RAMAN-SPECTROSCOPY STUDY OF THE STRUCTURE OF LITHIUM AND SODIUM ULTRAPHOSPHATE GLASSES

Anhydrous binary phosphate glasses containing from 0 to 50 mol% Li2O o r Na2C, have been prepared and examined by Raman scattering spectrosco py. The unpolarized Raman spectrum of vitreous P2O5 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 a t 1390 cm(-1) due to the symmetric stretch of the P=O terminal oxygens , (P=O)(sym). With the addition of alkali oxide to P2O5, a new feature appears in the Raman spectra near 1160 cm(-1) indicating the formatio n of Q(2) phosphate tetrahedra with two bridging and two non-bridging oxygens. The increase in relative amplitude of this new (PO2)(sym) ban d with increasing modifier content is consistent with a simple depolym erization of the phosphate network. From 20 to 50 mol% alkali oxide, t he position of the (P=O)(sym) Raman band decreases by similar to 130 c m(-1) whereas the freluency of the (POP)(sym) band increases by simila r to 60 cm(-1). These frequency shifts are the result of pi-bond deloc alization 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 pi-bond delocalizat ion on Q(3) tetrahedra is described by considering the interconnection s between neighboring Q(3) and Q(2) tetrahedra. The onset of pi-bond d elocalization on Q(3) species corresponds with the anomalous T-g minim um at 20 mol% alkali oxide in alkali ultraphosphate glasses. The incre ase in T-g between 20 and 50 mol% alkali oxide is attributed to the in creased ionic interconnection of what becomes a chain-like phosphate n etwork at higher alkali contents. Finally, the Raman spectra of severa l alkali ultraphosphate glasses show high frequency shoulders on the R aman bands attributed to the (PO2)(sym) and (PO2)(asym) vibrational mo des. These shoulders represent the presence of strained structural uni ts, possibly three- or four-membered rings. (C) 1998 Elsevier Science B.V.