The glass-forming region in the binary system SnF
2-P
2O
5 has been determined as the composition range 45-90 mol%SnF
2, and all of the glasses were clear, colourless, and durable. This wide region indicated that SnF
2 contributed to glass-forming as a network former (NWF). Glass transition temperature (
Tg), dilatometric softening temperature (
Tg), melting point (
Tm) of the glasses, and viscosity of the melts were measured. The variations in these properties as a function of SnF
2 content showed the maximum at 60SnF
2-40P
2O
5 as mixed composition, and Sn
4+/Sn
2+ in the glasses had the maximum value at this composition judging from the chemical shifts of SnL
β2 in the glasses. IR spectroscopic study of the glasses revealed that relative amount of P=O bond decreased and ν
3 vibration of PO
4 units became symmetric with increasing SnF
2 content. From these results, Sn in this system was assumed to be able to bridge between PO
4 units when SnF
2 changed to SnOF
2(Sn
4+) and interacted with P=O bonds in the glass. On the basis of these results, a structure model of Sn-P-O-F glass is proposed.
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