Solid state Si-29 and P-31 NMR study of gel derived phosphosilicate glasses

Solid state Si-29 and P-31 MAS NMR have been used to investigate the micros tructural changes occurring in phosphosilicate gels during their conversion from a gel to the corresponding gel-derived glasses by heating. The studie d gels have the molar compositions 10P(2)O(5). 90SiO(2) and 30P(2)O(5). 70S iO(2). It was found that the dried gels (100 degreesC) have very similar st ructures formed by a siloxane framework containing silanol groups and inclu ding trapped molecules of orthophosphoric acid along with a very small amou nt of pyrophosphoric acid. In spite of this initial similarity further heat ing causes markedly different structural rearrangements of their glassy mat rices. Namely, the co-polymerisation of phosphate and silicate tetrahedra t akes place at 300 degreesC for the gel with the higher phosphorus content w hereas this occurs only after heat treatment for 30 minutes at 400 degreesC for the gel with the lower phosphorus content. Moreover, the presence of s ix-coordinated silicon in the glassy matrix of the 30P(2)O(5). 70SiO(2) gel has been observed. The different evolution of the investigated gel microst ructures mirrors their different crystallization behaviour: 10P(2)O(5). 90S iO(2) keeps its amorphous nature up to 1000 degreesC while for 30P(2)O(5). 70SiO(2) crystallization starts after heat treatment for 30 minutes at 400 degreesC.