Characterizing Pressure-Induced Coordination Changes in CaAl2O4 Glass Using 27Al NMR

Al-27 NMR is used to quantify coordination changes in CaAl2O4 glass pressure cycled to 16 GPa. The structure and coordination environments remain unchanged up to 8 GPa, at which 93% of the recovered glass exists as four-fold Al, whereas the remaining population exists as Al-[5,Al-6]. Upon densification, Al-[5'6] comprises nearly 30% of observed Al, most likely through the generation of three-coordinated oxygen. Negligible changes are observed for the isotropic chemical shift (Al-[4] approximate to 78.8 ppm, Al-[5] approximate to 46.3 ppm, Al-[6] approximate to 12.6 ppm) and average quadrupole coupling strengths with pressure. 3QMAS spectra suggest less distortion within the newly formed Al environments in comparison with Al-[4]. The chemical shift of Al-[4] suggests that the aluminate network remains fully polymerized in glasses quenched from pressures up to 16 GPa.