SPIN-LATTICE RELAXATION IN AMMONIUM-COMPOUNDS WITH A COMPLEX MOLECULAR-DYNAMICS

Expressions are derived for the initial relaxation rate 1/T-1 of proto ns and deuterons of nontunnelling NH4 and ND4 groups reorienting about various symmetry axes in solids. The reorientation rates are modified by a trigonal, tetragonal or monoclinic distortion of the predominant ly cubic hindering potential. When the rates differ sufficiently from each other, two T-1 minima are observed with a characteristic ratio. E xperiments were performed in NH4VO3, (NH4)(2)S2O8, (NH4)(2)PtCl4, and their deuterated modifications, which all exhibit two T-1 minima. In N H4VO3 and ND4VO3 the relaxation and spectral data agree rather well wi th the model of trigonal distortion. Also (NH4)(2)S2O8 has a preferred threefold axis but there, the large tunnel splitting of protons has t o be taken into account before an agreement is reached. All the purely reorientational models fail with (NH4)(2)PtCl4, where, instead, the a mmonium groups are proposed to be ordered into domains at low temperat ures. The groups inside the domains and boundary regions give rise to the high- and low-temperature T-1 minima, respectively. The boundaries are also believed to give rise to the narrow component in the deutero n spectrum at low temperatures. Evidence for a proton tunnelling frequ ency of 32 MHz is found in (NH4)(2)PtCl4. (C) 1997 Elsevier Science B. V.