Medium-range structure of vitreous SiO2 obtained through first-principles investigation of vibrational spectra

Using a density-functional framework, we investigate the vibrational spectra of vitreous SiO2 to determine to what extent these spectra provide information about the medium-range structure of the oxide network. We carry out a comparative study involving three model structures, which all feature a nondefective network of corner-sharing tetrahedra but differ through their Si-O-Si bond-angle distributions and ring statistics. We first address the results of typical diffraction probes. Fair agreement with experiment is achieved for the total neutron and total x-ray structure factors of all models, indicating limited sensitivity of these structure factors to the medium-range structure. The same consideration also applies to the Si-O and O-O partial structure factors. At variance, the Si-Si partial structure factor is found to be highly sensitive to the Si-O-Si bond-angle distribution. We then address typical vibrational spectra, such as the inelastic neutron spectrum, the infrared spectra, and the Raman spectra. For the inelastic neutron spectrum and the infrared spectra, the comparison with experiment is fair for all models, indicating poor sensitivity to the structural arrangement of tetrahedra. The only noticeable exception is the feature at similar to 800 cm(-1) which shifts to higher frequencies with decreasing Si-O-Si angles. At variance, the Raman spectra are shown to be very informative about the medium-range organization of the network through their sensitivity to the concentrations of three-membered and four-membered rings. Our study indicates that the considered experimental data are globally consistent with a medium-range structure characterized by an average Si-O-Si bond angle of 148 degrees and with small-ring concentrations as derived from the intensities of the experimental Raman defect lines. To describe the infrared and Raman couplings, our work also introduces parametric models which reproduce well the spectra calculated from first principles.

First-principles investigation of the structural and vibrational properties of vitreous GeSe2

Luigi Giacomazzi, Alfredo Pasquarello

Using a density-functional scheme, we study the structural and vibrational properties of vitreous germanium diselenide (v-GeSe2). Through the use of classical and first-principles molecular-dynamics methods, we generate a set of structural models showing a varying degree of chemical disorder. In particular, two types of structural concepts are represented: one in which the tetrahedral order is preserved to a very large extent, and one which reproduces the high degree of disorder in first-neighbor shells found in first-principles molecular-dynamics simulations of the liquid. The investigated structural properties include the angular distributions, the atomic arrangements in the first-neighbor shells, and the pair-correlation functions. In reciprocal space, we have calculated the x-ray and neutron total structure factors and the partial structure factors. Comparison with experiment gives overall good agreement for the models of either structural conception. We then investigate the vibrational properties via the vibrational density of states and the inelastic neutron spectrum. The considered models yield similar spectra and agree with experimental data. We also obtain infrared and Raman spectra through a density-functional scheme based on the application of finite electric fields. For these spectra, significant differences appear among the models. The comparison with experiment favors a model showing a high degree of chemical order. The Raman intensity is analyzed in terms of the underlying atomic vibrations. The assignment of the Raman companion line to Se motions in edge-sharing tetrahedra is supported.

2007

First principles vibrational spectra of tetrahedrally-bonded glasses

Luigi Giacomazzi

Disordered oxides, such as vitreous silica (v-SiO2) and vitreous germania, (v-GeO2) and chalcogenide glasses, such as vitreous germanium diselenide (v-GeSe2) are currently key materials in many technological fields, ranging from Si-based microelectronic to optical fibers and information storage devices production. Thus an increasing interest is nowadays devoted to understanding the structural arrangements of atoms in these materials. Diffraction probes like x-ray and neutron diffraction applied to amorphous materials can only partially characterize their atomic structure, since these materials lack translational symmetry. A complementary class of experimental techniques is constituted by vibrational spectroscopies, since the latter are also sensitive to the underlying structure. Yet, extracting structural information from the associated experimental data is rather difficult and above all it requires an accurate theoretical modeling of the material under investigation. This thesis is dedicated to the first-principles investigation of vibrational spectra of the tetrahedrally bonded glasses v-SiO2, v-GeO2 and v-GeSe2. To this aim, we make use of methods based on density functional theory which allow us to calculate vibrational spectra for a set of selected model structures for each one of these materials. We take advantage of a recent development which consists in applying a finite electric field under the constraint of periodic boundary conditions to calculate coupling factors, and hence to derive the infrared and Raman spectra. Through a comparison with experimental data of several vibrational spectroscopies, and by taking into account the structural differences of our models, we interpret features of the considered vibrational spectra. The extracted structural information represents a refinement of our knowledge of the investigated materials.

EPFL2007

Vibrational spectra of vitreous germania from first-principles

Luigi Giacomazzi, Alfredo Pasquarello

We report on a first-principles investigation of the structural and vibrational properties of vitreous germania (v-GeO2). Our work focuses on a periodic model structure of 168 atoms, but three smaller models are also studied for comparison. We first carry out a detailed structural analysis both in real and reciprocal spaces. Our study comprises the partial pair correlation functions, the angular distributions, the total neutron correlation function, the neutron and x-ray total structure factors, and the Faber-Ziman and Bhatia-Thornthon partial structure factors. We find overall good agreement with available experimental data. We then obtain the vibrational frequencies and eigenmodes. We analyze the vibrational density of states in terms of Ge and O motions, and further in terms of rocking, bending, and stretching contributions. The inelastic neutron spectrum is found to differ only marginally from the vibrational density of states. Using a methodology based on the application of finite electric fields, we derive dynamical Born charge tensors and Raman coupling tensors. For the infrared spectra, we calculate the real and imaginary parts of the dielectric function, including the high-frequency and static dielectric constants. The Raman spectra are shown to be sensitive to the medium-range structure and support an average Ge-O-Ge angle of 135 degrees. We identify the shoulder X-2 as a signature of breathing O vibrations in three-membered rings. Four-membered rings are found to contribute to the main Raman peak. We advance an interpretation for the shoulder X-1 in terms of delocalized bond-bending modes. We derive bond polarizability parameters from the calculated Raman coupling tensors and demonstrate their level of reliability in reproducing the spectra. The calculated vibrational spectra all show good agreement with the respective experimental spectra.

2006