Raman and 11B Nuclear Magnetic Resonance Spectroscopic Studies of Alkaline-Earth Lanthanoborate Glasses
Glasses from the RO·La2O3·B2O3 (R = Mg, Ca, and Ba) systems have been examined. Glass formation is centered along the metaborate tie line, from La(BO2)3 to R(BO2)2. Glasses generally have transition temperatures >600°C and expansion coefficients between 60 × 10-7/°C and 100 × 10-7/°C. Raman and solid-state nuclear magnetic resonance spectroscopies reveal changes in the metaborate network that depend on both the [R]:[La] ratio and the type of alkaline-earth ion. The fraction of tetrahedral sites is generally reduced in alkaline-earth-rich glasses, with magnesium glasses possessing the lowest concentration of B. Raman spectra indicate that, with increasing [R]:[La] ratio, the preferred metaborate anion changes from a double-chain structure associated with crystalline La(BO2)3 to the single-chain and ring metaborate anions found in crystalline R(BO2)2 phases. In addition, disproportionation of the metaborate anions leads to the formation of a variety of other species, including pyroborates with terminal oxygens and more-polymerized species, such as diborates, with tetrahedral borons. Such structural changes are related to the ease of glass formation and some of the glass properties.
R. K. Brow et al., "Raman and 11B Nuclear Magnetic Resonance Spectroscopic Studies of Alkaline-Earth Lanthanoborate Glasses," Journal of the American Ceramic Society, American Chemical Society (ACS), Jan 1996.
The definitive version is available at http://dx.doi.org/10.1111/j.1151-2916.1996.tb08990.x
Materials Science and Engineering
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