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The kinetics of the site exchange reaction of Al 3+ cations between tetrahedral and octahedral sites in MgAl 2 O 4 and ZnAl 2 O 4 spinel in natural and synthetic powdered samples was determined by 27 Al MAS-NMR spectroscopy with a high-speed magic-angle-spinning probe using samples heattreated for different time periods. Three site fittings to the 27 Al NMR spectra, one tetrahedral site [4] Al and two octahedral sites ( [6] Al(1) and [6] Al(2)), reasonably reproduced the observed spectra. Among these three sites, [6] Al(2) and [4] Al showed a broad distribution of the nuclear-quadrupole coupling constant, which suggests a distorted coordination environment of the Al ions. In the ZnAl 2 O 4 spinel, the observed degree of cation disorder and its temperature variation are quite small, which is consistent with previous thermodynamic measurements. For the MgAl 2 O 4 spinel, the change in the concentration of [4] Al versus time was fitted well by a single exponential function. This suggests that the reaction [6] Al + [4] Mg → [4] Al + [6] Mg is a first-order reaction. An almost linear relationship between the inverse temperature and the logarithm of the reaction rate yielded an activation energy of 260 ± 30 kJ/ mol. The reverse rate for the reaction [4] Al + [6] Mg → [6] Al + [4]Mg was also determined. The reverse rate was also shown to be first order and the reaction rates were 2 orders greater than those of the forward reaction. The ratio of the rate constants for the forward and reverse reactions gives a reasonable nonconfigurational free energy change for the cation exchange of MgAl 2 O 4 spinel.

The structure and properties of Na-Si-0-N oxynitride glasses have been studied by molecular dynamics calculations using a pair potential of the Busing approximation of the Born-Mayer-Huggins type. Nitrogen atoms bonded to one, two, and three silicon atoms coexist in the glass structure. The mean of the number of silicon atoms bonded to a nitrogen atom ranges from 2.4 to 2.1, decreasing with increasing Na,O content from 15 to 30 mol%. It has been assumed that nitrogen atoms bonded to two or fewer silicon atoms are formed when nitrogen atoms substitute for nonbridging atoms. The bond angle LSi-N-Si exhibits a bimodal distribution around 105-135" and 140-170", roughly corresponding to the nitrogen atoms bonded to three and two silicon atoms, respectively. The dependences of the density, the bulk thermal expansion, and the bulk modulus on the nitrogen content are consistent with those observed in real systems.

Local structure around silicon and nitrogen atoms in Na-Si-O-N oxynitride glasses has been studied by me ans of 29Si MAS NMR spectroscopy. The NMR spectra have been analyzed on the assumption that the resonances of singly nitrified silicate tetrahedra shift towards less shielded direction by about 1.5ppm com pared with those of non-nitrided ones. It has been esti mated that the number of silicon atoms around nitro gen is about 2.5, suggesting that about a half of the total nitrogen atoms may be bonded to two silicon atoms.

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