Cation clustering and formation of free oxide ions in sodium and potassium lanthanum silicate glasses: nuclear magnetic resonance and Raman spectroscopic findings.
Alkali silicate glasses contg. lanthanum oxide are useful model systems for understanding the structural role of rare earth cations in optical and other types of materials. We report 29Si and Raman spectra of sodium and potassium silicate glasses, both with added La2O3 and with La2O3 substituted for Na2O or K2O on an equal-oxygen basis. In the former series, silicate speciation changes show the formation of more non-bridging oxygens (NBO) as more of the network-modifying La2O3 is added. In the latter series, however, in which the nominal ratio of NBO to Si is const., silicate speciation changes indicate that the actual ratio decreases significantly as La is substituted for 3 Na or K. The simplest explanation of this finding is that up to several percent of the oxygen in the La-rich glasses is not bonded to any Si, but instead forms 'free oxide' ions that are part of La-rich domains. Although the size of these domains remains unconstrained, the lack of evidence for phase sepn. and continuity of trends in structure with compn. suggest that the metastable liq. structure at the glass transition contains substantial intermediate-range heterogeneity.
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