Estimating oxygen tricluster populations from NMR experiments on aluminosilicate glasses

Oxygen sites coordinating three network-forming cations in aluminosilicate (AS) glasses—i.e., O [ 3 ] coordination species referred to as “oxygen triclusters”—was proposed 60 years ago and has since then been heavily debated because irrefutable experimental proof for their existence is still lacking. We introduce a protocol for predicting the O [ 3 ] population from the average coordination number of Al ( Z ¯ Al ) and the fraction of non-bridging O (NBO) species, which are available from routine 27 Al and 17 O NMR experiments, respectively. Although O [ 3 ] species were proposed for balancing the negatively charged [AlO 4 / 2 ] − groups that typically dominate the Al speciation in AS glasses, recent computational modeling reveals a stronger propensity for the higher-coordination Al [ 5 ] (AlO 5 ) and Al [ 6 ] (AlO 6 ) species to coordinate O triclusters for charge-compensation. We discuss the structural roles of the O [ 3 ] and Al [ 5 ] / Al [ 6 ] entities, all of which remain unclear despite extensive previous research. For a large set of AS glasses involving various M + , M 2 + , and M 3 + cations, the fractional O [ 3 ] population ( x O [ 3 ] ) increases concurrently with the net number of Al [ 5 ] and Al [ 6 ] sites in the glass, but decreases for increasing M z + cation potential. Both O [ 3 ] and Al [ 5 ] /Al [ 6 ] species have been suggested to enhance the glass hardness and crack resistance. For AS glasses involving various trivalent rare-earth cations, the microhardness correlated better with the average coordination number of Al ( Z ¯ Al ) than with x O [ 3 ] , as did the crack resistance of a limited set of Ca AS glasses. Nevertheless, contributions from O [ 3 ] sites cannot be excluded for the latter glasses and further studies are needed for firm conclusions.