X-Ray and neutron diffraction study of alums: II. The crystal structure of methylammonium aluminium alum III. The crystal structure of ammonium aluminium alum

Study on the phase transitions by nuclear magnetic resonance of alpha-type RbAl(SO4)2 x 12H2O and beta-type CsAl(SO4)2 x 12H2O single crystals

The thermodynamic properties, spin-lattice relaxation times, T(1), and spin-spin relaxation times, T(2), of the (27)Al, (87)Rb, and (133)Cs nuclei in MAl(SO(4))(2)x12H(2)O (M=Rb and Cs) crystals were investigated, and the two crystals were found to lose H(2)O with increases in temperature. From our results for T(1) and T(2), we conclude that the discontinuities near T(d) in the T(1) curves of the two crystals correspond to structural changes. In both crystals, below T(d) the water molecules surrounding the Al(3+) and M(+) nuclei form distorted octahedra, whereas above T(d) the water molecules around the Al(3+) and M(+) nuclei form regular octahedra and the environment of the Al(3+) and M(+) nuclei has cubic symmetry. Further, the T(1) for the (27)Al and (87)Rb nuclei in RbAl(SO(4))(2)x12H(2)O below T(d) were found to increase with increasing temperature, whereas the T(1) for the (27)Al and (133)Cs nuclei in CsAl(SO(4))(2)x12H(2)O were found to decrease. It is possible that this difference is due to the different characteristics of alpha- and beta-type crystals.

Solid-state 33S MAS NMR of inorganic sulfates

Solid-state (33)S MAS NMR spectra of a variety of inorganic sulfates have been obtained at magnetic field strengths of 4.7, 14.1, 17.6, and 18.8 T. Some of the difficulties associated with obtaining natural abundance (33)S NMR spectra have been overcome by using a high magnetic field strength and magic angle spinning (MAS). Multiple factors were considered when analyzing the spectral linewidths, including magnetic field inhomogeneity, dipolar coupling, chemical shift anisotropy, chemical shift dispersion, and quadrupolar coupling. In most of these sulfate samples, quadrupolar coupling was the dominant line broadening mechanism. Nuclear electric quadrupolar coupling constants (C(q)) as large as 2.05 MHz were calculated using spectral simulation software. Spectral information from these new data are compared with X-ray measurements and GAUSSIAN 98W calculations. A general correlation was observed between the magnitude of the C(q) and the increasing difference between S-O bond distances within the sulfate groups. Solid-state (33)S spin-lattice (T(1)) relaxation times were measured and show a significant reduction in T(1) for the hydrated sulfates. This is most likely the result of the modulation of the time-dependent electric field gradient at the nuclear site by motion of water molecules. This information will be useful in future efforts to use (33)S NMR in the compositional and structural analysis of sulfur containing materials.