Modification of silicophosphate glass composition, structure, and properties via crucible material and melting conditions

Ceramic crucibles are known to corrode in contact with glass melts. Here, we investigate the effect of alumina and fused silica crucibles on the composition, structure, and properties of silicophosphate glasses. Glasses in the system 0.3 Na 2 O‐0.6 P 2 O 5 ‐0.1 SiO 2 were melted in platinum, alumina, or fused silica crucibles at 900°C or 1200°C for 0.5‐12 hours. Al 2 O 3 and SiO 2 were found to leach from the crucibles into the glass melt and alter the glass composition: Al 2 O 3 content increased with melting temperature and time, resulting in up to 10 mol% Al 2 O 3 ; SiO 2 from fused silica crucibles was also introduced into the glass, resulting in a 25% higher SiO 2 content compared to the nominal composition. Glass density, transition temperature, thermal expansion, and mechanical properties were strongly affected by these compositional changes. Based on vibrational spectroscopy, this is explained by increasing numbers of P–O–Al or P–O–Si bonds, resulting in a depolymerization of the phosphate network, and ionic cross‐linking by high field strength aluminum or silicon ions. With increasing alumina content, P–O–Si bonds were replaced by P–O–Al bonds. 31 P and 27 Al MAS NMR spectra revealed that aluminum is present in sixfold coordination exclusively and fully bonded to phosphate species, connecting phosphate groups by P–O–Al–O–P bonds.