Structural, elastic, electronic, optical and vibrational properties of single-layered, bilayered and bulk molybdenite MoS2-2H

In recent years, transition metal dichalcogenides have received great attention since they can be prepared as two-dimensional semiconductors, presenting heterodesmic structures incorporating strong in-plane covalent bonds and weak out-of-plane interactions, with an easy cleavage/exfoliation in single or multiple layers. In this context, molybdenite, the mineralogical name of molybdenum disulfide, MoS2, has drawn much attention because of its very promising physical properties for optoelectronic applications, in particular a band gap that can be tailored with the material's thickness, optical absorption in the visible region and strong light-matter interactions due to the planar exciton confinement effect. Despite this wide interest and the numerous experimental and theoretical articles in the literature, these report on just one or two specific features of bulk and layered MoS2 and sometimes provide conflicting results. For these reasons, presented here is a thorough theoretical analysis of the different aspects of bulk, monolayer and bilayer MoS2 within the density functional theory (DFT) framework and with the DFT-D3 correction to account for long-range interactions. The crystal chemistry, stiffness, and electronic, dielectric/optical and phonon properties of single-layered, bilayered and bulk molybdenite have been investigated, to obtain a consistent and detailed set of data and to assess the variations and cross correlation from the bulk to single- and double-layer units. The simulations show the indirect-direct transition of the band gap (K-K' in the first Brillouin zone) from the bulk to the single-layer structure, which however reverts to an indirect transition when a bilayer is considered. In general, the optical properties are in good agreement with previous experimental measurements using spectroscopic ellipsometry and reflectivity, and with preliminary theoretical simulations

The effect of long-range interactions on the infrared and Raman spectra of aragonite (CaCO3, Pmcn) up to 25 GPa

Long-range interactions are relevant in the physical description of materials, even for those where other stronger bonds give the leading contributions. In this work, we demonstrate this assertion by simulating the infrared and Raman spectra of aragonite, an important calcium carbonate polymorph (space group Pmcn) in geological, biological and materials science fields. To this aim, we used Density Functional Theory methods and two corrections to include long-range interactions (DFT-D2 and DFT-D3). The results were correlated to IR spectroscopy and confocal Raman spectrometry data, finding a very good agreement between theory and experiments. Furthermore, the evolution of the IR/Raman modes up to 25 GPa was described in terms of mode-Grüneisen's parameters, which are useful for geological and materials science applications of aragonite. Our findings clearly show that weak interactions are of utmost importance when modelling minerals and materials, even when they are not the predominant forces

Monte Carlo strategy for SEM-EDS micro-nanoanalysis of geopolymer composites

The development, optimization and application of new geopolymer composite materials must necessarily go through a precise and accurate physico-chemical and mineralogical characterization down to the micro and nanoscale. In this regard, SEM-EDS X-ray microanalysis is widely and successfully employed by the scientific community and industry. However, the nano-to-micrometre sized architecture of many geopolymer composites introduces many difficulties and issues in SEM-EDS quantification, with potential large sources of error that should carefully be taken into account and investigated. In this work, a SEM-EDS Monte Carlo approach is proposed to study the complex physical phenomena at the basis of the quantification issues and errors, through the investigation of: (i) a not completely reacted sodium-poly(sialate-siloxo) geopolymer, and (ii) a geopolymer composite with a potassium-poly(sialate-siloxo) matrix and basalt-derived glass fibres reinforcement. The Monte Carlo simulation evinced a strong influence of the nano-microsized specimen architecture (e.g., basalt fibre size and shape, different elemental composition between fibre and matrix) on the measured X-ray intensity, with contributions also depending on the SEM electron beam energy. The proposed approach provided fundamental indications for selecting optimal operative conditions depending on the type of geopolymer sample, shape, size with the specific SEM-EDS setup and silicon drift EDS detector here used

Ab initio quantum mechanical investigation of structural and chemical-physical properties of selected minerals for minero-petrological, structural ceramic and biomaterial applications

The purpose of this thesis is the atomic-scale simulation of the crystal-chemical and physical (phonon, energetic) properties of some strategically important minerals for structural ceramics, biomedical and petrological applications. These properties affect the thermodynamic stability and rule the mineral-environment interface phenomena, with important economical, (bio)technological, petrological and environmental implications. The minerals of interest belong to the family of phyllosilicates (talc, pyrophyllite and muscovite) and apatite (OHAp), chosen for their importance in industrial and biomedical applications (structural ceramics) and petrophysics. In this thesis work we have applicated quantum mechanics methods, formulas and knowledge to the resolution of mineralogical problems ("Quantum Mineralogy”). The chosen theoretical approach is the Density Functional Theory (DFT), along with periodic boundary conditions to limit the portion of the mineral in analysis to the crystallographic cell and the hybrid functional B3LYP. The crystalline orbitals were simulated by linear combination of Gaussian functions (GTO). The dispersive forces, which are important for the structural determination of phyllosilicates and not properly con-sidered in pure DFT method, have been included by means of a semi-empirical correction. The phonon and the mechanical properties were also calculated. The equation of state, both in athermal conditions and in a wide temperature range, has been obtained by means of variations in the volume of the cell and quasi-harmonic approximation. Some thermo-chemical properties of the minerals (isochoric and isobaric thermal capacity) were calculated, because of their considerable applicative importance. For the first time three-dimensional charts related to these properties at different pressures and temperatures were provided. The hydroxylapatite has been studied from the standpoint of structural and phonon properties for its biotechnological role. In fact, biological apatite represents the inorganic phase of vertebrate hard tissues. Numerous carbonated (hydroxyl)apatite structures were modelled by QM to cover the broadest spectrum of possible biological structural variations to fulfil bioceramics applications.Scopo della presente tesi di dottorato è la simulazione su scala atomica delle proprietà cristallochimiche e fisiche di alcuni minerali di importanza strategica per applicazioni ceramiche strutturali, biomediche e petrologiche. Tali proprietà influenzano le caratteristiche di stabilità termodinamica e guidano fenomeni all’interfaccia minerale-ambiente, con importanti ricadute economiche, (bio)tecnologiche, petrologiche e ambientali. I minerali di interesse appartengono alla famiglia dei fillosilicati (talco, pirofillite e muscovite) e delle apatiti (idrossiapatite), scelti per la loro importanza in ambito industriale, biomedico e petrofisico. In questo lavoro di tesi abbiamo applicato metodi, formule e conoscenze della meccanica quantistica a problemi di natura mineralogica (“Mineralogica Quantistica”). L’approccio teorico scelto è la Density Fuctional Theory (DFT), adoperata insieme a condizioni periodiche al contorno per limitare la porzione di minerale in analisi alla sola cella cristallografica e al funzionale ibrido B3LYP. Gli orbitali cristallini sono stati simulati mediante una combinazione lineare di funzioni gaussiane (GTO). Le forze dispersive, importanti per la determinazione strutturale dei fillosilicati e non propriamente considerate dal metodo DFT puro, sono state incluse mediante una correzione semi-empirica. Inoltre, sono state calcolate le proprietà fononiche e meccaniche. L’equazione di stato, sia in condizioni atermiche, sia in un ampio intervallo di temperature, è stata ricavata mediante variazioni dei volumi di cella e approssimazione quasi-armonica. Alcune proprietà termo-chimiche dei minerali (capacità termiche isocora e isobara) sono state calcolate, in quanto di notevole importanza in ambito applicativo. Per la prima volta sono forniti grafici tridimensionali relativi a queste proprietà a diverse pressioni e temperature. L’idrossiapatite è stata studiata dal punto di vista strutturale e fononico per il ruolo ricoperto dal minerale in ambito biotecnologico. Infatti, l’apatite biologica rappresenta la fase inorganica dei tessuti duri degli organismi vertebrati. Sono stati realizzati numerosi modelli di (idrossi)apatite carbonatata per coprire il più ampio spettro di possibili variazioni strutturali biologiche per applicazioni bioceramiche

Simulated infrared and Raman spectroscopy, complex dielectric function and refractive index dataset of monoclinic C2/m stoichiometric clinochlore Mg6Si4O10(OH)8 as obtained from Density Functional Theory

This article reports a simulated dataset of the vibrational (infrared and Raman) and optical properties (complex dielectric function and refractive index) of clinochlore, an important mineral belonging to the phyllosilicate family [1]. The data here reported were calculated from ab initio Density Functional Theory (DFT) simulations at B3LYP level, including a correction for the dispersive forces (B3LYP-D* approach) and all-electron Gaussian-type orbitals basis sets. This dataset was calculated between 0 cm–1 and 4000 cm–1 and comprises infrared, reflectance and Raman spectra, frequency-dependent complex dielectric function and complex refractive index of clinochlore. The data was validated against available experimental spectroscopic results reported in literature and can be of help in several application fields, for instance fundamental georesource exploration and exploitation, in applied mineralogy, geology, material science, and as a reference to assess the quality of other theoretical approaches

Monte Carlo SEM-EDS Nano-Microanalysis Strategy of Historical Mineral Pigments: The Simulation of the Egyptian Blue from Pompeii (Italy) as an Example

A correct determination of the mineral and chemical composition of specimens is of the utmost importance to answer questions regarding the Cultural Heritage field. Because of the preciousness and often very low quantity of sample available, with textures and sizes in the nano-to-micrometric range, scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) is one of the most suited and exploited nano-microanalytical techniques. In these cases, to avoid severe mistakes and quantification errors in SEM-EDS, it is mandatory to consider several effects related to the transport of electrons and X-rays in the material, which in turn are dependent on the SEM-EDS setup. In the present work, a Monte Carlo SEM-EDS nano-microanalytical simulation strategy is proposed and applied to a practical selected case. The Egyptian blue mineral pigment, which is found in Pompeian murals, is used here as an example and model system to show the effects of real size variations (0.1–10 µm), basic geometrical shapes of the pigment (prismatic and spherical) and typical SEM setups, sample holders and substrates. The simulations showed a great—sometimes not intuitive—dependence of the X-ray intensity on the thickness and shape of the samples and SEM-EDS parameters, thereby influencing the analysis and quantification. The critical overview of the results allowed the determination of the correct procedure and technical SEM-EDS parameters and indicated how to apply the Monte Carlo simulation strategy to other Cultural Heritage cases

Crystal-chemical and structural data related to the equation of state and second-order elastic constants of portlandite Ca(OH)2 and brucite Mg(OH)2

This data article reports crystal-chemical and structural data (unit cell parameters and internal coordinates) of two hydroxyl minerals, namely brucite [magnesium hydroxide, Mg(OH)2] and portlandite [calcium hydroxide, Ca(OH)2], which were calculated and employed to derive the mechanical behavior of these solid phases under hydrostatic compression (Ulian and Valdrè, 2018). The dataset has been obtained by ab initio quantum mechanical means, by employing Density Functional Theory methods, in particular the B3LYP hybrid functional, all-electron Gaussian-type orbitals basis sets and a correction to take into account the effects of dispersive forces. Equilibrium and expanded/compressed models of both minerals were realized and geometrically optimized within two space group settings, P3¯m1 and P3¯

SEISMIC, a Python-based code of the Quantas package to calculate the phase and group acoustic velocities in crystals

The present work reports the theoretical background and the capabilities of SEISMIC, a Python code specifically developed to calculate the propagation of the sound waves inside crystalline materials. SEISMIC is a tool integrated in the QUANTAS package and provides a series of useful information for engineers and geophysicists, such as the phase and group velocities, the power flow angle, the enhancement factors, and the polarization vectors, using as input the elastic moduli and the density of the material. Numerical treatments of the derivatives were avoided, using analytical methods to obtain numerically stable results. The code relies only on Python numerical and graphical libraries to ensure a full cross-platform usability

Anisotropy and directional elastic behavior data obtained from the second-order elastic constants of portlandite Ca(OH)2 and brucite Mg(OH)2

This article reports data on the anisotropy and directional elastic behavior, namely Young׳s modulus E, linear compressibility β, shear modulus μ, Poisson׳s ratio ν and wave velocities Vs1, Vs2 and Vp, of brucite (magnesium hydroxide, Mg(OH)2) and portlandite (calcium hydroxide, Ca(OH)2), calculated from their second order elastic constants at different hydrostatic compressions (Ulian and Valdrè, in press). The dataset has been obtained by ab initio quantum mechanical means, by employing density functional theory methods, in particular the B3LYP hybrid functional, all-electron Gaussian-type orbitals basis sets and a correction to take into account the effects of dispersive forces

Density functional investigation of the thermophysical and thermochemical properties of talc [Mg3Si4O10(OH)2]

The knowledge of the P, T behavior of talc is very important in mineralogical–petrological and geophysical research fields because talc can be considered a hydrous phase that can recycle water into the Earth’s mantle and also an important mineral in both industrial and technological applications. However, very few works have been presented to fully characterize the thermodynamic properties of this mineral, especially at atomic scale. In a previous work, we modeled the structural and mechanical properties of talc using the B3LYP-D* hybrid density functional, which included a correction for the dispersive forces and all-electron Gaussian-type orbital basis sets. The results were in good agreement with single-crystal X-ray and neutron diffraction experimental data. Here, we extend the investigation to the thermochemical and thermophysical properties of talc using the same density functional approach and the quasi-harmonic approximation, providing the thermal equation of state, the heat capacity and the entropy of the mineral at different P, T conditions