Raman spectra and structure of fluoroaluminophosphate glasses

Fluoroaluminophosphate glasses are promising materials for rare earth doping, but their structure is not yet well known. The structural changes which take place in the 39AlF3–11NaF–10LiF–(40−x)(CaF2–MgF2–SrF2–BaF2)–xNaPO3 system, with increasing NaPO3 content (0–15 mol%), have been studied by polarised Raman and also infrared reflectance spectroscopies. The Raman spectra 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 into three Gaussian functions near 515, 570 and 620 cm−1, assigned to octahedral [AlF6], five-coordinated [AlF5] and tetrahedral [AlF4] structural units, respectively. The addition of NaPO3 appeared to promote the formation of [AlF5] units up to 3 mol% and to increase the conversion of [AlF4] and [AlF5] into [AlF6] octahedra, beyond 3 mol% of NaPO3. A picture has emerged for the structure of these ionic glasses, in which, for the low-phosphate content, the largest fraction of structural units are [AlF4] tetrahedra, containing both bridging and non-bridging fluorine, some of which are replaced by [AlF6] octahedra, surrounded mostly by non-bridging fluorine, as the NaPO3 content increases, bonded to short phosphate chains.