Abstract

Synthetic letovicite (NH4)3H(SO4)2 has been investigated using 1H static, low-speed MAS, and 15N MAS NMR spectroscopy. Experiments were carried out in the temperature range of 215–425 K. The 1H MAS NMR spectra show three different resonances. The resonance assigned to the ammonia protons is broad and spinning sidebands cannot be resolved in the low-speed MAS NMR spectra. On the other hand, the acidic protons in the ferro- and paraphase show narrow signals with sideband patterns that enable a chemical shift anisotropy analysis. The chemical shift parameters of the free protons in the paraphase (δiso = 13.2 ppm, δaniso = 4.5 ppm, η = 0.0) differ completely from those of the protons in the ferrophase (δiso = 14.1 ppm, δaniso = 8.5 ppm, η = 1.0). The lowering of the chemical shift anisotropy δaniso by a factor of two and the change of the asymmetry parameter η imply a tetrahedral site jump mechanism of the protons. Three different ammonia tetrahedra can be distinguished by 15N MAS NMR spectroscopy in the P2/n phase below 273 K. Two resonances are prominent for the ferrophase (space group C2/c) corresponding to the two different crystallographic sites. Both resonances move together into a single resonance in the high-temperature phase that can be interpreted as fast dynamics of ammonia groups and its local environment so that the two crystallographic sites are locally nearly equal.

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