Molecular Dynamics Simulations of Borate Glasses

Structural and dynamic properties of ionic borate glasses, xM₂O-(1-x)B₂O₃ (M=Li,Cs, 0.2 x 0.5) are investigated by means of molecular dynamics simulations. This activity emanates from and completes the principal experimental research direction of the Materials Synthesis & Physical Chemistry group on inorganic glasses by vibrational and impedance spectroscopic techniques. We apply standard molecular dynamics techniques in collections of 1024 particles, interacting via the Born-Mayer-Huggins potential augmented with a three-body harmonic term to ensure the stability of the basic network structural units.

• STRUCTURAL PROPERTIES
• DYNAMIC PROPERTIES
• IONIC CONDUCTIVITY MECHANISM

STRUCTURAL PROPERTIES

Borate glasses constitute a relatively simple and computationally tractable model system, where the results derived from its study can be utilized in more complex glassy systems. Short range order (SRO) is nontrivial due to t he existence of three types of negatively charged borate structural units, tetrahedral BØ₄^- and triangular ΒØ₂Ο ^-, ΒØΟ₂^2- units which appear with two different boron coordination numbers (Ø denotes a bridging oxygen atom whereas O a non-bridging oxygen).

The structural properties relate to:

a) The detailed mapping of the SRO network structure: It was found that the SRO structure depends strongly on composition, temperature and nature of the alkali ion [1,2].
The trends concerning the dependence of SRO structure on composition and temperature are in close agreement with NMR and infrared structural investigations.
On the contrary, the differentiation of the SRO structure on the nature of the alkali ion is still a disputed issue in the literature [2].

b) Nature of cation hosting sites : it was found that long-lived distinct cation hosting sites are formed by: a) bridging oxygen atoms (b-type of sites), or b) bridging and non-bridging oxygen (NBO) atoms of the borate network (nb-type of sites).

       bridging(b-) type of site                                non-bridging( nb-) type of site

[1] Molecular dynamics investigation of lithium borate glasses: local structure and ion dynamics, C.P.E. Varsamis, A. Vegiri and E.I. Kamitsos, Phys. Rev. B 65, 104203 (2002).

[2] Composition and temperature dependence of cesium - borate glasses by molecular dynamics simulations, A. Vegiri, C.P.E. Varsamis and E.I. Kamitsos , J . Chem . Phys., in press (2005).

DYNAMIC PROPERTIES

Ions that reside predominantly, i.e. more than 75% of the total simulation time, in b-type sites were labelled M^b and those residing in nb-type sites as M ^nb. With this distinction, it was found that the structural heterogeneities are reflected on distinct dynamic responses of the two types of cations in both their short- and long-time dynamics [1].

a) short-time dynamics: the calculated vibrational density of states (VDOS) for cations M^b and M^nb lie in the far-infrared range and are well-separated, with the response of nb-type of ions being always at higher frequencies.

Top frames : experimental far-infrared absorption coefficient spectra for 0.3Li₂O-0.7B₂O₃ (left) and 0.3Cs₂O-0.7B₂O₃ glasses (right) and their deconvolution in two Gaussian component bands (L and H)..

Bottom frames : Calculated VDOS for M^b and M ^nb ions at room temperature for the same compositions.

This result provided a microscopic explanation for the origin of far-infrared spectral line-shapes of borate glasses.

b) long-time dynamics: The Mean Square Displacements (MSD) for ions M^b and M ^nb suggest that ions in nb-type sites are more mobile than those in b-type sites. This result suggests that ionic conductivity in borate glasses is mostly NBO-assisted.

IONIC CONDUCTIVITY MECHANISM

Cluster size distribution curves for NBO atoms in xLi₂O-(1-x)B₂O₃ glasses.
The shoulders correspond to the percolating clusters.

Due to the important role of NBO atoms in the diffusive properties of metal cations, we further investigate their spatial distribution in the glassy network. This is accomplished by calculating the NBO cluster distribution curves, which obey laws implying a percolative type of diffusion for cations in their vicinity. The curves follow an exponential decay for compositions below the percolation threshold (x=0.2 and 0.3 in the following Figure); this changes to a power law for higher alkali contents. This agrees with results for the Li ions [3].

[3] "Clustering and percolation in lithium borate glasses", A. Vegiri and C.P.E. Varsamis, J. Chem. Phys. 120, 7689 (2004).

A snapshot of a percolating cluster is shown below [1]. The formation of regions rich in NBO-Li and channels suitable for ion migration is in line with the Modified Random Network model.