VARIABLE-TEMPERATURE H-1, NA-23, AND SI-29 MAS NMR-STUDIES ON SODIUM-SILICATE HYDRATES OF COMPOSITION NA2O-CENTER-DOT-SIO2-CENTER-DOT-NH2O (N=9, 6, 5) - LOCAL-STRUCTURE IN CRYSTALS, MELTS, SUPERCOOLED MELTS, AND GLASSES

The local structure in crystals, melts, supercooled melts, and glasses of sodium silicate hydrates of composition Na2O . SiO2 . nH2O (n = 9, 6, 5) is studied by variable temperature H-1, Na-23, and Si-29 MAS NMR spectroscopy. Detailed in situ investigations on the melting process of the crystalline materials reveal the importance of H2O motion in the melting mechanism. Depending on the local coordination, crystallographically distinct Na sites show different behaviour during the melting process. Upon melting, the monomer silicate anions present in the crystalline hydrates undergo condensation reactions to oligomeric silicate anions. No recrystallization but glass formation occurs at low temperature if the melts were heated initially about 10 K above the melting point. In the glasses also oligomeric silicate anions are present with a preference for cyclotrimer species. In situ MAS NMR investigations and electric conductivity measurements of the melts, supercooled melts, and glasses suggest the distinction of three temperature ranges characterized by different local structure and dynamics of the sodium cations, water and silicate anions. These ranges comprise a glass and glass transition range A at low temperatures, an aggregation region B at intermediate temperatures, and a solution or electrolyte region C at high temperatures. In region B aggregation of sodium water complexes to hydrated polycation clusters is suggested, the dynamic behaviour of which is clearly different to that of the silicate anions, indicating that no long-lived contact ion pairs between sodium cations and silicate anions are formed.