The effect of network-modifying cations on the structure and disorder in peralkaline Ca–Na aluminosilicate glasses: O-17 3QMAS NMR study

Abstract

We report the detailed atomic configurations around the oxygens in peralkaline Ca–Na aluminosilicate glasses {[(CaO) 1 − x (Na 2O) x ] 1.5(Al 2O 3) 0.5 (SiO 2) 6 with varying Na/Ca ratios [X Na2O = Na 2O/(Na 2O + CaO)], CNAS} using 2D 17O 3QMAS (triple-quantum magic angle spinning) NMR at 9.4 and 14.1 T. The high-resolution O-17 NMR spectra reveals peaks due to several distinct bridging oxygens (BO, Si–O–Al and Si–O–Si) and three types of non-bridging oxygens (NBO, Ca-NBO, Na-NBO, and mixed {Ca,Na}–NBO). The local environment around the oxygens provides previously unknown details of the extent of chemical and topological disorders in quaternary silicate glasses. The chemical shielding of the NBO peak increases with the Ca contents (chemical shift decreases), and a significant fraction of the mixed-NBO peak is observed in intermediate compositions, suggesting a considerable extent of mixing between Ca and Na around NBO. While the framework disorder due to the mixing of [4]Si and [4]Al and degree of polymerization (i.e., fraction of NBO) is almost invariant with the Ca content, topological disorders (and thus, the topological entropy) in Si–O–Si and Si–O–Al due to bond angle and bond length distributions in the CNAS (CaO–Na 2O–Al 2O 3–SiO 2) glasses increases with the Ca content, indicating that the non-framework cations influence the framework topology of BOs (particularly, with Si–O–Al) and NBOs. The increase in the topological disorder in Si–O–Al with the Ca content is non-linear in that the degree of disorder abruptly increases near the Ca-end member. This trend, together with the non-linear variation of O-17 isotropic chemical shift of Si–O–Al, implies that Na + may have proximity to Si–O–Al at intermediate compositions; thus, Na + plays a preferential role as a charge-balancing cation, while Ca 2+ can act as a network-modifying cation in the glass composition studied here (non-randomness in the partitioning of non-framework cations between NBO and Si–O–Al), manifesting a deviation from the randomness in the complex silicate glasses and melts.