Molecular dynamics investigation of lithium borate glasses: Local structure and ion dynamics

Abstract

The effect of alkali content and temperature on the microstructure of lithium borate glasses, xLi 2 O-(1 Ϫx)B 2 O 3 , has been investigated for glass compositions xϭ0.2-0.5 and temperatures up to 1250 K. The molecular dynamics technique has been applied, with Ewald summation and periodic boundary conditions, to a collection of ca. 256 particles confined in a primitive cubic cell and interacting through a Born-Mayer-Huggins-type potential augmented with three-body angular terms. The short-range order ͑SRO͒ structure was found to consist of boron-oxygen tetrahedral, BO " 4 Ϫ ͓O " ϭbridging oxygen atom ͑BO͔͒, and triangular units with variable number of nonbridging oxygen ͑NBO͒ atoms, BO " 3 , BO " 2 O Ϫ , and BO " O 2 2Ϫ (O Ϫ ϭNBO). The relative abundance of SRO units was determined and found to depend on both glass composition and temperature. Increasing Li 2 O content at constant temperature or increasing temperature at a fixed composition was shown to cause rearrangements of the SRO structure and to lead towards BO " O 2 2Ϫ units in the range of compositions and temperatures investigated. Such changes were expressed in terms of chemical equilibria involving the SRO units. The local environments hosting the Li ions were investigated and distinguished in two main types: the first type of site is formed by BO's, while the second type involves the participation of NBO's. The vibrational response of Li ions in the two types of site was computed and found to correlate very well with the experimental far-infrared profiles. Calculation of diffusion coefficients of Li ions showed that diffusion is carried out predominantly through NBO sites. In addition, glass regions rich in Li/NBO were found to develop with increasing lithium oxide content and to percolate eventually into microchannels suitable for ion migration.

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.…”

“…Considering the important role of NBOs in metal ion network bonding and transport properties, 29,45,46 it is of interest to examine the nature of NBO-containing structural …”

“…1 and are compared with corresponding RDFs obtained previously for Li borate glasses. 29 This first peak in the g B-O ͑R͒ arises from the nearestneighbor B-O correlations and should be taken as resulting from the overlapping contributions of B-O distances in SRO units, i.e., borate triangles ͑BØ 3 ,BØ 2 O − , and BØO 2 2− ͒ and…”

“…[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.…”