NEUTRON SPECTROSCOPIC STUDY OF SYNTHETIC ALUNITE AND OXONIUM-SUBSTITUTED ALUNITE

Abstract

Synthetic, polycrystalline samples of alunite [K 0.88 (H 3 O) 0.12 Al 2.64 (SO 4 ) 2 (OH) 4.92 (H 2 O) 1.08 ] and oxonium-substituted alunite [H 3 OAl 2.87 (SO 4 ) 2 (OH) 5.61 (H 2 O) 0.39 ] have been investigated using incoherent, inelastic neutron-scattering (IINS) methods in order to determine the nature of the non-OH “H 2 O”. 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 H 2 O 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.