Crystallization of sodium molybdate-phosphate and tungstate-phosphate glasses

The glass to crystal transformation was studied for two glasses with composition 25Na2O·50MoO3.25P2O5 and 25Na2O·50WO3.25P2O5. During the heat treatment process, two crystalline compounds were formed, NaMoO2PO4 and NaWO2PO4, respectively. Physico-chemical properties of the corresponding glasses and crystals were compared, as well as their Raman and 31P MAS NMR spectra. Differential thermal analysis revealed crystallization peaks at 483 °C for the Mo-containing glass and at 624 °C for the W-containing glass; melting points of crystalline compounds were determined as 695 ± 2 °C for NaMoO2PO4 and 858 ± 2 °C for NaMoO2PO4. Sodium tungstate-phosphate glass has lower solubility in comparison with the sodium molybdate-phosphate glass. The study of the crystallization mechanism showed a prevailing surface nucleation. 31P MAS NMR spectra of the two glasses revealed the same shape and almost the same width, which reflects a similar local environment for P in these glasses. The position of the resonance for W-containing glass at a more negative chemical shift than the Mo-containing one is attributed to the different electrical field strength of W atoms than Mo atoms. 31P MAS NMR spectra of crystals revealed two resonances at +17 and − 0.5 ppm for the Mo glass and at −0.1 and − 1.9 ppm for the W glass, reflecting the presence of two different phosphorus short range environments, in agreement with their crystal structure. Raman spectra showed similarity in structural features for Mo and W glasses and their isomorphic compounds. Splitting of the dominant Raman bands in the crystal spectra reflects a distortion of MoO6 and WO6 octahedra. Raman spectra also suggested the breaking of Mo-O-Mo and W-O-W linkages during crystallization.