A MOLECULAR DYNAMICS STUDY OF Li-DOPED BORATE GLASSES

Abstract

The objective of this work is to investigate the effect of alkali content and temperature on the microstructure of lithium borate glasses, xLi 2 O · (1 − x)B 2 O 3 . We have applied the molecular dynamics technique with Ewald summation and periodic boundary conditions to a collection of ca. 256 particles confined within a primitive cubic cell and interacting through a BornMayer-Huggins type of potential augmented with three-body angular terms. The results of this study have been discussed in relation to experimental structural data obtained by NMR and infrared spectroscopies.

Composition and temperature dependence of cesium-borate glasses by molecular dynamics

“…24 The effect of temperature on structure is studied here by "preparing" simulated Cs borate glasses at 1250 K. In comparison with the MD data at 300 K ͑Fig. 2͒ it is clear that X 4 decreases with increasing temperature for all Cs borate compositions, in agreement with our earlier simulations of Li borate glasses 28,29 and neutron-scattering experiments on alkali diborate glasses. 24 The effect of temperature is particularly drastic for Cs contents x = 0.3 and 0.4, since for these compositions the BØ 4 − tetrahedra have been completely transformed into NBO-containing borate triangular units at 1250 K. The nature of these latter units will be discussed in the following section.…”

“…2 where they are compared with the theoretical curve x / ͑1−x͒, the room-temperature NMR data for Cs borate glasses, 16,25 and the X 4 data from the MD simulation of Li borate glasses. 28,29 At T = 300 K both experimental and MDderived X 4 values deviate from the theoretical curve, and this effect becomes more pronounced for the x = 0.3 and 0.4 Cs borate compositions, implying an increasing fraction of NBO-containing units at these higher Cs modification levels.…”

“…[28][29][30] The analysis of the MD data allowed the evaluation of the relative population of the local structural units as a function of lithium oxide content and temperature. In addition, the microstructure of sites hosting Li ions was studied, and the corresponding Li ion site vibrational response was computed and compared with experimental far-infrared spectra.…”

Clustering and percolation in lithium borate glasses

“…25,26 This ''two-site'' model 25 for metal ions was invoked in conductivity mechanisms for single [27][28][29] and mixed 30,31 alkali glasses. Furthermore, our recent molecular dynamics simulation in lithium borate glasses 20,21,32 corroborated the distinction in two types of Li cation hosting sites: the first type of site is formed by bridging oxygen ͑BO͒ atoms of the network ͑b site͒, whereas the second one involves also the participation of nonbridging oxygen ͑NBO͒ atoms ͑nb site͒. Accordingly, a Li cation was labeled Li b or Li nb if it was found to reside predominantly in b type or nb type of sites, respectively.…”

“…6 It is interesting to note that inhomogeneous transport dynamics involving the formation of favored pathways, or pathway clusters of cations suitable for ion migration, was also invoked in other models, like the diffusion pathway model, 7,8 the cluster model, [9][10][11] the cluster-bypass model, 12,13 and the dynamic structure model. 14,15 The formation of channels suitable for ion migration-i.e., percolating clusters of cations-was also visualized in previous molecular dynamics ͑MD͒ calculations of alkali silicate 16 -19 and borate 20,21 glasses and in reverse Monte Carlo structural models of silver ion conducting glasses and crystals. 22,23 Finally, the formation of clusters, which eventually percolate above a certain concentration, was deduced from the analysis of highfrequency dielectric spectra in Li-containing borate glasses.…”