Proton dynamics in letovicite, (NH4)(3)H(SO4)(2): A H-1 and N-14 NMR spectroscopic study

The improper ferroelastic phase letovicite (NH4)(3)H(SO4)(2) has been studi ed by H-1 MAS NMR as well as by static N-14 NMR experiments in the temperat ure range of 296-425 K. The H-1 MAS NMR resonance from ammonium protons can be well distinguished from that of acidic protons. A third resonance appea rs just below the phase transition temperature which is due to the acidic p rotons in the paraelastic phase. The lowering of the second moment MZ for t he ammonium protons takes place in the same temperature range as the format ion of domain boundaries, while the signals of the acidic protons suffer a line narrowing in the area of T-c. The static N-14 NMR spectra confirm the temperature of the motional changes of the ammonium tetrahedra. Two-dimensi onal 1H NOESY spectra indicate a chemical exchange between ammonium protons and the acidic protons of the paraphase. (C) 2000 Academic Press.