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Proton dynamics in letovicite: Part I. Static ¹H and ¹⁵N NMR MAS experiments and lineshape simulations

¹ Institut für Geologie, Mineralogie und Geophysik der Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany
² Institut für Mineralogie der Universität Hannover, Welfengarten 1, 30167 Hannover, Germany
³ Mineralogisch-Petrographisches Institut der Universität Hamburg, Grindelallee 48, 20146 Hamburg, Germany

Correspondence: ^* E-mail: Michael.Fechtelkord{at}ruhr-uni-bochum.de

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 (_iso = 13.2 ppm, _aniso = 4.5 ppm, = 0.0) differ completely from those of the protons in the ferrophase (_iso = 14.1 ppm, _aniso = 8.5 ppm, = 1.0). The lowering of the chemical shift anisotropy _aniso by a factor of two and the change of the asymmetry parameter 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.

Key Words: Letovicite • proton conduction • ferroelastic • ¹H, ¹⁵N, chemical shift anisotropy • lineshape analysis • reorientation • MAS • solid state NMR spectroscopy • phase transition

This article has been cited by other articles:

				M. Fechtelkord, A. Diekmann, and U. Bismayer Proton dynamics in letovicite: Part II. Static 2H NMR experiments and lineshape simulations American Mineralogist, November 1, 2007; 92(11-12): 1827 - 1832. [Abstract] [Full Text] [PDF]