arXiv subjects

Simona Ispas

Publications and source records attributed to Simona Ispas.

At least 19 recordsLinked to original sources

arXiv2026

Percolation Criticality of Amorphous-Amorphous Transitions in Compressed Glasses

The low-to-high-density transition in compressed silica glass is investigated using percolation theory. Large-scale molecular dynamics simulations of SiO$_2$ glasses, with system sizes of up to 10$^6$ atoms and pressures ranging from 0 to 35 GPa, were carried out to investigate the emergence of structural motifs and their growth to system-spanning length scales under compression. On this basis, we introduced long-range descriptors that complement conventional local and medium-range structural measures. The results reveal critical percolation transitions of SiO$_Z$-SiO$_Z$ clusters with increasing coordination number $Z$. The critical exponents slightly deviate from the standard (random) correlation, a behavior that seems to be more pronounced for higher coordinated polyhedra than for tetrahedra, suggesting a possible rigidity percolation mechanism. SiSi$_z$-SiSi$_z$ clusters were also analyzed using the non-bonded approach. Bonded and non-bonded approaches complement each other in a particularly illuminating way for describing pressure-induced structural transformations and common mechanisms shared by bonded glasses, such as SiO$_2$, and non-bonded glasses, such as amorphous ice.

cond-mat.dis-nn↗

arXiv2026

Nexus-CAT: A Computational Framework to Define Long-Range Structural Descriptors in Glassy Materials from Percolation Theory

Nexus-CAT (Cluster Analysis Toolkit) is an open-source Python package for cluster detection and percolation analysis of atomistic simulation trajectories. Standard structural tools, such as the pair distribution function or structure factor, fail to capture the long-range connectivity changes underlying amorphous-amorphous transitions in glassy materials. Nexus-CAT addresses this gap by reading extended XYZ trajectory files and identifying clusters via a Union-Find algorithm with path-compression. Four clustering strategies, i.e., distance-based, bonding, coordination-filtered, and shared-neighbor, are implemented through a Strategy Factory design pattern, enabling the treatment of diverse network topologies. The program computes key percolation properties with percolation detection based on a rigorous period vector algorithm. The package is validated against theoretical predictions and applied to glasses with different bonding environments, namely vitreous silica, vitreous ice, and amorphous silicon. One original result is the observation of a percolation transition prior to crystallization in the latter, indicating that pressure-induced crystallization is initially driven by an amorphous transformation with similar coordination number. The code is also designed to be readily extended to gels, cements, and other disordered materials. Nexus-CAT is fully available on GitHub and PyPI.

cond-mat.dis-nn↗

arXiv2023

Surface properties of alkali silicate glasses: Influence of the modifiers

Using large-scale molecular dynamics simulations, we investigate the surface properties of lithium, sodium, and potassium silicate glasses containing 25 mole % of alkali oxide. The comparison of two types of surfaces, a melt-formed surface (MS) and a fracture surface (FS), demonstrates that the influence of the alkali modifier on the surface properties depends strongly on the nature of the surface. The FS exhibits a monotonic increase of modifier concentration with increasing alkali size while the MS shows a saturation of alkali concentration when going from Na to K glasses, indicating the presence of two competing mechanism that influence the properties of a MS. For the FS, we find that larger alkali ions reduce the concentration of under-coordinated Si atoms and increase the fraction of two-membered rings, implying an enhanced chemical reactivity of the surface. For both types of surfaces, the roughness is found to increase with alkali size, with the effect being more pronounced for the FS than for the MS. The height-height correlation functions of the surfaces show a scaling behavior that is independent of the alkali species considered: The ones for the MS are compatible with the prediction of the frozen capillary wave theory while the ones for the FS show a logarithmic growth, i.e., on the nanoscale these surfaces are not self-affine fractals. The influence of the modifier on the surface properties are rationalized in terms of the interplay between multiple factors involving the size of the ions, bond strength, and charge balance on the surface.

cond-mat.dis-nn↗

arXiv2022

Fracture of silicate glasses: Micro-cavities and correlations between atomic-level properties

We use large-scale simulations to investigate the dynamic fracture of silica and sodium-silicate glasses under uniaxial tension. The stress-strain curves demonstrate that silica glass is brittle whereas the glasses rich in Na show pronounced ductility. A strong composition dependence is also seen in the crack velocity which is on the order of 1800 m/s for glasses with low Na concentration and decreases to 700 m/s if the concentration is high. We find that during the fracture of Na-rich glasses very irregular cavities as large as 3-4 nm form ahead of the crack front, indicating the presence of nanoductility in these glasses. Before fracture occurs, the local composition, structure, and mechanical properties are heterogeneous in space and show a strong dependence on the applied strain. Further analysis of the correlations between these local properties allows to obtain a better microscopic understanding of the deformation and fracture of glasses and how the local heating close to the crack tip, up to several hundred degrees, permits the structure to relax.

cond-mat.dis-nn↗

arXiv2021

Origin of the non-linear elastic behavior of silicate glasses

For small tension the response of a solid to an applied stress is given by Hooke's law. Outside this linear regime the relation between stress and strain is no longer universal and at present there is no satisfactory insight on how to connect for disordered materials the stress-strain relation to the microscopic properties of the system. Here we use atomistic computer simulations to establish this connection for the case of silicate glasses containing modifiers. By probing how in the highly non-linear regime the stress-strain curve depends on composition, we are able to identify the microscopic mechanisms that are responsible for the complex dependence of stress on strain, notably the presence of an unexpected quasi-plateau in the tangent modulus. We trace back this dependence to the mobility of the modifiers which, without leaving their cage or modifying the topology of the network, are able to relieve the local stresses. Since the identified mechanism is general, the results obtained in this study will also be helpful for understanding the mechanical response of other disordered materials.

cond-mat.dis-nn↗

arXiv2021

New interaction potentials for alkaline earth silicate and borate glasses

Structure and properties of magnesium silicate and borate melts and glasses were investigated by using newly parameterized interaction potentials in molecular dynamics simulations and compared with those of calcium silicate and borate. The competition between the depolymerization of the silica network and the formation of new bonds with modifier ions leads to the enhancement of the elastic moduli with increasing modifier content in alkaline earth silicate glasses. Compared with calcium silicate, the higher elastic moduli of magnesium silicate result from a higher connectivity of the overall glass network due to the incorporation of fourfold coordinated magnesium and a more rigid connection between SiO4 tetrahedra and modifier ions. In contrast to the silicates, the effect of modifier content on the elastic moduli of alkaline earth borates is dominated by the fraction of fourfold coordinated boron (N4). Calcium borate with higher N4 shows a more rigid network structure and higher elastic moduli.

cond-mat.mtrl-sci↗

arXiv2021

First principles study of the surface of silica and sodium silicate glasses

We use \textit{ab initio} molecular dynamics simulations to investigate the properties of the dry surface of pure silica and sodium silicate glasses. The surface layers are defined based on the atomic distributions along the direction ($z-$direction) perpendicular to the surfaces. We show that these surfaces have a higher concentration of dangling bonds as well as two-membered (2M) rings than the bulk samples. Increasing concentration of Na$_2$O reduces the proportion of structural defects. From the vibrational density of states, one concludes that 2M rings have a unique vibrational signature at a frequency $\approx850$~cm$^{-1}$, compatible with experimental findings. We also find that, due to the presence of surfaces, the atomic vibration in the $z-$direction is softer than for the two other directions. The electronic density of states shows clear the differences between the surface and interior and we can attribute these to specific structural units. Finally, the analysis of the electron localization function allows to get insight on the influence of local structure and the presence of Na on the nature of chemical bonding in the glasses.

cond-mat.mtrl-sci↗

arXiv2021

The critical role of the interaction potential and simulation protocol for the structural and mechanical properties of sodosilicate glasses

We compare the ability of various interaction potentials to predict the structural and mechanical properties of silica and sodium silicate glasses. While most structural quantities show a relatively mild dependence on the potential used, the mechanical properties such as the failure stress and strain as well as the elastic moduli depend very strongly on the potential, once finite size effects have been taken into account. We find that to avoid such finite size effects, samples of at least 75,000 atoms are needed. Finally we probe how the simulation ensemble influences the fracture properties of the glasses and conclude that fracture simulations should be carried out in the constant pressure ensemble.

cond-mat.dis-nn↗

arXiv2020

Roughness and scaling properties of oxide glass surfaces at the nanoscale

Using atomistic computer simulations we determine the roughness and topographical features of melt-formed (MS) and fracture surfaces (FS) of oxide glasses. We find that the topography of the MS is described well by the frozen capillary wave theory. The FS are significant rougher than the MS and depend strongly on glass composition. The height-height correlation function for the FS shows an unexpected logarithmic dependence on distance, in contrast to the power-law found in experiments. We thus conclude that on length scales less than 10 nm FS are not self-affine fractals.

cond-mat.dis-nn↗

arXiv2020

Structure and vibrational properties of sodium silicate glass surfaces

Using molecular dynamics simulations we investigate the dependence of the structural and vibrational properties of the surfaces of sodo-silicate glasses on the sodium content as well as the nature of the surface. Two types of glass surfaces are considered: A melt-formed surface (MS) in which a liquid with a free surface has been cooled down into the glass phase and a fracture surface (FS) obtained by tensile loading of a glass sample. We find that the MS is more abundant in Na and non-bridging oxygen atoms than the FS and the bulk glass, whereas the FS has higher concentration of structural defects such as two-membered rings and under-coordinated Si than the MS. We associate these structural differences to the production histories of the glasses and the mobility of the Na ions. It is also found that for Na-poor systems the fluctuations in composition and local atomic charge density decay with a power-law as a function of distance from the surface while Na-rich systems show an exponential decay with a typical decay length of $\approx2.3$~Å. The vibrational density of states shows that the presence of the surfaces leads to a decrease of the characteristic frequencies in the system. The two-membered rings give rise to a pronounce band at $\approx880$~cm$^{-1}$ which is in good agreement experimental observations.

cond-mat.dis-nn↗

arXiv2019

New interaction potentials for borate glasses with mixed network formers

We adapt and apply a recently developed optimization scheme used to obtain effective potentials for aluminosilicate glasses to include the network former boron into the interaction parameter set. As input data for the optimization, we used the radial distribution functions of the liquid at high temperature generated by ab initio molecular dynamics simulations, and density, coordination and elastic modulus of glass at room temperature from experiments. The new interaction potentials are shown to reproduce reliably the structure, coordination and mechanical properties over a wide range of compositions for binary alkali borates. Furthermore, the transferability of these new interaction parameters allows mixing to reliably reproduce properties of various boroaluminate and borosilicate glasses.

physics.comp-ph↗

arXiv2019

Vibrational properties of sodosilicate glasses from first-principles calculations

The vibrational properties of three sodosilicate glasses have been investigated in the framework of Density Functional Theory. The pure vibrational density of states has been calculated for all systems and the different vibrational modes have been assigned to specific atoms or structural units. It is shown that the Na content affects several vibrational features as the position and intensity of the $R$ band or the mixing of the rocking and bending atomic motions of the Si-O-Si bridges. The calculated Raman spectra have been found to agree with experimental observations and their decomposition indicated the dominant character of the non-bridging oxygen contribution on the spectra, in particular for the high-frequency band, above 800~cm$^{-1}$. The decomposition of the high-frequency Raman feature into vibrations of the depolymerized tetrahedra (i.e. $Q_n$-units) has revealed spectral shapes of the partial contributions that cannot be accounted for by simple gaussians as frequently assumed in the treatment of experimentally obtained Raman spectra.

cond-mat.mtrl-sci↗

arXiv2018

New interaction potentials for alkali and alkaline-earth aluminosilicate glasses

We apply a recently developed optimization scheme to obtain effective potentials for alkali and alkaline-earth aluminosilicate glasses that contains lithium, sodium, potassium, or calcium as modifiers. As input data for the optimization, we used the radial distribution functions of the liquid at high temperature generated by means of ab initio molecular dynamics simulations and density and elastic modulus of glass at room temperature from experiments. The new interaction potentials are able to reproduce reliably the structure and various mechanical and vibrational properties over a wide range of compositions for binary silicates. We have tested these potentials for various ternary systems and find that they are transferable and can be mixed, thus allowing to reproduce and predict the structure and properties of multi-component glasses.

physics.comp-ph↗

arXiv2018

New optimization scheme to obtain interaction potentials for oxide glasses

We propose a new scheme to parameterize effective potentials that can be used to simulate atomic systems such as oxide glasses. As input data for the optimization, we use the radial distribution functions of the liquid and the vibrational density of state of the glass, both obtained from ab-initio simulations, as well as experimental data on the pressure dependence of the density of the glass. For the case of silica, we find that this new scheme facilitates finding pair potentials that are significantly more accurate than previous ones even if the functional form is the same, thus demonstrating that even simple two-body potentials can be superior to more complex three-body potentials. We have tested the new potential by calculating the pressure dependence of the elastic moduli and find a good agreement with the corresponding experimental data.

cond-mat.mtrl-sci↗

arXiv2016

First-principles simulations of glass-formers

In this article we review results of computer simulation of glasses carried out using first principles approaches, notably density functional theory. We start with a brief introduction to this method and compare the pros and cons of this approach with the ones of simulations with classical potentials. This is followed by a discussion of simulation results of various glass-forming systems that have been obtained via ab initio simulations and that demonstrate the usefulness of this approach to understand the properties of glasses on the microscopic level.

cond-mat.dis-nn↗

arXiv2015

Effective interaction potential for amorphous silica from ab initio simulations

We discuss a novel approach that allows to obtain effective potentials from ab initio trajectories. Our method consists in fitting the weighted radial distribution functions obtained from the ab initio data with the ones obtained from simulations with the effective potential, and using the parameters of the latter as fitting variables. As a case study, we consider the example of amorphous silica, a material that is highly relevant in the field of glass science as well as in geology. Our approach is able to obtain an effective potential that gives a better description with respect to structural and thermodynamic properties than the potential proposed by van Beest, Kramer, and van Santen, and that has been very frequently used as a model for amorphous silica. In parallel, we have also used the so-called "force matching" approach proposed by Ercolessi and Adams to obtain an effective potential. We demonstrate that for the case of silica this method does not yield a reliable potential and discuss the likely origin for this failure.

cond-mat.dis-nn↗

arXiv2014

First principles study of a sodium borosilicate glass-former I: The liquid state

We use ab initio simulations to study the static and dynamic properties of a sodium borosilicate liquid with composition 3Na_2O-B_2O_3-6SiO_2, i.e. a system that is the basis of many glass-forming materials. In particular we focus on the question how boron is embedded into the local structure of the silicate network liquid. From the partial structure factors we conclude that there is a weak nanoscale phase separation between silicon and boron and that the sodium atoms form channel-like structures as they have been found in previous studies of sodo-silicate glass-formers. Our results for the X-ray and neutron structure factor show that this feature is basically unnoticeable in the former but should be visible in the latter as a small peak at small wave-vectors. At high temperatures we find a high concentration of three-fold coordinated boron atoms which decreases rapidly with decreasing T, whereas the number of four-fold coordinated boron atoms increases. Therefore we conclude that at the experimental glass transition temperature most boron atoms will be four-fold coordinated. We show that the transformation of [3]B into [4]B with decreasing T is not just related to the diminution of non-bridging oxygen atoms as claimed in previous studies, but to a restructuration of the silicate matrix. The diffusion constants of the various elements show an Arrhenius behavior and we find that the one for boron has the same value as the one of oxygen and is significantly larger than the one of silicon. This shows that these two network formers have rather different dynamical properties, a result that is also confirmed from the time dependence of the van Hove functions. Finally we show that the coherent intermediate scattering function for the sodium atoms is very different from the incoherent one and that it tracks the one of the matrix atoms.

cond-mat.dis-nn↗

arXiv2014

First principles study of a sodium borosilicate glass-former II: The glass state

We use ab initio simulations to investigate the properties of a sodium borosilicate glass of composition 3Na_2O-B_2O_3-6SiO_2. We find that the broadening of the first peak in the radial distribution functions g_BO(r) and g_BNa(r) is due to the presence of trigonal and tetrahedral boron units as well as to non-bridging oxygen atoms connected to BO_3 units. In agreement with experimental results we find that the [3]B units involve a significant number of non-bridging oxygens whereas the vast majority of [4]B have only bridging oxygens. We determine the three dimensional distribution of the Na atoms around the [3]B and [4]B units and use this information to explain why the sodium atoms associated to the latter share more oxygen atoms with the central boron atoms than the former units. From the distribution of the electrons we calculate the total electronic density of states as well its decomposition into angular momentum contributions. The vibrational density of states shows at high frequencies a band that originates from the motion of the boron atoms. Furthermore we show that the [3]B and [4]B units give rise to well defined features in the spectrum which thus can be used to estimate the concentration of these structural entities. The contribution of [3]B can be decomposed further into symmetric and asymmetric parts that can also be easily identified in the spectrum. We show that certain features in the spectrum can be used to obtain information on the type of atom that is the second nearest neighbor of a boron in the [4]B unit. We calculate the average Born charges on the bridging and non-bridging oxygen atoms and show that these depend linearly on the angle between the two bonds and the distance from the connected cation, respectively. Finally we have calculated the frequency dependence of the dielectric function as well as the absorption spectra.

cond-mat.dis-nn↗