Infrared absorption and structural investigation of fluorine phosphate and phosphate glasses

Abstract

Structural conclusions are obtained for various homologous series of the above-mentioned glasses by IR spectroscopy. Alterations in vibration frequencies of the fluorine phosphate tetrahedra as a function of chemical composition are interpreted and indicate, that fluorine ions are present in the glass network as POnF4-n units. P-F vibrations show the participation of fluorine in the phosphorus coordination with network forming character. Another remarkable influence of the fluorine ions is the delocalization of the P = O double bonding, which is typical and localized for pure phosphate glasses, and thus, only bridging and non-bridging oxygen atoms are left in the fluorine phosphate glasses. This delocalization is connected with an increase in frequency of all remaining P-O vibrations (stronger P-O bondings), which prevents the phosphorus atoms from carrying a higher charge. Neighbouring aluminum atoms get a chance for a four-fold coordination without the presence of alkali ions by a charge transfer from P5+ to Al3+ via the p-orbital of oxygen. From other frequency shifts is evident the network forming influence of Al3+, Mg3+ and the network modifying character of Na+, Ca2+ and Ba2+.