Deuteron NMR study of the diverse mobility of the ammonium ions in the ordered phase of (ND4)2PtCl4.

A detailed description of the diverse mobility of the ND4+ ions in the low-temperature ordered phase of (ND4)2PtCl4 is developed on the basis of single-crystal deuteron NMR spectra and site-selective T1 measurements. The ordered phase of (ND4)2PtCl4 consists of two kinds of domains in which the orientation of the ND4+ tetrahedra differs by a 90 degrees rotation about an axis which otherwise is a two-fold symmetry axis of the tetrahedra. Inside the domains, the ND4+ ions do not reorient at low temperatures. The domains are separated by domain walls which contain, according to the deuteron NMR spectra, about 10% of all ND4+ ions. These ions are highly mobile even at 10 K. On rising the temperature, the thickness of the domain walls increases, that is, the ions in more and more layers become mobile. Moreover, we provide evidence for fluctuations of the locations of the domain walls. The central resonance of the domain-wall ions shows a complicated structure below 36 K. On the basis of a tunnelling hypothesis we make an attempt to account for this structure. There are indications that the tunnelling process is incoherent.