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

Abstract

The structural aspects of xCs 2 O-͑1−x͒B 2 O 3 glasses have been investigated by molecular dynamics as functions of Cs 2 O content ͑x = 0.2, 0.3, and 0.4͒ and temperature ͑T = 300 and 1250 K͒. The tetrahedral ͑BØ 4 − ͒ and triangular ͑BØ 3 ,BØ 2 O − , and BØO 2 2− ͒ short-range order borate units were found to be the structure-building entities of the simulated glasses ͓Ø=bridging oxygen ͑BO͒ and O − =nonbridging oxygen ͑NBO͒ atom͔. The increase of Cs 2 O content results in the progressive increase of the NBO-containing triangle population at the expense of the BØ 4 − tetrahedral units. The same effect is caused by temperature increase at a fixed Cs 2 O content, and this was associated with the "fragile" characteristics of alkali borate glasses. A comparison of simulated Cs and Li borates showed very similar structures at x = 0.2, but dissimilar ones when the alkali content exceeds this composition. In particular, for x Ͼ 0.2 Cs borates exhibit a preference for NBO formation relative to Li borates. Differences in the microstructure of sites hosting Cs ions were found, and this permits their classification into bridging ͑b type͒ and nonbridging type ͑nb type͒ of sites. b-type sites consist exclusively of BO atoms, while both BO and NBO atoms participate in nb-type sites. These differences in Cs-site local bonding characteristics were found to be reflected on the Cs-O͑site͒ vibration frequencies. Also, the computed Cs-O vibrational responses for simulated Cs borates were found to compare well with experimental far-infrared spectra.

Probing alkali coordination environments in alkali borate glasses by multinuclear magnetic resonance

“…Heavier alkali ions appear to stabilize non-bridging oxygens, whereas lighter alkalis -particularly lithium -appear to stabilize tetrahedral boron. These results are in qualitative agreement with recent molecular dynamics simulations of cesium and lithium borate glasses, which reveal smaller numbers of NBO-bearing borons for lithium borates [38,39]. One possible explanation for this observation is related to charge balance in high-alkali glasses, where linkages between anionic tetrahedral borons become unavoidable in the absence of non-bridging oxygens.…”

“…Likely due to all of these challenges with 87 Rb MAS NMR, this appears to be the first report of rubidium NMR on an oxide glass. However, like 39 K, direct spectral comparison shows a clear peak shift to higher frequencies with increased rubidium loading ( Table 2).…”

“…The heavier alkali cations (K, Rb, Cs) have also traditionally been considered 'difficult' nuclei to probe because of their unfavourable NMR characteristics. 39 K has a very low magnetogyric ratio, c, and hence suffers from low sensitivity and practical acquisition problems [19]. 87 Rb can have very large quadrupole couplings for low-symmetry sites, resulting in extremely broad peaks, if observable.…”

Boron speciation and non-bridging oxygens in high-alkali borate glasses

“…Triple-quantum filtered calculations were done using SIMPSON [12] on a 3.1 GHz PIV. 30 mol% the lighter and heavier alkali cations generate different ratios of charge-balancing anionic units [13][14][15]. Lithium borate glasses possess a higher fraction of fourcoordinate boron than do cesium borate glasses at a given composition, and show excellent agreement with previous 10 B wideline NMR results of Feller et al [16] for the lithium borate glasses (Table 1).…”

“…The speciation estimates given in Table 1 are subject to assumptions about which species are likely to be present at a given composition. Recent molecular dynamics simulations of lithium and cesium borate glasses confirm this preference for [4] B sites, and reveal the presence of T 1 sites at compositions as low as 30 mol% Cs 2 O and 40 mol% Li 2 O [14,15]. Although these studies are known to underestimate N 4 , due to the higher fictive temperatures of the simulations relative to experimental quenching conditions, the relative proportions of different NBO-bearing borons (T 2 , T 1 and T 0 ) are thought to be reliable.…”