Glass formation and structure in the MgSiO3-Mg2SiO4 pseudobinary system: From degraded networks to ioniclike glasses

A series of glasses xMgO-(1-x)SiO2 with compositions from enstatite MgSiO3 (x=0.5) to forsterite Mg2SiO4 (x=0.667) in mole fraction intervals of x approximate to 0.02 have been prepared by containerless levitation techniques and CO2 laser heating. Polarized and depolarized Raman spectra measured at ambient conditions for all these glasses show systematic and smooth band intensity changes with composition. Analysis of the Raman band contours in terms of vibrations due to different oxygen bridged SiO4 tetrahedra (Q(i), species analysis) undoubtedly shows that bridging oxygens are present in all glasses studied even in the limit of the forsterite composition where bridged Si2O76- ionic dimers are formed. Furthermore the relative amounts of the Q(i) species change smoothly with composition while at high MgO content free oxygens are present presumably forming Mg-O-Mg bridges, which contribute to the glass stability at these compositions. Raman spectra measurements at different temperature below T-g show small alterations in the Q(i) species in the MgSiO3 region while no changes were observed in the Mg2SiO4 region. The Boson peak frequency is practically invariant on both composition and temperature and this is in contrast to the systematics followed by most silicate glasses. It is suggested that at compositions near the forsterite ioniclike glasses are formed arising from a very fragile liquid. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3225431]