Inference of Principal Species in Caustic Aluminate Solutions Through Solid-State Spectroscopic Characterization

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Tetrahedrally coordinated aluminate Al(OH)4- and dialuminate Al2O(OH)62- anions are considered to be major species in aluminum-rich alkaline solutions. However, their relative abundance remains difficult to spectroscopically quantify due local structure similarities and poorly understood effects arising from extent of polymerization and counter-cations. To help unravel these relationships here we report detailed characterization of three solid-phase analogues as structurally and compositionally well-defined reference materials. We successfully synthesized a cesium salt of the aluminate monomer, CsAl(OH)4ยท2H2O, for comparison to potassium and rubidium salts of the aluminate dimer, K2Al2O(OH)6, Rb2Al2O(OH)6. Single crystal and powder X-ray diffraction methods clearly reveal the structure and purity of these materials for which a combination of 27Al MAS-NMR, Al K-edge X-ray absorption and Raman/IR spectroscopies was then used to fingerprint the two major tetrahedrally coordinated Al species. The resulting insights into the effect of Al-O-Al bridge formation, between aluminate tetrahedra on spectroscopic features may also be more general to the many materials that are based on this motif.