Vibrational properties of a sodium tetrasilicate glass: Ab initio versus classical force fields

Abstract

We have determined the vibrational properties of a sodium tetrasilicate (Na 2 Si 4 O 9 ) glass model generated by molecular dynamics simulations. The study has been carried out using a classical valence force fields approach as well as an ab initio approach in the framework of the density functional theory. The total and partial vibrational densities of states (VDOS) are presented, as well as some characteristics of the vibrational modes (participation ratios, correlation lengths). For the low-frequency bands below 500 cm −1 , we find that the shapes of the two calculated VDOS as well as those of their corresponding partial VDOS are quite similar. For the intermediate-and high-frequency ranges, we observe larger discrepancies between the two calculations. Using the eigenmodes of the dynamical matrix we also calculate the polarized Raman spectra within the bond-polarizability approximation. We find an overall agreement between the calculated parallel polarized (VV) Raman spectra and the corresponding experimental spectrum. Regarding the perpendicular depolarized (VH) Raman spectrum, the comparison of the calculated spectra to the experimental data indicates a need for an adjustment of the VH bond-polarizability parameters.

Structural and vibrational properties of a calcium aluminosilicate glass: classical force-fields vs. first-principles

“…At low frequency, the contribution of the Ca atoms is also important (between 50 and 400 cm −1 and also at 600 cm −1 ) and resembles the one found for Na in sodo-silicate in Ref. (20). Finally the high frequency peak is due to bending and stretching modes involving mainly Si, BO and NBO atoms.…”

“…11e). For this species, a main low frequency band is observed around 150 cm −1 for all samples, which has a similar origin as the low-frequency band observed in the Na partial VDOS of the Na 2 O-4SiO 2 glass (20). This band has a tail toward high frequencies which, for the "full" CP sample, vanishes at ≈ 400 cm −1 .…”

“…The main features of these VDOS can be found in a large variety of silicate glasses, such as the sodo-silicate ones. In particular, the VDOS obtained from very similar simulations carried out by Ispas et al on a Na 2 O-4SiO 2 glass presented the same features at the same frequencies (20). Only the gap between the 700 cm −1 band and the other ones seem to be less pronounced in the CAS glass than in the sodo-silicate one.…”

“…In contrast to this, all the EP+CP samples (100-atoms and 200-atoms) show an additional peak around ≈ 600-700 cm −1 . These relatively high frequency peak for the Ca atoms is quite intriguing since no similar features were found in the Na 2 O-4SiO 2 glass (20).…”

Rationalizing the Biodegradation of Glasses for Biomedical Applications Through Classical and Ab-initio Simulations

“…An hybrid approach involves using classical MD to produce an initial structure for a further AIMD refinement or other quantummechanical calculations such as electronic, vibrational and NMR spectra [52][53][54][55]. It is important to note that while the AIMD refinement can introduce limited local changes to the bonding environments (e.g., interatomic distances and angles) created through classical MD [55][56][57], substantial medium-range rearrangements to the glass structure (involving for instance different silicate network topology, connectivity and Q n and ring size distribution) would require overcoming significant energy barriers and are not normally observed over the relatively short AIMD time scale. In other words, the AIMD simulation will conserve the main medium-range structural features determined by the classical potential.…”