Vibrational spectra and structure of H+(H2O)nβ- alumina

Single crystals and polycrystalline samples of β-alumina containing hydrated proton H+(H2O)n have been studied by infrared and, to some extent, by Raman spectroscopy. The spectra of fully hydrated and more of less deyhdrated samples and their deuterated derivatives have been compared to those of Na+ β-alumina and of mixed crystals with different M+/H+ ratios (M=Na, K, Ag). It can be shown that the samples heated above 400 °C contain H3O+ ions occupying Beevers–Ross sites as dominant species; midoxygen sites with single and double occupancy are less populated. The spectroscopic data are consistent with nearly nonhydrogen-bonded H3O+ ions having a slightly distorted C3v pyramidal structure. The fully hydrated β-alumina, on the other hand, appears to contain also higher species such as H5O2+ and H7O3+. The former is believed to have an unsymmetrical structure, H3O+H2O, with terminal groups hydrogen bonded to the oxygen atoms of spinel blocks. The latter is characterized by OH stretching bands similar to those of H7O3+ClO4−.