SI-29 MAS NMR-SPECTRUM AND SUPERSTRUCTURE OF MODULATED TRIDYMITE L3-T-O(MX-1)

The Si-29 MAS NMR spectrum of the third room-temperature modification of tridymite (L3-T-o(MX-1)) exhibits ten partly resolved signals, whic h are assigned to 24 symmetrically independent silicon atoms. The inco mmensurate superstructure was refined with single-crystal x-ray diffra ction data, showing that the modulation consists of a complex system o f tilted tetrahedra, which can be described as a combination of transv erse and longitudinal wave-like shifts of the atoms without severe dis tortions of the tetrahedral units themselves.