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

Abstract

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 C_3v 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⁻.