterojunctions for optimized light emission

We present coupled classical and quantum simulations of 1 to 2 nm Si nanocrystals (NCs) embedded in amorphous SiO(2) and we show that by tuning the density of the oxide matrix one may change the relative alignment of Si NC and SiO(2) electronic states at the interface. We find that interfacial strain plays a key role in determining the variation of the nanaoparticle gap as a function of size, as well as of conduction band offsets with the oxide. In particular, our results show that it is the variation of the valence band offset with size that is responsible for the gap change. Our findings suggest that the elastic properties of the embedding matrix may be tuned to tailor the energy levels of small Si NCs so as to optimize their performance in optoelectronic devices and solar cells.

A Semi-Empirical Methodology to Study Bulk Silica System

A semi-empirical methodology [1,2] developed to model and simulate covalently bonded networked systems is modified to study the heteroatomic mixtures of silica. This methodology is capable of grasping the essential qualitative and quantitative features of the coupling between the electronic coordinates and the geometric structure. The methodology is used to simulate and to probe the structural and thermodynamic properties of the bulk crystalline, amorphous solid and the melt states of silica.

First-principles study of enhancement of transport properties of silica melt by water

First-principles molecular dynamics simulations show that water (8.25 wt%) dramatically affects the transport properties of SiO2 liquid increasing the diffusivity and decreasing the viscosity by an order of magnitude. At 3000 K, the diffusivity of Si, O, and H, and the viscosity vary anomalously with pressure. Highly mobile protons make the hydrous liquid a potential superionic conductor. The predicted dynamical changes are associated with structural depolymerization and water speciation, which changes from being dominated by hydroxyls at low pressure to extended structures at high pressure.

First-principles codes for computational crystallography in the Quantum-ESPRESSO package

The Quantum-ESPRESSO package is a multi-purpose and multi-platform software for ab-initio calculations of condensed matter (periodic and disordered) systems. Codes in the package are based on density functional theory and on a plane wave/pseudopotential description of the electronic ground state and are ideally suited for structural optimizations (both at zero and at finite temperature), linear response calculations (phonons, elastic constants, dielectric and Raman tensors, etc.) and high-temperature molecular dynamics. Examples of applications of the codes included in the package are briefly discussed.

Structural models of bioactive glasses from molecular dynamics simulations

The bioactive mechanism, by which living tissues attach to and integrate with an artificial implant through stable chemical bonds, is at the core of many current medical applications of biomaterials, as well as of novel promising applications in tissue engineering. Having been employed in these applications for almost 40 years, soda-lime phosphosilicate glasses such as 45S5 represent today the paradigm of bioactive materials. Despite their strategical importance in the field, the relationship between the structure and the activity of a glass composition in a biological environment has not been studied in detail. This fundamental gap negatively affects further progress, for instance, to improve the chemical durability and tailor the biodegradability of these materials for specific applications. This paper reviews recent advances in computer modelling of bioactive glasses based on molecular dynamics simulations, which are starting to unveil key structural features of these materials, thus contributing to improve our fundamental understanding of how bioactive materials work.

Structure and speciation in hydrous silica melts. 1. Temperature and composition effects

Monte Carlo simulations were used to investigate the phase behavior of hydrated liquid silica as a function of temperature and overall water mole fraction, x w. Simulations using the Feuston-Garofalini potential were performed in the isobaric-isothermal ensemble at p = 1 GPa for 15 temperatures (2000 < or = T < or = 9000 K) and 25 compositions (0.0 < or = x w < or = 0.4). The unusual volume minimum exhibited by tetrahedrally coordinated liquid silica is found to persist up to x w approximately 0.267, although the temperature of the volume minimum decreases with increasing water content. Structural properties of the pure and hydrated systems are compared and the addition of water to liquid silica disrupts the silica network more dramatically than temperature alone. The simulations yield very low concentrations of molecular water, e.g. only about 1.2% of the oxygen atoms are bound to exactly two hydrogen atoms at x w = 0.4 and T = 3000 K.

Band offsets at semiconductor-oxide interfaces from hybrid density-functional calculations

Band offsets at semiconductor-oxide interfaces are determined through a scheme based on hybrid density functionals, which incorporate a fraction alpha of Hartree-Fock exchange. For each bulk component, the fraction alpha is tuned to reproduce the experimental band gap, and the conduction and valence band edges are then located with respect to a reference level. The lineup of the bulk reference levels is determined through an interface calculation, and shown to be almost independent of the fraction alpha. Application of this scheme to the Si-SiO2, SiC-SiO2, and Si-HfO2 interfaces yields excellent agreement with experiment.

Structural properties of amorphous GeSe2

By using first-principles molecular dynamics within density functional theory, we study the structural properties of amorphous GeSe₂ at T = 300 K. The amorphous configurations have been obtained via cooling from the liquid state followed by extensive relaxation (22 ps) at T = 300 K. The agreement with neutron diffraction experiments is very satisfactory, in particular for the pair correlation functions in real space and the partial structure factors in reciprocal space describing the Ge-Se and the Se-Se correlations. Some residual differences between theory and experiment are found for Ge-Ge correlations. The network organizes itself through the predominant presence of GeSe₄ tetrahedra. However, other coordinations exist in non-negligible proportions for both Ge and Se. Homopolar bonds are found for Se, and, in a very limited extent, also for Ge. The number of edge-sharing connections reproduces the experimental data.

Model for the Water−Amorphous Silica Interface: The Undissociated Surface

The physical and chemical properties of the amorphous silica-water interface are of crucial importance for a fundamental understanding of electrochemical and electrokinetic phenomena, and for various applications including chromatography, sensors, metal ion extraction, and the construction of micro- and nanoscale devices. A model for the undissociated amorphous silica-water interface reported here is a step toward a practical microscopic model of this important system. We have extended the popular BKS and SPC/E models for bulk silica and water to describe the hydrated, hydroxylated amorphous silica surface. The parameters of our model were determined using ab initio quantum chemical studies on small fragments. Our model will be useful in empirical potential studies, and as a starting point for ab initio molecular dynamics calculations. At this stage, we present a model for the undissociated surface. Our calculated value for the heat of immersion, 0.3 J x m(-2), falls within the range of reported experimental values of 0.2-0.8 J x m(-2). We also study the perturbation of water properties near the silica-water interface. The disordered surface is characterized by regions that are hydrophilic and hydrophobic, depending on the statistical variations in silanol group density.

Structure of liquid SiO2: a measurement by high-energy x-ray diffraction

The x-ray structure factor for liquid SiO2 has been measured by laser heating of an aerodynamically levitated droplet. The main structural changes of the melt compared to the room temperature glass are associated with an increase in the size of the SiO4 tetrahedra, indicating a small reduction in the average Si-O-Si bond torsion angle and an expansion of the network between 5 and 9 A. Strong directional bonds with little high temperature broadening and a high degree of intermediate range order are found to persist in the liquid state.

Ab Initio Molecular Dynamics Study of 45S5 Bioactive Silicate Glass

Bioglass 45S5, the prototype of bioactive melt-quenched silicate glasses, was modeled by means of Car-Parrinello molecular dynamics (CPMD) simulations. Although long-range structural properties cannot be modeled by using this ab initio approach, the accuracy of CPMD simulations is exploited here to provide insight into the short-range structure and to analyze vibrational and electronic properties of this biomaterial. Detailed structural analysis in the short-range scale provided insight into the local environment of modifier Na and Ca ions: a possible key role of these cations in organizing the glass network by connecting different chains and fragments into specific, rather flexible geometries was proposed. The individual contributions of different species to the vibrational density of states were separated and discussed, allowing the identification of specific features in the vibrational spectrum, such as those related to phosphate groups. The components of the electronic density of states were also analyzed, enabling us to identify correlations between the electronic structure and the structural properties, such as the different bonding character of Si-O bonds involving bridging or nonbridging oxygen atoms.

Proton diffusion mechanism in amorphous SiO2

We study proton diffusion in amorphous SiO2 from the atomic scale to the long-range percolative regime. Ab initio molecular dynamics suggest that the dominant atomic process consists in cross-ring interoxygen hopping assisted by network vibrations. A statistical analysis accounting for the disorder in amorphous SiO2 yields relations between transition energies and interoxygen distances for both cross-ring and nearest-neighbor hopping. The percolative regime is then addressed through large-size model systems reproducing these relations. Cross-ring hopping is confirmed as the dominant diffusion mechanism and supported by a good agreement with experiment for the activation energy.

Mixed Wannier-Bloch functions for electrons and phonons in periodic systems

We introduce mixed Wannier-Bloch functions for studying electronic and vibrational spectra of periodic systems. These functions carry both spatial localization and limited spectral broadening, thereby combining the advantages of descriptions based on energy eigenstates (Bloch states) and position eigenstates (Wannier states). For the analysis of vibrational modes, a lattice position operator is introduced, analogous to the electronic Berry-phase position operator. Application to vitreous SiO2 demonstrates that mixed Wannier-Bloch functions constitute a powerful tool for tracking fingerprints of short- and medium-range structural order in electronic and vibrational spectra.

Simulated structural and thermal properties of glassy and liquid germania

Structural, dynamical, and thermal properties of germanium dioxide are investigated with classical molecular dynamics simulations from the amorphous to the liquid state. Pair correlation functions and coordination numbers are computed under pressure change and show the progressive conversion of the tetrahedral network into an octahedral network, in agreement with experiments. The thermodynamical behavior of the liquid is investigated by means of an equation of state that allows a precise estimation of the compressibility. At low temperature, the diffusion constant D shows an Arrhenius law that progressively deviates when the temperature is increased. The overall comparison with simulated silica permits finally to outline not only the differences in the physical behavior of these two similar systems but also to stress the limitation of the employed germania potential.