Direct detection of Al-O-Al structure in aluminosilicate specimens: A use of homo-nuclear DQMAS NMR

A general strategy of Al-O-Al structure in various aluminosilicate was evaluated by combining triple-quantum magic angle spinning (3QMAS) and double-quantum homo-nuclear correlation under magic angle spinning (DQMAS) solid-state nuclear magnetic resonance (NMR) measurements with the aid of high magnetic field NMR (800 MHz for ¹H Larmor frequency). The results show that in many cases the direct detection of Al-O-Al sites in aluminosilicate crystals and glasses is possible; hence the extent of aluminum avoidance can be directly elucidated. Specifically, experimental evidence of Al-O-Al linkages in several aluminosilicate materials with Si/Al [1 was straightforwardly confirmed; and the existence of Al-O-Al is considered to have little correlation with the Si/Al ratio, but it may be strongly related to the cation and local structural arrangement. In addition, the presence of tri-clusters of (Si, Al)O₄-tetrahedra in aluminosilicate framework was proposed, which was thought to act as nuclei for formation and incorporation of cations to achieve charge neutrality.