Congruent dissolution of Al2O3 in a K-disilicate melt, with constraints on the reaction mechanism

High resolution O 1s X-ray Photoelectron Spectroscopy (XPS) was employed to determine oxygen speciation in 31 mol% K ₂O silicate glass containing 0.0, 1.0 and 3.0 mol% dissolved Al ₂O ₃. Highly resolved bridging oxygen (BO) and non-bridging oxygen (NBO) peaks were observed. The NBO peak intensity decreased and the BO increased with dissolution of Al ₂O ₃. The enhanced BO intensity is localized at two binding energies (BE), one characteristic of a Si-O-Si moiety (e.g., quartz, or v-SiO ₂), the other of a Si-O-Al moiety (e.g., kyanite, Al ₂SiO ₅). The dissolution reaction proceeds in two steps; NBO ⁻ of a Q ³ species attacks and bonds to an under-coordinated surface Al atom of crystalline Al ₂O ₃. The reaction produces a Si-O-Al surface species thus converting the attacking NBO to a BO and altering the reactant Q ³ species to a Q ⁴ product species: [Q ³] ⁻ _melt + [AlO _1.5] _surface → [Q ⁴] ⁰ _melt + [AlO ₂] ⁻ _melt The second step involves reaction of [AlO ₂] ⁻ _melt with K ⁺converting it from a modifier cation to a charge compensator, resulting in the polymerization reaction: [K-Q ³] ⁰ _melt + [AlO ₂] ⁻ _melt → [Q ⁴] ⁰ + [KAlO ₂] ⁰ _melt + 1/2O ²⁻ The stoichiometric reaction is the sum of the two: [Q ³] ⁻ _melt + [K-Q ³] ⁰ _melt + [AlO _1.5] _surface → 2[Q ⁴] ⁰ + [KAlO ₂] ⁰ _melt + 1/2O ²⁻ Al species detached from the solid surface initially must be bonded to some NBO. Within the melt, nucleophilic substitution attack of the Al-bearing species by Q ³ species converts NBO to BO, resulting in [KAlO ₂] _melt being bonded four BOs. This reaction may not go to completion in all melts, depending on dissolved Al content and melt composition.