Sodium cation dynamics in nitrate cancrinite: A low and high temperature Na-23 and H-1 MAS NMR study and high temperature Rietveld structure refinement
M. Fechtelkord et al., Sodium cation dynamics in nitrate cancrinite: A low and high temperature Na-23 and H-1 MAS NMR study and high temperature Rietveld structure refinement, AM MINERAL, 86(1-2), 2001, pp. 165-175
Static and Na-23 MAS and H-1 MAS NMR experiments at temperatures down to 10
0 K as well as high temperature static and 23Na MAS NMR experiments up to 8
73 K were carried out to study the dynamics of the sodium cation in nitrate
cancrinite. In addition, X-ray powder data were recorded at 296 K and 673
K to obtain information about long-range order changes in the structure. Lo
w temperature Na-23 NMR experiments show almost no change in signal, wherea
s the H-1 MAS NMR spec tra show the freezing of cage water motion. The two
different crystallographic sodium sites cannot be distinguished in the 23Na
MAS NMR spectra at low and room temperature. In contrast, two Na-23 NMR si
gnals exist at high temperatures above 673 K, a quadrupolar pattern and a L
orentzian signal. Rietveld structure refinements of powders show no change
in the local environment of sodium. The strong quadrupolar coupling results
from the loss of cage water and the near planar O atom coordination of Na
in the epsilon -cage, while the interaction of Na+ in the channel remains s
imilar. Thus the change in quadrupolar interaction is mainly caused by the
absence of electrostatic shielding of cage water after dehydration.