Raman spectra and structure of fluoro aluminophosphate glasses

Fluoroaluminophosphate glasses are promising materials for rare earth dopin g, but their structure is not yet well known. The structural changes which take place in the 39AlF(3)-11 NaF-10LiF-(40 - x)(CaF2-MgF2- SrF2-BaF2)-xNaP O(3) system, with increasing NaPO3 content (0-15 mol%), have been studied b y polarised Raman and also infrared reflectance spectroscopies. The Raman s pectra of the glasses, at low-phosphate content, presented a polarised band peaking near 520 cm(-1), with a shoulder on the high-frequency side, which was the band with the largest amplitude. This band could be deconvoluted i nto three Gaussian functions near 515, 570 and 620 cm(-1), assigned to octa hedral [AIF(6)], five-coordinated [AIF(5)] and tetrahedral [AIF(4)] structu ral units, respectively. The addition of NaPO3 appeared to promote the form ation of [AIF(5)] units up to 3 mol% and to increase the conversion of [AIF (4)] and [AIF(5)] into [AIF(6)] octahedra, beyond 3 mol% of NaPO3. A pictur e has emerged for the structure of these ionic glasses, in which, for the l ow-phosphate content, the largest fraction of structural units are [AIF(4)] tetrahedra, containing both bridging and non-bridging fluorine, some of wh ich are replaced by [AIF(6)] octahedra, surrounded mostly by nonbridging fl uorine, as the NaPO3 content increases, bonded to short phosphate chains. ( C) 2001 Elsevier Science B.V. All rights reserved.