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1.
Infrared spectra in amorphous alumina
Luigi Giacomazzi, Nikita S Shcheblanov, Mikhail E Povarnitsyn, Yanbo Li, Andraž Mavrič, Barbara Zupančič, Jože Grdadolnik, Alfredo Pasquarello, complete scientific database of research data

Abstract: We present a combined study based on experimental measurements of infrared (IR) dielectric function and first-principles calculations of IR spectra and vibrational density of states (VDOS) of amorphous alumina (am-Al₂O₃). In particular, we show that the main features of the imaginary part of the dielectric function ε₂(ω) at ~380 and 630 cm-¹ are related to the motions of threefold coordinated oxygen atoms, which are the vast majority of oxygen atoms in am-Al₂O₃. Our analysis (involving three model structures) provides an alternative point of view with respect to an earlier suggested assignment of the vibrational modes, which relates them to the stretching and bending vibrational modes of AlOₙ (n = 4, 5, and 6) polyhedra. Our assignment is based on the additive decomposition of the VDOS and ε₂(ω) spectra, which shows that: (i) the band at ~380 cm-¹ features oxygen motions occurring in a direction normal to the plane defined by the three nearest-neighbor aluminum atoms, i.e. out-of-plane motions of oxygen atoms; (ii) Al-O stretching vibrations (i.e. in-plane motions of oxygen atoms) appear at frequencies above ~500 cm-¹, which characterize the vibrational modes underlying the band at ~630 cm-¹.
Keywords: amorphous alumina, infrared spectra, first-principles calculations
Published in RUNG: 15.09.2023; Views: 606; Downloads: 7
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2.
Infrared spectra in amorphous alumina : a combined ab initio and experimental study
Luigi Giacomazzi, Nikita S. Shcheblanov, Mikhail E. Povarnitsyn, Yanbo Li, Andraž Mavrič, Barbara Zupančič, Jože Grdadolnik, Alfredo Pasquarello, 2023, original scientific article

Abstract: We present a combined study based on the experimental measurements of an infrared (IR) dielectric function and first-principles calculations of IR spectra and the vibrational density of states (VDOS) of amorphous alumina (am−Al2O3). In particular, we show that the main features of the imaginary part of the dielectric function ε2(ω) at ∼380 and 630 cm−1 are related to the motions of threefold-coordinated oxygen atoms, which are the vast majority of oxygen atoms in am-Al2O3. Our analysis provides an alternative point of view with respect to an earlier suggested assignment of the vibrational modes, which relates them to the stretching and bending vibrational modes of AlOn (n=4, 5, and 6) polyhedra. Our assignment is based on the additive decomposition of the VDOS and ε2(ω) spectra, which shows that (i) the band at ∼380cm−1 features oxygen motions occurring in a direction normal to the plane defined by the three nearest-neighbor aluminum atoms, i.e., out-of-plane motions of oxygen atoms; (ii) Al-O stretching vibrations (i.e., in-plane motions of oxygen atoms) appear at frequencies above ∼500cm−1, which characterize the vibrational modes underlying the band at ∼630cm−1. Aluminum and fourfold-coordinated oxygen atoms contribute uniformly to the VDOS and ε2(ω) spectra in the frequency region ∼350–650 cm−1 without causing specific features. Our numerical results are in good agreement with the previous and presently obtained experimental data on the IR dielectric function of am−Al2O3 films. Finally, we show that the IR spectrum can be modeled successfully by assuming isotropic Born charges for aluminum atoms and fourfold-coordinated oxygen atoms, while requiring the use of three parameters, defined in a local reference frame, for the anisotropic Born charges of threefold-coordinated oxygen atoms.
Keywords: dielectric properties, microstructure, amorphous materials, density functional calculations, infrared techniques, aluminium oxide
Published in RUNG: 10.05.2023; Views: 1049; Downloads: 6
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3.
4.
Unravelling the P[sub]3 centers origin in alkali phosphate glasses
Luigi Giacomazzi, Nikita S. Shcheblanov, Layla Martin-Samos, Matjaž Valant, Nadege Ollier, Nicolas Richard, 2021, published scientific conference contribution abstract

Keywords: EPR, DFT, NaPO3, LiPO3, alkali phosphate glasses, point defects
Published in RUNG: 29.09.2021; Views: 1682; Downloads: 7
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5.
Unravelling the P[sub]3 centers origin in alkali phosphate glasses
Luigi Giacomazzi, Nikita S. Shcheblanov, Layla Martin-Samos, Matjaž Valant, Nadege Ollier, Nicolas Richard, 2021, published scientific conference contribution abstract

Keywords: EPR, DFT, NaPO3, LiPO3, alkali phosphate glasses, point defects
Published in RUNG: 29.09.2021; Views: 1703; Downloads: 0
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6.
Vibrational spectra of am-Al 2 O 3 : tuning a parametric model. : Written report: in fulfillment of Diploma Seminar 1FAF29 requirements
Kurtović Jasmin, 2021, research project (high school)

Abstract: The present diploma seminar work has been dedicated first to the implementation of a procedure to calculate the vibrational density of states of two structural models of vitreous SiO 2 (v- SiO 2 ) and amorphous Al 2 O 3 (am-Al 2 O 3 ) models, and next to tune a parametric model for the calculation of the infrared (IR) spectra of am-Al 2 O 3 , in particular of the imaginary part of the dielectric function. The ground state of both structural models is obtained by relaxing the atomic structure by using the conjugate gradient method as implemented in the LAMMPS code. Vibrational frequencies and modes are obtained, in the harmonic approximation, by diagonalizing the dynamical matrices calculated for the given v-SiO 2 and am-Al 2 O 3 structural models. Dynamical matrices are obtained through a finite differences approach and vibrational density of states are plotted by applying Gaussian broadening. The calculation of the dielectric function requires the knowledge of the vibrational frequencies as well as the knowledge of the dynamical (or Born) charge tensors related to the atoms of the am-Al 2 O 3 structural model. For the latter model, a parametrization of the ab-initio Born charge tensors has been carried out with the purpose to allow for the fast calculation of the IR spectrum of any other am- Al 2 O 3 model without the need to calculate for it the Born charge tensors using expensive ab- initio methods. The parametrization of Born charge tensors takes into account, for aluminium atoms, only of the isotropic charge which depends on coordination number and average Al- O bond length of aluminium atoms, while for oxygen atoms coordinated to three Al atoms (75%), dynamical charges are parametrized also by the area bounded by aluminium atoms nearest neighbours of the 3-coordinated oxygen atom. Moreover for analyzing the dynamical charge tensors of 3-coordinated oxygen atoms a decomposition in terms of the representations of the spatial rotations was used. The IR spectrum obtained by means of the above described parametrization provides a good approximation to the IR spectrum obtained by using the ab- initio calculated dynamical charges, as it differs from it, on average, by around 5.1% which is much better (12%) than using a average isotropic charge model (i.e. diagonal Born charge tensors where each diagonal element is one third of the average isotropic charge).
Keywords: Vibrational spectra, am-Al 2 O 3, parametric model
Published in RUNG: 15.09.2021; Views: 1815; Downloads: 0
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7.
8.
Vers une connaissance microscopique de la structure de verres de phosphate P2O5
Nicolas Richard, Luigi Giacomazzi, Nikita S. Shcheblanov, Layla Martin-Samos, Nadege Ollier, 2020, popular article

Abstract: Les propriétés d’amplification de la lumière des ions terres rares permettent en les incluant comme dopants dans le cœur des fibres optiques d’obtenir des sources laser et des amplificateurs de lumière compacts. Pour réduire la tendance à l’agrégation des terres rares qui diminuent leurs propriétés amplificatrices, le phosphore est employé comme co-dopant dans ces fibres. Cet article montre comment les méthodes de simulation à l’échelle atomique peuvent aider a' l'optimisation de ce co-dopage en contribuant a' une meilleure comprehension de la structure des verres phosphates [1] et des défauts qui leur sont liés [2]. Au CEA-DAM, ces resultats serviront aussi dans le cadre d'etudes sur le developpement de fibres optiques pour des mesures de dosimetrie radiative.
Keywords: glasses, P2O5, point defects, ab-initio
Published in RUNG: 18.09.2020; Views: 2721; Downloads: 0
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9.
Collective dipole effects in ionic transport under electric fields
Matjaž Valant, Salles Nicolas, Giacomazzi Luigi, 2020, original scientific article

Keywords: ionska prevodnost, dipoli, kolektivni vplivi
Published in RUNG: 03.07.2020; Views: 2665; Downloads: 115
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10.
v-P[sub]2O[sub]5 "micro-clusters" in P-doped silica studied by a first principles Raman investigation
Luigi Giacomazzi, Layla Martin-Samos, Matjaž Valant, Nicolas Richard, 2019, published scientific conference contribution abstract

Keywords: r-POHC, silica, Raman, first-principles
Published in RUNG: 19.12.2019; Views: 3049; Downloads: 0
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