NMR and NQR investigations of nuclear quadrupole interactions in alums II. Internal motions and the temperature dependence of the electric field gradient

Neutron diffraction study of alums L The crystal structure of hydroxylammonium aluminium alum L The crystal structure of hydroxylammonium aluminium alum

The crystal structure of the hydroxylammonium aluminium sulfate dodecahydrate alum, (NH3OH)Al(SO4)2 · 12H2O, has been determined from three dimensional neutron diffraction data. The compound crystallizes cubic with a - 12.328(4) Å in the space group Pa3. The (NH3OH) + complex around ½½½, the centre of symmetry, is arranged with equal probability over two orientations related to each other by a centre of symmetry. The elastic diffraction experiment gives a space and time-average picture of two orientations obtained by a rotation of the (NH3OH)+ complex around an axis perpendicular to the N − O bond. The positional and thermal parameters have been refined by full-matrix least-squares analysis to a weighted R-value of 1.9%. Disorder in the (SO4)2− complex has been considered and only 3% disorder has been found.

Effects of T2-relaxation in MAS NMR spectra of the satellite transitions for quadrupolar nuclei: a 27Al MAS and single-crystal NMR study of alum KAl(SO4)2.12H2O

Asymmetries in the manifold of spinning sidebands (ssbs) from the satellite transitions have been observed in variable-temperature 27Al MAS NMR spectra of alum (KAl(SO4)2.12H2O), recorded in the temperature range from -76 to 92 degrees C. The asymmetries decrease with increasing temperature and reflect the fact that the ssbs exhibit systematically different linewidths for different spectral regions of the manifold. From spin-echo 27Al NMR experiments on a single-crystal of alum, it is demonstrated that these variations in linewidth originate from differences in transverse (T2) relaxation times for the two inner (m=1/2<-->m=3/2 and m=-1/2<-->m=-3/2) and correspondingly for the two outer (m=3/2<-->m=5/2 and m=-3/2<-->m=-5/2) satellite transitions. T2 relaxation times in the range 0.5-3.5 ms are observed for the individual satellite transitions at -50 degrees C and 7.05 T, whereas the corresponding T1 relaxation times, determined from similar saturation-recovery 27Al NMR experiments, are almost constant (T1=0.07-0.10 s) for the individual satellite transitions. The variation in T2 values for the individual 27Al satellite transitions for alum is justified by a simple theoretical approach which considers the cross-correlation of the local fluctuating fields from the quadrupolar coupling and the heteronuclear (27Al-1H) dipolar interaction on the T2 relaxation times for the individual transitions. This approach and the observed differences in T2 values indicate that a single random motional process modulates both the quadrupolar and heteronuclear dipolar interactions for 27Al in alum at low temperatures.