Neutron Spectroscopic Study of Synthetic Alunite and Oxonium-substituted Alunite

Synthetic, polycrystalline samples of alunite [K0.88(H3O)0.12Al2.64(SO4)2(OH)4.92(H2O)1.08] and oxonium-substituted alunite [H3OAl2.87(SO4)2(OH)5.61(H2O)0.39] have been investigated using incoherent, inelastic neutron-scattering (IINS) methods in order to determine the nature of the non-OH “H2O”. IINS measurements were made on non-deuterated samples at 20 K using the HRMECS chopper spectrometer with 250 and 600 meV incident energies. Alunite and oxonium-substituted alunite exhibit similar spectral features over the energy range of 120–550 meV, where assignments for local vibrations can be made based on the presence of OH and H2O groups. Salient differences in the intensities and positions of the observed low-energy vibrational bands reflect the effects of chemical substitution on the structural environment of the monovalent cation site and neighboring Al and OH sites in the framework.