Unexpectedly high compressibility of Na6Al2Si6O18 glass with a large excess of Na2O relative to Al2O3 (Na2O/Al2O3=3) in the pressures range up to 6.0 GPa

Abstract The refractive index and the relative changes in the density (compressibility) for Na6Al2Si6O18 glass, with the ratio [Na2O]/([Na2O]+[Al2O3])=0.75, in the pressure range up to 6.0 GPa were obtained using a polarization-interference microscope, an apparatus with diamond anvils and the theory of photoelasticity. The experimentally obtained compressibility, (d-d0)/d, of Na6Al2Si6O18 glass is higher than one would expect from the glass with a large excess of Na2O relative to Al2O3 (Na2O/Al2O3=3). This phenomenon is discussed in the light of the quantitative mismatch of modifier cations and voids in the glass network and the existence of microheterogeneity due to the formation of microcrystalline sodium-rich areas with a low degree of polymerization. Highly polymerized regions, rich in silicon- and aluminum-containing structural units, are responsible for the compressibility of glass. The calculated degree of depolymerization in these regions, NBO/T = 0.28, corresponds to the experimentally obtained compressibility of Na6Al2Si6O18 glass.