Zt. Lalowicz et al., SPIN-LATTICE RELAXATION IN AMMONIUM-COMPOUNDS WITH A COMPLEX MOLECULAR-DYNAMICS, Solid state nuclear magnetic resonance, 8(2), 1997, pp. 89-107
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.