The 29Si MASNMR spectra of P2O5-SiO 2 binary glasses were measured before and after annealing to examine the local structure around Si atoms in the glasses. The glasses were composed of SiO6 octahedra and Q4 (SiO4 tetrahedra with four bridging oxygens). By increasing the P2O5 content, the fractions of SiO6 octahedra increased and those of Q4 decreased. The fractions of SiO6 octahedra increased by 6-8% in the range of 29.7 to 47.4 mol % P2O5 by annealing for 5 h. This result was found for the first time in binary glasses. On the basis of the results obtained, the local structure around Si atoms in the glasses and the structural change by annealing are discussed.
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