Crystallization of sodium molybdate-phosphate and tungstate-phosphate glasses

Abstract The glass to crystal transformation was studied for two glasses with composition 25Na 2 O·50MoO 3 .25P 2 O 5 and 25Na 2 O·50WO 3 .25P 2 O 5 . During the heat treatment process, two crystalline compounds were formed, NaMoO 2 PO 4 and NaWO 2 PO 4 , respectively. Physico-chemical properties of the corresponding glasses and crystals were compared, as well as their Raman and 31 P 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 NaMoO 2 PO 4 and 858 ± 2 °C for NaMoO 2 PO 4 . 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. 31 P 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. 31 P 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 MoO 6 and WO 6 octahedra. Raman spectra also suggested the breaking of Mo-O-Mo and W-O-W linkages during crystallization.