Abstract:
A ~si MAS NMR study of spin-lattice relaxation behaviour
in paramagnetic-doped crystalline silicates was undertaken,
using synthetic magnesium orthosilicate (forsterite) and
synthetic zinc orthosilicate (willemite) doped with 0.1% to
20% of Co(II), Ni(II), or CU(II), as experimental systems.
All of the samples studied exhibited a longitudinal
magnetization return to the Boltzmann distribution of nuclear
spin states which followed a stretched-exponential function of
time:
Y=exp [- (tjTn) n], O<n<l
For the most reliable experimental data, there is a bias
toward n=O.Sj the few genuine cases of deviation from 1/2power
are for dopant concentrations equal to or exceeding 2.5
percent dopant.
In some cases we find agreement with theory, and observe
a direct proportionality between the spin-lattice relaxation
time and paramagnetic dopant ion concentration, with
Tni[M2+]i=Tnj[M2+]j for a given dopant and mineral. There are many
cases where this correlation is not apparent, however, and
this is attributed to the structural, phase, and ion
distribution complexities inherent in many of these systems.
