Local Structure and Tunneling Kinetics of Ammonium Aluminum Alum by Infrared Hole-Burning

The title compound, crystallized with a few percent of deuterium, contains some NH3D+ and HOD. At low temperatures, five N−D stretch bands are observed. These belong to two ammonium sites with apparent C s symmetry in the low-temperature phase. The N−D bands can be hole-burned with an infrared laser. Burning bands belonging to the A-sites transforms some of them to the higher-energy B-sites. The A- and B-sites probably differ from each other by the arrangement of water and sulfate about the ammonium.