Raman spectroscopy study of the structure of lithium and sodium ultraphosphate glasses

Abstract

Anhydrous binary phosphate glasses containing from 0 to 50 mol% Li 2O or Na 2O have been prepared and examined by Raman scattering spectroscopy. The unpolarized Raman spectrum of vitreous P 2O 5 has intense bands near 640 cm −1, attributed to the symmetric stretching mode of POP bridging oxygens, (POP) sym, between Q 3 phosphate tetrahedra, and at 1390 cm −1 due to the symmetric stretch of the PO terminal oxygens, (PO) sym. With the addition of alkali oxide to P 2O 5, a new feature appears in the Raman spectra near 1160 cm −1 indicating the formation of Q 2 phosphate tetrahedra with two bridging and two non-bridging oxygens. The increase in relative amplitude of this new (PO 2) sym band with increasing modifier content is consistent with a simple depolymerization of the phosphate network. From 20 to 50 mol% alkali oxide, the position of the (PO) sym Raman band decreases by ∼ 130 cm −1 whereas the frequency of the (POP) sym band increases by ∼ 60 cm −1. These frequency shifts are the result of π-bond delocalization on Q 3 species that effectively lengthens the PO terminal oxygen bond and strengthens the POP linkages with increasing alkali oxide content. The compositional dependence of the π-bond delocalization on Q 3 tetrahedra is described by considering the interconnections between neighboring Q 3 and Q 2 tetrahedra. The onset of π-bond delocalization on Q 3 species corresponds with the anomalous T g minimum at 20 mol% alkali oxide in alkali ultraphosphate glasses. The increase in T g between 20 and 50 mol% alkali oxide is attributed to the increased ionic interconnection of what becomes a chain-like phosphate network at higher alkali contents. Finally, the Raman spectra of several alkali ultraphosphate glasses show high frequency shoulders on the Raman bands attributed to the (PO 2) sym and (PO 2) asym vibrational modes. These shoulders represent the presence of strained structural units, possibly three- or four-membered rings.