Network structure of sodium aluminoborophosphate glasses from solid-state NMR spectroscopy and Pauling bond strengths

Abstract

The intermixing of Al, B and P structural units produces a very complex network in aluminoborophosphate glasses. We use Pauling bond strengths to assess the likelihood of different connectivities inferred from 11B, 31P, 27Al and 23Na MAS NMR spectra. Aluminophosphate glasses exhibit dimers and branched phosphates bonded principally to [5/6]Al and [4]Al units, respectively. Borophosphate glasses possess a high fraction of [4]BO[4]B which are stabilized by P5+ due their capacity to contribute variable bond strengths to underbonded bridging oxygens. The addition of B to aluminophosphate glasses forces partial conversion of [6]Al to [4]Al, but retains a stable and significant [6]Al fraction to charge-compensate underbonded bridging oxygens. Increasing the B/Al ratio increases the number of POP linkages, which bond to [4]B(Al) units and stabilize [4]BO[4]B linkages. This work demonstrates the value of Pauling bond strengths to aid in understanding speciation and connectivity in multicomponent glasses.